INCREASED BIOGAS PRODUCTION BY ANAEROBIC CO-DIGESTION OF
WASTEWATER SLUDGE WITH FRUIT AND VEGETABLE WASTE, AND BY
SLUDGE PRE-TREATMENT

By

Nathan D. Park
B.Sc., University of Northern British Columbia, 2008

THESIS SUBMITTED IN PARTIAL FULFILLMENT OF
THE REQUIREMENTS FOR THE DEGREE
OF MASTER OF SCIENCE
IN
NATURAL RESOURCES AND ENVIRONMENTAL STUDIES
(ENVIRONMENTAL SCIENCE)

UNIVERSITY OF NORTHERN BRITISH COLUMBIA
January 2012

© Nathan D. Park, 2012

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Full scale anaerobic co-digestion of fruit and vegetable waste (FVW) and municipal
wastewater primary sludge significantly increased biogas production. Digester operation
remained stable. Undigested FVW was visible in dewatered sludge suggesting that FVW
should be added to the first stage digester to prevent short-circuiting and increase the
hydraulic retention time (HRT) of the FVW. Batch lab results confirmed that co-digestate
addition to first stage sludge (FSS) is preferred to second stage sludge (SSS). FSS
produced significantly more methane (514 ± 57 L CH4 kgVS"1added) than SSS (392 ± 16 L
CH4 kgVS 'added)- In a related study, combined alkaline and ultrasonic pre-treatment of

thermomechanical pulp mill sludge (PMS) significantly increased the soluble TS, VS,
and COD of the PMS over non-treated sludge. Pre-treatment did not significantly
improve biogas production over 28 d, but did increase VS reduction, and the initial rate
of methane production. Overall, biogas production from PMS was inconsistent.

TABLE OF CONTENTS
ABSTRACT

ii

TABLE OF CONTENTS

iii

LIST OF TABLES

vi

LIST OF FIGURES

viii

LIST OF APPENDICES

xi

ACKNOWLEDGEMENT

xii

CHAPTER 1: Introduction

1

1.1 A short history of anaerobic digestion
1.2 The biology of anaerobic digestion
1.3 Parameters affecting anaerobic digestion
1.4 Inhibition of anaerobic digestion
1.5 Advantages of using anaerobic digestion to treat wastes
1.6 Disadvantages and limitations of anaerobic digestion
1.7 Anaerobic digestion, global warming, and renewable energy
1.8 Anaerobic digestion in the municipal sector
1.9 Food waste and co-digestion
1.10 Anaerobic digestion in the pulp and paper industry
1.11 Pre-treatment of pulp sludge
1.12 Alkali pre-treatment
1.13 Ultrasound pre-treatment

1
4
8
14
19
22
23
24
25
32
32
35
36

CHAPTER 2: Increased biogas production in a wastewater treatment plant by
anaerobic co-digestion of fruit and vegetable waste and sewer sludge — A full scale
study
41
2.1 Abstract
2.2 Introduction
2.3 Materials and Methods
2.3.1 The Lansdowne wastewater treatment process
2.3.2 Fruit and vegetable waste (FVW) collection and addition
2.3.3 FVW laboratory analysis
2.3.4 Statistical data treatment
2.4 Results and Discussion
2.4.1 Fruit and vegetable waste addition
2.4.2 FVW and SSS characteristics
2.4.3 Digester performance and response
2.4.4 Operation and maintenance

41
41
44
44
46
46
47
47
47
48
49
54
iii

2.4.5 Biosolids quality
2.5 Conclusions
2.6 Acknowledgements

55
55
56

CHAPTER 3. Comparison of methane production co-digesting fruit and vegetable
waste with first stage and second stage anaerobic digester sludge from a two stage
digester
58
3.1 Abstract
3.2 Introduction
3.3 Materials and Methods
3.3.1 Lansdowne Wastewater Treatment Plant
3.3.2 Food waste
3.3.3 Sewage sludge
3.3.4 Batch digestions
3.3.5 Biogas analysis
3.3.6 Statistical data treatment
3.4 Results and Discussion
3.4.1 Initial characteristics of sewage sludge and FVW
3.4.2 Biogas generation
3.4.3 Methane generation
3.4.4 Methane content of biogas
3.4.5 COD reduction
3.5 Conclusions
3.6 Acknowledgements

58
58
61
61
61
62
62
63
63
64
64
65
67
69
71
71
72

CHAPTER 4. Combined alkaline and ultrasound pre-treatment of pulp mill waste
activated sludge for improved anaerobic digestion
74
4.1 Abstract
4.2 Introduction
4.3 Materials and Methods
4.3.1 Pulp sludge
4.3.2 Pre-treatment
4.3.3 Sludge characterization
4.3.4 Inoculum sludge
4.3.5 Batch digestions
4.3.6 Biogas analysis
4.3.7 Specific resistance to filtration
4.3.8 Statistical data treatment
4.4 Results and Discussion
4.4.1 Effect of sludge pretreatment
4.4.2 Biogas production
4.4.3 Methane production
4.4.4 Post digestion sludge characteristics
4.4.5 Sulphur reduction

74
74
78
78
78
79
79
79
80
80
81
82
82
85
86
87
89
iv

4.4.6 Economics
4.5 Conclusions
4.6 Acknowledgements

90
91
92

CHAPTER 5. Conclusions and Recommendations

93

LITERATURE CITED

98

v

LIST OF TABLES
Table 1.0 Time frame for the hydrolysis of various polymers to their respective
monomers (Deublin and Steinhauser, 2008)

5

Table 1.1 Major acids and alcohols produced during acidogenesis (Gerardi, 2003)
5
Table 1.2 Methanogenic species type differentiated by substrate utilization
formation (Deublin and Steinhauser, 2008)

7

Table 1.3 Components and potential impurities in biogas (Deublin and Steinhauser,
2008)
8
Table 1.4 Common temperature ranges for methane forming bacteria

9

Table 1.5 The effect of ammonia on anaerobic digestion

15

Table 1.6 Concentrations (mg L'1) of alkali cations that will stimulate, inhibit and
strongly inhibit anaerobic digestion (McCarty, 1964; Deublin and
Steinhauser, 2008)

18

Table 1.7 Full scale co-digestion studies utilizing organic waste that have been
reported in the literature since the year 2000. (OFMSW-Organic fraction of
municipal solid waste. SS- sewage sludge. OLR- organic loading rate. FVWfruit and vegetable waste. FW-Food waste. BW- Brewery waste. ARAgricultural residues)
28
Table 1.8 Lab scale co-digestion studies utilizing organic waste that have reported in
the literature since the year 2000
29
Table 1.9 Studies anaerobically digesting pre-treated pulp mill sludge since the year
2000 (PPS- Primary pulp sludge. SPS- Secondary pulp sludge. SMSulphite mill. KM- Kraft mill. NS- Not significant)
38
Table 2.0 Characteristics of FVW added to the second stage digester

48

Table 2.1 Characteristics of FVW and SSS

49

Table 3.0 Initial characteristics of FVW and SS collected from the primary and
secondary digester used in batch experiments

62

Table 3.1 Biogas and methane production rates between the SSS and PSS. Standard
deviations are in parenthesis. Means with different letters indicate
66
significant differences (p < 0.05)

vi

Table 3.2 COD reduction of SS over 10 days of digestion. Standard deviations of
three measurements are in parenthesis. Means with different letters
indicate significant differences (p < 0.05)

71

Table 4.0 Characteristics of thickened (6.5% TS), non-thickened (2.5%TS), and
inoculum sludge pre-treated with NaOH and ultrasound. Numbers in
brackets represent standard deviations, n=2)

82

Table 4.1 Sludge characteristics post anaerobic digestion. Values in brackets
represent standard deviations, n=3

87

Table 4.2 Sludge sulphur reduction

89

vii

LIST OF FIGURES
Figure 1.0 The history of anaerobic digestion

3

Figure 1.1 Comparison of sludge production between anaerobic treatment and
aerobic treatment (Adapted from Gerardi, 2003)

19

Figure 1.2 Land application of biosolids and the nutrient loop

22

Figure 1.3 Effects of disintegration on pulp mill waste activated sludge and mixed
primary and secondary waste activated sludge particle size distribution
after a) no treatment b) various intensities of microwave pretreatment
and c) various intensities of ultrasound pretreatment (Saha et al., 2011)

35

Figure 1.4 Development and collapse of the cavitation bubble (Sonotronic
Technologies)

36

Figure 2.0 The LWWTP plant process

44

Figure 2.1 Set-up for digester and biogas end use. Biogas is either utilized in boiler
(1) or micro-turbines (2), both with excess biogas flared

46

Figure 2.2 Average daily biogas production during the addition of FVW (July 12- Aug
20, 2010) compared to previous years without addition (Means with the
same letter are not significant at p < 0.05)
50
Figure 2.3 Average daily flow of crude sewage sludge to the first stage digester
during the addition of FVW (July 12- Aug 20, 2010) compared to previous
years without addition (Means with the same letter are not significant at
p < 0.05)
51
Figure 2.4 First and second stage digester sludge pH over the course the study

52

Figure 2.5 Second stage digester biogas carbon dioxide concentration over the course
the study (July 12 - August 20, 2010). The dashed line represents carbon
dioxide concentration. The solid line indicates the mass of FVW inserted... 53
Figure 2.6 First and second stage digester sludge volatile acids/alkalinity ratio

54

Figure 2.7 Left- Pre experiment biosolids Right- Biosolids centrifuged during the
course of the experiment

55

Figure 3.0 Cumulative biogas production at STP. • FSS + FVW. o FSS control.
A SSS + FVW. 0 SSS control. Error bars represent the standard deviation
of three measurements of the daily biogas generation
65
viii

Figure 3.1 Cumulative methane production for: • FSS + FVW. o FSS control.
A SSS + FVW. 0 SSS control. Error bars represent the standard deviation
of three measurements of the daily biogas generation. All gas volumes
were corrected to STP (0 °C, 1 atm)
67
Figure 3.2 Biogas methane concentration of: • PSS + FVW. o PSS control. A SSS +
FVW. 0 SSS control. Error bars represent the standard deviation of three
measurements
69
Figure 3.3 Daily methane production rate of sludges. • Primary digester sludge and
FVW. o Primary digester sludge control. A Secondary digester sludge and
FVW. 0 Secondary digester sludge control. Error bars represent the standard
deviation of three measurements of the daily methane generation
70
Figure 4.0 Difference in soluble TS, VS, and COD between pre-treated and
non-treated sludge. Error bars represent the standard deviation of 2 (TS
and VS), or 3 (COD) measurements

83

Figure 4.1 Net cumulative biogas generation. • 6.5% pre-treated A 6.5% non-treated
o 2.5% pre-treated 0 2.5% non-treated. Error bars represent the standard
deviation of three replicates. Biogas generated from inoculum only blanks
was subtracted from the data
85
Figure 4.2 Net cumulative methane generation. • 6.5% pre-treated A 6.5% non-treated
o 2.5% pre-treated 0 2.5% non-treated. Error bars represent the standard
deviation of three replicates. Methane generated from inoculum only blanks
86
was subtracted from the data
Figure A Figure A- Plot of filtration time/filtrate volume (t/V) vs. filtrate volume
(V) for inoculum, 2.5% TS non-treated, 2.5%TS pre-treated, 6.5%TS nontreated, 6.5%TS pre-treated pulp mill sludge post anaerobic digestion
113

Figure B1 Chromatogram of methane standard and human breath. Gas chromatography
conditions are reported in section 4.3.6
116

Figure B2 Chromatogram of biogas. Inoculum replicates 1, 2 and 3, 9:00 am, June
19th, 2011. Gas chromatography conditions are reported in section 4.3.6....117
Figure CI Cumulative methane generation for inoculum at standard temperature and
pressure
118
Figure C2 Cumulative methane generation for 2.5%TS, not pre-treated, at standard
ix

temperature and pressure

118

Figure C3 Cumulative methane generation for 2.5%TS, pre-treated, at standard
temperature and pressure

119

Figure C4 Cumulative methane generation for 6.5%TS, not pre-treated, at standard
temperature and pressure

119

Figure C5 Cumulative methane generation for 6.5%TS, pre-treated, at standard
temperature and pressure

120

x

LIST OF APPENDICIES
Appendix A Sample calculations

112

Appendix B Sample chromatograms

118

Appendix C Chapter 4 biogas production raw data

120

xi

ACKNOWLEDGEMENT

Firstly, and most importantly, I would like to thank Dr. Ron Thring. I chose Dr.
Thring as my supervisor due to a small gesture he made acknowledging an undergraduate
project I presented in his class. At the time I did not know how important of a decision
this was, but I am glad I made such an excellent choice. I appreciate his understanding,
advice, direction, and independent guidance. He was always looking out for my best
interest. I am grateful for his patience as I jump from topic to topic during a conversation.
I am especially glad that he is always pushing me to do things outside of my comfort
zone and ensuring that I get the most out of my degree as possible in the least amount of
time. Thank you.
I am also very thankful for the help that was given to me by all of my committee
members. I appreciate the extra effort Dr. Mike Rutherford put in working on our side
projects and for always being welcoming when 1 interrupted him. I would also like to
thank Dr. Steve Helle for always having a question that I didn't know the answer to.
These questions tended to be crucial. Thank you too to Mr. Randy Garton. His years of
direct experience in the wastewater treatment process provided a wealth of knowledge. I
appreciate him allowing me to mess with his wastewater treatment plant.
Thank you to all of my friends: Jordan Keim, Adam Middleton, Bobby Chavarie,
Harpuneet Singh and Mateusz Partyka for always reminding me that I can do it
tomorrow. Some things are more important than work.
Lastly, I would like to thank Amy for being the motivation to better myself, for
always being positive and helpful, listening to me, and always believing in me. I'm sorry
I was never home for dinner.
xii

CHAPTER 1. Introduction
1.1 A short history of anaerobic digestion
Anaerobic digestion (AD) has been used by humans to produce energy as far back
as the 10th century. It is believed that the ancient Assyrians used methane gas produced
from animal wastes to heat bath water (Stepan, 2008). Lessandro Volta, in the 18th
century, was the first scientist to report that gas collected from aquatic sediments was
combustible (McCarty, 2001). He concluded that the organic matter contained in aquatic
sediments was producing "combustible air" (McCarty, 2001). Faraday, Dalton, Henry,
and Davy continued to work with this "combustible air" until in 1821 Amedeo Avogadro
concluded that the chemical structure of the flammable gas was CH4 (Deublin and
Steinhauser, 2008). One of the first practical applications of anaerobic digestion was
applied to wastewater in 1860 (McCarty, 2001). The "Mouras Automatic Scavenger",
invented by M.L. Mouras, was an airtight tank that was used for the stabilization of
sewage. The invention was reported in the French Journal Cosmos and claimed to be the
"complete solution to the problem which for centuries had been an insolent menace
hurled in the face of humanity" (Moigno and Mouras, 1882). In 1895, methane gas
produced from waste was being utilized as an energy source in Exeter, England. The
methane gas produced from treatment of wastewater was used for heating and lighting at
the sewage disposal plant (McCarty, 2001). Around the same time at a leper colony in
Bombay, India, sewage disposal tanks were being fitted with gas collectors in order to
store the methane produced for use in engines (AEBIOM, 2010).
In the 20th century, research and development of AD was concerned with
wastewater stabilization and sludge solids reduction more than energy production.

1

Around 1905, Imhoff and Travis each designed a two stage sludge digestion system (that
bears their names) for the stabilization of solids collected from wastewater (McCarty,
2001). From 1930-1950 the optimization of the AD process was investigated by studying
parameters such as temperature, pH, mixing and retention times. In the 1970's more
complex AD systems were invented in order to increase waste stabilization and methane
gas production. These systems included; the anaerobic contact process, the anaerobic
filter, and the up flow anaerobic sludge blanket reactor. A short timeline summarizing the
main events in the development of anaerobic digestion is presented in Figure 1.
Although AD has been in use for well over 100 years for the treatment of
wastewater sludges, its potential for energy production is only now being fully
recognized. Concerns over fossil fuel reserves, energy security, and anthropogenic
climate change have sparked interest in the use of AD to produce renewable energy from
wastes. Various agricultural, municipal, and industrial wastes have been successfully
processed by anaerobic digestion on large scales (Alatriste-Mondragon et al., 2006).
European nations have become a leader in this technology with over 6000 AD facilities
currently operating within the European Union (AEBIOM, 2010). Canada by
comparison, with its abundance of cheap fossil fuels, and lack of government legislation
pertaining to waste disposal, has approximately 17 digesters in operation (IEA, 2010).
However, Canada is a budding market with over 34 new anaerobic digestion plants
expected to be complete by 2012 (IEA, 2010).

2

Volta describes: "combustable
air"

_ Fair & More: Digester seeding
and pH control

- Moras Automatic Scavenger
_ Bechamp: Mixed population of
microorganisms

Dalton, Henry, Davy: Chemical
structure of CH4

Stadner: Imprtance of SRT for
reducing size

Herter: Acetate forms CH4 and
C02

•• Poebel: Extensive co-digestion

Anaerobic filter
_ Avogadro: Final chemjcal
structure of CH4

_ Moncreiff: Hybrid AD tpnk
constructed

Cheap oil prices reduce biogas
value
Schroepfer: Anaerobic contact
process

t AD used in Exeter England

_ Bryant: Minimum of two kinds
of bacteria

Methane from AD ^ised in
engines in Bombay

j Oil Crisis drives biogas demand
Anaerobic filter for dilute
wastewater

1775 1800 1825 1850 1875 19Clo 1925 1950 1ST5 2000
_ Storage and compression of
gas
Ruvrverbad: Sludge heating
_ Methane sold to public gas
works
Buswell: Industrial and
agricultural research
About 75 Travis-Imhoff tanks
in use
- Travis-Imhoff tank invented
- Clark: AD of sludge

Waste A[yoidence Management
Act
Speece: Trace elements
_ McCarty: Anaerobic baffled
reactor
- Recognization of AD granules

- Letbnga: USAB

Figure 1.0 The history of anaerobic digestion (Deublin and Steinhauser, 2008; Khanal, 2008; McCarty, 2001)

3

1.2 The biology of anaerobic digestion
Anaerobic digestion is the biological process by which organic matter is broken
down into methane and carbon dioxide in an anoxic environment. It is a natural process
that occurs in mud, ocean sediments, and the guts of ruminants. AD is an extremely
complex, sequential, synergistic process, carried out in cooperation by Archaea, bacteria,
fungi, yeast, and protozoa. A typical anaerobic digester can contain over 120 different
species of bacteria having a density greater than 1016 bacterial cells mL"1 (Gerardi, 2003).
The process can be divided into four sequential stages each having its own flora of
organisms. The four stages are described in detail below.
1.2.1 Hydrolysis
The first step of the AD process is termed hydrolysis. Large, insoluble polymers
are broken down by enzymes such as lipases, cellulases, and proteases into smaller more
soluble monomers. Solubilization is necessary in order for the molecules to enter the
bacterial cell and therefore the majority of the enzymes used are released extracellularly.
The process is very complex with bacteria, fungi, and protozoa all contributing. The
various rates of breakdown for large polymers are summarized in Table 1.0. When
digesting waste streams made up of particulate matter, or complex materials that are
difficult to degrade, hydrolysis is considered the rate limiting step of the methane forming
process (Jeihanipour et al., 2011; Noike et al., 1985).

4

Table 1.0 Time frame for the hydrolysis of various polymers to their respective
monomers (Deublin and Steinhauser, 2008).
Polymer
Carbohydrates
Fats

—*
—»

Proteins
Lignocellulosic
Compounds

—*
—•

Monomer

Timeframe

Sugars
Fatty acids,
Glycerine
Amino acids
Sugars

Hours
Days
Days
Slowly/Incompletely

1.2.2 Acidogenesis
The next step in the anaerobic digestion process is the conversion of the soluble
monomers formed by the hydrolysis of complex molecules into short chain organic acids
(volatile fatty acids), alcohols, carbon dioxide, and hydrogen. This phase is termed
acidogenesis. As seen in Table 1.1, these short chain acids and alcohols are mostly less
than five carbon atoms in length (Deublin and Steinhauser, 2008).
Table 1.1 Major acids and alcohols produced during acidogenesis (Gerardi, 2003).
Name
Acetate
Butanol
Butyrate
Caproic acid
Formate
Ethanol
Lactate
Methanol
Propanol
Propionate
Succinate

Formula
CH3COOH

CH3(CH2)2CH2OH
CH3(CH2)2CH2COOH
CH3(CH2)4COOH
HCOOH
CH3CH2OH
CH3CHOHCOOH
CH3OH

CH3CH2CH2OH
CH3CH2COOH
HOOCCH2CH2COOH

Many of the bacteria involved in hydrolysis are also involved in acidogenesis. Therefore,
the first two steps of the AD process are sometimes combined and termed, "anaerobic
fermentation". Bacteria involved in this step have quick growth rates compared to

5

Methanogens and are much less sensitive to environmental conditions such as
temperature, pH, and inhibiting compounds (Ahring et al., 2001).
1.2.3 Acetogenesls
Acetogenesis is the process by which the volatile fatty acids (VFA's) and alcohols
produced by the acidogenic bacteria are converted to acetate and hydrogen.
Short chain acids + alcohols + H2O —» Acetate + H+ + H2 (AG= + 9.6-76.1 kJ)
The oxidation of the short chain acids is not thermodynamically favorable. Only when the
partial pressure of H2 is low will the reaction occur. The acetogenic organisms must
therefore live symbiotically with the methanogenic bacteria that consume H2 in order for
the reaction to take place. This phenomenon is known as "interspecies hydrogen
transfer", or syntropic interaction (Deublin and Steinhauser, 2008; Amani et al., 2010).
The second group of organisms present in this phase is the homoacetogens.
Homoacetogens reduce H2 and CO2 to acetate.

2C02 + 4H2

CH3COOH + H20

1.2.4 Methanogenesis
The methanogenic class of organisms is not made up of bacteria at all, but are
organisms from the kingdom Archaea. Archaea are descendents of some of the most
primitive life on earth when the atmosphere was much reduced (Novaes, 1986). Archaea
are distinguished from bacteria by their lack of peptidoglycan, membrane lipids, and
distinct RNA (Khanal, 2008). There are three main groups of methanogenic organisms
that are used for the formation of methane and they are distinguished by which substrate
they utilize.

6

Table 1.2 Methanogenic species type differentiated by substrate utilization. (Deublin and
Steinhauser, 2008).
Type

Example Reaction

CO2 Type
Methyl Type
Acetate type

C02 + 4H2 — CH4 + 2H20
CH3OH + H2 —> CH4 + H20
CH3COO- + H20 CH4 + HCO3
—*

The majority of methane, about 70%, is produced from the acetate pathway whereas 30%
is generated from the reduction of C02 (Khanal, 2008; Deublin and Steinhauser, 2008).
The CO2 pathway is favored energetically, but limited due to the amount of hydrogen
present in the digester (Amani et al., 2010). If sulphate is present in the waste, a group of
organisms called, "sulphur reducing bacteria (SRB)" compete for substrates and reduce
sulphur to form hydrogen sulphide gas (Khanal, 2008). If nitrate is present, it will act as
an electron acceptor and become reduced to nitrogen gas. Bacteria that reduce nitrogen
are called, "nitrate-reducing bacteria (NRB)".
1.2.5 Biogas
Microbial action causes the reduction and stabilization of the wastewater sludge
and produces biogas. The concentrations of different compounds in the biogas can vary
greatly depending on the characteristics of the waste used in digestion. Some of the
various components and their respective concentrations can be seen in Table 1.3.

7

Table 1.3 Components and potential impurities in biogas (Deublin and Steinhauser,
2008).
Component

Concentration

Origin

Methane
Carbon Dioxide
Hydrogen sulphide
Ammonia
Nitrogen
Water
Siloxanes

50- 75% (vol.)
25-50% (vol.)
0-5% (vol.)
0-0.05% (vol.)
0-5% (vol.)
1-5% (vol.)
0-50 mg m"3

Reduction of organic waste
Reduction of organic waste
Reduction of sulphate by SRB
Reduction of N compounds, influent
Reduction of nitrate by NRB, air
Due to heating water
Degradation of cosmetics/detergents

Depending on the amount of methane present in the biogas it has an energy content of
6.0-6.5 kWh m'3 of biogas and the fuel equivalent of 0.6-0.65 L oil m*3 of biogas at STP
(0°C, 1 atm) (Deublin and Steinhauser, 2008).
1.3 Parameters effecting anaerobic digestion
1.3.1 Temperature
Like any biological system AD is dependant of temperature. Even small
temperature changes of ± 2° C can temporarily stunt biogas production (Chae et al.,
2008). Methanogens are the most sensitive group of organisms to temperature changes.
Hydrolysis and acidogenesis can occur over a much wider temperature range (Gerardi,
2003). There are three main temperature ranges that AD can be performed at (Table 1.4).
Each temperature is preferred by a different array of methanogenic organisms.
Mesophilic digestion is the most popular due to its lower energy requirement than
thermophilic digestion, shorter retention time than psychrophilic digestion, and the larger
variety mesophilic Methanogens. In general, more species of Methanogens prefer
mesophilic temperatures (Deublin and Steinhauser, 2008; Gerardi, 2003).

8

Table 1.4 Common temperature ranges for methane forming bacteria.
Name
Hyperthemophiles
Thermophilic
Mesophilic
Psychrophilic

Temperature (°C)
>65
48-55
35-40
5-15

Thermophilic digestion is beneficial when destruction of pathogens is necessary, or
higher loading rates are required, however; it costs more to operate, it is more sensitive to
temperature changes, and is more sensitive to toxic compounds. Psychrophilic digestion
is most often performed in small scale sewage treatment units such a septic tanks
(Gerardi, 2003). Large scale psychrophilic AD has advantages in colder climates, but
research is very limited in the field (Lettinga et al., 2001).
1.3.2 pH/Alkalinity
The pH and alkalinity are crucial operating parameters of the anaerobic digestion
process. The buffering systems that maintain a neutral pH in the digester occur naturally.
Even slight changes in digester pH indicate imbalances in activity and are an early
warning for digester failure. A drop in digester pH, and a rise in biogas CO2
concentration, indicates a problem with methanogenesis. Methanogens are most sensitive
to environmental factors and if their activity decreases the level of volatile acids in the
digester can raise causing acidification and more subsequent inhibition. Optimum
Methanogen productivity is achieved at a pH of 6.7-7.5 (Deubiin and Steinhauser, 2008).
Thus, digester pH should be maintained at a pH near 7.0. The main buffering reaction in
the digester that controls the digester becoming too acidic is the carbon dioxidebicarbonate-carbonate shift (Deubiin and Steinhauser, 2008). For this reason, an

9

atmosphere of at least 30% carbon dioxide is required to maintain the buffering system
(McCarty, 1964).

C02 <-» H2CO3 <- H+ + HCO3" ~ 2H+ +2C032"
When the pH is low, due to an increase in volatile acids, more carbon dioxide is dissolved
in the water. When the pH increases the carbon dioxide turns into carbonic acid and
lowers the pH. The secondary buffering system that controls the digester becoming too
basic is the ammonia-ammonium system (Deublin and Steinhauser, 2008).
NH3 + H+ <-• NH»+
A digester system should maintain an alkalinity of 1000 to 5000 mg L'1 to maintain a
stable pH and allow sufficient time to correct imbalances (McCarty, 1964). Often, the
ratio of volatile acids to alkalinity is used to monitor the buffering capacity. A ratio of
0.1-0.25 is favorable, 0.3-0.4 signifies digester upset, and at a ratio higher than 0.8
methanogenesis will be disrupted (Kahnal, 2008). Some factors that can cause pH
unbalance in the reactor are; change in temperature, change in organic loading, change in
nature of waste, and the presence of toxic materials (McCarty, 1964). A digester
experiencing an upset in pH can be returned to stable operation by (Deublin and
Steinhauser, 2008):
-

-

adding chemicals (lime, calcium hydroxide, sodium hydroxide)
lowering digester loading rate
dilution
increasing hydraulic retention time (HRT)
co-digesting substrate with high buffering capacity

1.3.3 Nutrients
Nutrient requirements for anaerobic digestion are relatively low compared to
aerobic treatment since only approximately 10% of the organic matter is utilized for

10

biomass synthesis (Kahnal, 2008). It is generally accepted that the optimal C/N ratio of
waste for anaerobic digestion is 16-25:1 (Deublin and Steinhauser, 2008; Yen and Brune,
2007; Wu et al., 2010; Ward et al., 2008). If the C/N ratio is too high the microbes will
not have the required nitrogen for growth; if the C/N ratio is too low ammonia inhibition
can occur. Co-digestion of substrates with low (manure, sewage sludge) carbon to
nitrogen ratios with high (paper, vegetables) carbon to nitrogen ratios can help to
maximize methane production (Yen and Brune, 2007; Wu et al., 2010; AlattristeMondgradon et al., 2006). Often, nitrogen and phosphorous can be added to nutrient
deficient systems by the addition of urea, ammonia, or phosphoric acid (Kahnal, 2008).
The phosphorous requirement of a digester is approximately 1/7-1/5 that of the nitrogen
(Kahnal, 2008).
There are numerous micronutrients required by the anaerobic system. Ni, Co, Fe,
S2", Se, W, Mg have all been shown to be required micronutrients for methane formation
(Singh et al., 1999). Specifically, cobalt and nickel are essential components of vitamin
B12 and factor F430 which produce methane (Gerardi, 2003). Micronutrients are
generally high in municipal sewage sludge, but can be lacking in some industrial
wastewater treatment scenarios. Elements must be in a soluble form in order to be
absorbed by bacteria. Deficiencies in micronutrients are often mistaken as a form of
toxicity (Gerardi, 2003).
1.3.4 Hydrogen partial pressure
Hydrogen-producing acetogenic bacteria and hydrogen-consuming Methanogens
work together in a symbiotic relationship during anaerobic digestion. If the concentration
of hydrogen is too high acetogenic bacteria will not produce hydrogen; if it is too low

11

Methanogens will not have enough hydrogen for methane production. The maximum
partial pressure of hydrogen is dependant on the specific bacteria present as well as the
substrate (Deublin and Steinhauser, 2008).
1.3.5 Oxidation-reduction potential
The oxidation-reduction potential, or redox potential, must be maintained around 200-350 mV (Khanal, 2008). It is for this reason that the concentration of oxidizing
agents, such as oxygen, sulphate, and nitrates should be as low as possible in the digester
(Deublin and Steinhauser, 2008).
1.3.6 Mixing
Proper mixing of the anaerobic digestion process is required to ensure that
nutrients, substrates, enzymes, and bacteria are able to remain in contact. However, it
should not be too vigorous as to disrupt the spatial symbiotic relationship between
Acetogens and Methanogens (Deublin and Steinhauser, 2008). Also, mixing ensures a
consistent sludge temperature throughout the reactor, that biogas is removed from
solution, that floating/sinking sludge layers are combined, and that fresh substrate is
inoculated with bacteria (Deublin and Steinhauser, 2008). Mixing should be complete,
yet minimal, to maintain spatial relationships between organisms and to reduce energy
requirements (Stroot et al., 2001; Gerardi, 2003). Most often mixing is achieved through
gas recirculation, sludge heating recirculation, or propellers.
1.3.7 Organic loading rate, SRT, and HRT
Organic loading rate (OLR) is usually expressed in volatile solids (VS) m"3 d"1.
The greater the organic loading rate, the greater the biogas production. However, if
digester feed rate is increased too quickly overproduction of VFA's can occur. Volatile

12

fatty acids will not be utilized due to the slow regeneration time of methanogenic bacteria
(days to weeks) compared to the quick regeneration rate of acidogenic bacteria (hours).
This imbalance can cause the digester pH to drop and inhibition of methanogenesis will
occur. For this reason, the OLR of a digester must be increased slowly. The OLR of a
digester can be increased until substrate feed is approximately 12% total solids. At this
consistency, mixing and pumping become difficult, water content is too low for cell
growth, and material transfer is retarded (Deublin and Steinhauser, 2008). With a very
low OLR microbial populations are low, biogas production is low, and the process is
uneconomical. Usually digesters are designed for loading rates of 3.2-7.2 kg VS m"3 d"1,
but due to the low solids content of most wastewaters 0.5-0.6 kg VS m"3 d"1 is more
typical in a municipal setting (Gerardi, 2003).
The SRT, or solids retention time, is the average time that the bacteria stay in the
digester. The HRT, or hydraulic retention time, is length of time that the sludge remains
in the digester. The HRT is determined by the ease of degradation of the substrate. Easily
digested substrates, like glucose and cellulose, require a short HRT. More difficult to
degrade substrates, such as lignin and hemi-cellulose, require a longer HRT. The
maximum OLR of a digester is determined by its SRT. The SRT is a measure of the
digesters ability to maintain a certain biodegradation rate (Khanal, 2008). A high SRT
means a higher population of bacteria inside the digester. A high SRT is favorable
because it improves the rate of organics removal, provides resistance to toxicity,
increases buffering capacity of the sludge, makes the digester less susceptible to shock
loads, and reduces the required size of the digester (Khanal, 2008; Gerardi, 2003). A
variety of anaerobic digestion processes have been devised to maintain high SRT's by

13

either recycling digested sludge, or maintaining biomass on a fixed bed to prevent
washout.
1.4 Inhibition of anaerobic digestion
A variety of environmental factors can inhibit AD at any of the four stages. All
stages of anaerobic digestion must proceed together. If one stage is inhibited substrates
for the remaining stages will not be available. Methanogens are by far the most sensitive
to inhibition. Various organic, inorganic, and environmental inhibitors of the anaerobic
digestion process are described below.
1.4.1 Oxygen
Oxygen is not inhibitory to the acidogenic bacteria; however, it is to the
methanogenic (Deublin and Steinhauser, 2008). Inhibition of methanogenic bacteria
begins at oxygen concentration of 0.1 mg L"1 (Deublin and Steinhauser, 2008).
1.4.2 Light
Light severely inhibits methanogenesis (Deublin and Steinhauser, 2008).
1.4.3 Ammonia/ammonium
Ammonia is formed by the degradation of proteins and urea. Ammonia
(inhibiting) forms ammonium ions (less inhibiting) in solution. Free ammonium is used a
nitrogen source for the microorganisms and free ammonia is toxic (Gerardi, 2003).
NH4+<^NH3 + H+
The ratio of ammonia to ammonium is pH dependant and increases with an increase in
pH. A rise in temperature also shifts the equilibrium towards ammonia increasing
inhibition. The concentrations at which ammonia is detrimental is shown in Table 1.5
(McCarty, 1964):

14

Table 1.5 The effect of free ammonia on anaerobic digestion.
Concentration (mg L"1)
50-200
200-1000
1500-3000
3000+

Effect
Beneficial
No effect
Inhibitory when pH is high
Toxic

Ammonia may inhibit biogas formation through inhibiting methane forming enzymes
directly, or by direct diffusion into cells disrupting proton/potassium transport
(Kayhanian, 1999). Two practical methods to mitigate ammonia inhibition include;
dilution of digester contents with water and adjustment of feedstock C/N ratio using a
high carbon substrate (Kayhanian, 1999).
1.4.4 Sulphur compounds
Sulphur compounds originate from amino acids (cysteine and methionine) in high
protein waste streams, or they can be found in industrial wastes such as pulp mill
wastewater due to chemical addition. Sulphur is often found in wastewater as sulphate
and does not inhibit methanogenesis directly. There are two major groups of sulphur
reducing bacteria that reduce sulphate to sulphide (Chen et al., 2008). SRB can disrupt
anaerobic digestion in a number of ways. Firstly, SRB can compete with the organisms
involved in anaerobic digestion for various substrates in the sludge (Chen et al., 2008).
There is the possibility that they may become dominant in the digester because they
require less energy and do no need a symbiotic relationship with other organisms in order
to grow (Deublin and Steinhauser, 2008). Secondly, dissolved hydrogen sulphide that is
indirectly produced by the SRB can be poisonous to cells at concentrations above 50 mg
15

L"1 (Deublin and Steinhauser, 2008). The mechanism of toxicity is not fully understood,
but it is known to be pH dependant because hydrogen sulphide diffuses across
membranes more easily than sulphide (Gerardi, 2003). Hydrogen sulphide and its various
disassociated forms are shown below.
H2S <-+HS" + H+ «-• S2- +2H+
Thirdly, H2S can inhibit anaerobic digestion indirectly by precipitating trace metals, such
as Fe and Co, which are essential for methanogenic enzyme function (Deublin and
Steinhauser, 2008). Sulphur inhibition can be overcome by acclimatization, by increasing
the COD/sulphate ratio, by addition of metal salts to form precipitates, or by the addition
of molybdate in order to inhibit sulphur consuming organisms (Delee et al., 1998).
1.4.5 VFA's
The short chain organic acids that are formed during acidogenesis are normally
consumed by acetogentic/methanogenic organisms as they form. Normally, their
concentrations are in the range of 50-250 mg L"1 (Sawyer et al., 2003). As long as the pH
of the digester remains neutral they can be maintained at levels as high as 10 g L"1
(Amani et al., 2010). If there is a disturbance in methanogenesis, VFA's can build up
lowering the digester pH. If the pH drops too low methanogenesis can be inhibited. If
buffering capacity of the digester is maintained, and the pH of the system is neutral, the
specific VFA concentration isn't of concern. Addition of alkaline, such as NaOH, can
help to raise the pH and correct acidification due to overproduction of organic acids.
1.4.6 Long chain organic acids
Long chain fatty acids (LCFA's) are an energy dense carbon source for the
microbes involved in anaerobic digestion. Theoretically, due to their high carbon density,

16

they yield the most methane of any substrate. However, at increased concentrations they
can have a toxic effect (Khanal, 2008). LCFA's cause problems due to their
hydrophobicity. They can dissolve into bacterial cell walls and inhibit cell activity,
specifically transport and protection (Gerardi, 2003; Rinzema et al., 1994). Most long
chain fatty acids are in the form of insoluble calcium salts and are therefore of no concern
(McCarty, 1964). However, large sporadic inputs of oil of grease can cause problems
with biogas production (Neves et al., 2009). A concentration of LCFA's greater than 500
g L"1 can cause toxicity in an anaerobic digester (Gerardi, 2003).
1.4.7 Heavy metals
Soluble heavy metals are more toxic than their insoluble forms. Although
necessary in trace amounts, heavy metals can be toxic when higher concentrations are
reached. Since heavy metals do not biodegrade there is the potential for accumulation
over time to toxic levels inside the digester. In general, high concentrations of cadmium,
cobalt, copper, zinc, nickel and chromium can lead to disturbances in methane production
(Deublin and Steinhauser, 2008; Khanal, 2008; Gerardi, 2003). They are listed in order of
toxicity below,
Ni > Cu > Pb > Cr > Zn
Heavy metals adsorb to the surface of bacterial cells and inactivate enzymes by binding
to thiol groups or inactivate enzymes by replacing naturally occurring enzymatic metals
(Gerardi, 2003; Vallee and Ulner, 1972). Operators of anaerobic digesters can control
heavy metal toxicity by diluting the digester, removing toxic materials form the waste,
forming insoluble precipitates with sulphides, or deactivating the biological activity by
the addition of complexing agents such as EDTA (Deublin and Steinhauser, 2008;

17

McCarty, 1964). It requires approximately 0.5 mg of sulphide to precipitate 1.0 mg of
heavy metal (Khanal, 2008). Therefore, maintaining an adequate sulphide level inside the
digester is crucial for preventing heavy metal toxicity.
1.4.8 Light metals
High salt levels can cause bacterial cells to dehydrate due to disturbances in
osmotic pressure. Light metals such as sodium, potassium, calcium and magnesium are
required nutrients at low levels (Table 1.6). At concentrations above 1000 mg L"1 alkali
salts can cause inhibition (McCarty, 1964). Acclimation to high salt concentrations can
greatly reduce inhibitory effects.
Table 1.6 Concentrations (mg L'1) of alkali cations that will stimulate, inhibit, and
strongly inhibit anaerobic digestion (McCarty, 1964; Deublin and Steinhauser, 2008).

Cation

Stimulatorv

Moderately Inhibitory

Toxic

Sodium
Potassium
Calcium
Magnesium

100-200
200-400
100-200
75-150

3500-5500
2500-5000
2500-7000
1000-2400

8000
12000
8000
3000

1.4.9 Synthetic organic compounds
Various synthetic organic compounds can inhibit anaerobic digestion. Benzene
ring compounds (tannins), chlorinated hydrocarbons, and halogenated aliphatics, and Nsubstituted aromatics have all been shown to be toxic at varying concentrations, exposure
times, temperatures, and biomass concentrations (Chen et al., 2008). Poorly soluble
organics tend to accumulate on the surface of sludge solids causing cell membranes to
swell and leak (Heipieper et al., 1994). Compounds such as chlorophenols cause toxicity
by disrupting proton gradients and interfering with energy transduction (Chen et al.,

18

2008; Sikkema et al., 1995). Halogenated alphatics, such as chloroform, have been shown
to be inhibitory to methanogenesis at levels as low as O.Olmg L"1 (Stickley, 1970). Nsubstituted aromatics inhibit anaerobic digestion by interfering with specific enzymes and
chemical pathways (Balderston and Payne, 1976). It should be noted that with most
synthetic organic compounds it has been shown that acclimation can reduce inhibition
greatly (Chen et al., 2008).
1.5 Advantages of a using anaerobic digestion to treat wastes
The advantages of using anaerobic treatment over aerobic treatment are presented
below.
i. Anaerobic treatment of waste produces less sludge. During aerobic digestion 50% of
the substrate is converted to biomass and 50% of it to carbon dioxide and water.
Anaerobic respiration is much less efficient than aerobic respiration with only 10% of the
substrate converted into biomass and 90% converted to wastes and end products.
Therefore much less sludge is produced anaerobically than aerobically (Figure 1.1).
Bacterial Cells (Sludge)

Bacterial Cells
(Shidge)

10%

50%

Organic
Waste

Aerobic Oxidation

Organic
Waste

Respffatory
Wastes
Eod Products
50%

Anaerobic Respiration
90%

Figure 1.1 Comparison of sludge production between anaerobic treatment and aerobic
treatment (Adapted from Gerardi, 2003)
19

ii. Anaerobic digestion requires fewer nutrients. Due to the production of less biomass
anaerobic digestion requires less nitrogen and phosphorous than aerobic digestion. Often
N and P must be added for the proper aerobic treatment of wastes lacking these nutrients.
This is the case with many industrial effluents such as pulp mill effluent.
iii. Anaerobic digestion is less malodorous. The digestion of substrates must occur in a
closed system to exclude oxygen. Therefore, malodorous compounds such as hydrogen
sulphide, methylsulphide, and dimethylsulphide are trapped and cleaned from the biogas,
or combusted.
iv. Anaerobic digestion reduces the pathogen content of the waste. Pathogen levels in
wastes can be reduced as the bacteria, protozoa, and fungi in the digester compete with
and overgrow pathogenic organisms (Deublin and Steinhauser, 2008). Thermophilic
anaerobic digestion has the added advantage of thermally destroying microbial pathogens
present (Smith et al., 2005). Mesophilic anaerobic digestion can also reduce the number
of pathogens present in the sludge to a lesser degree (Kearney et al., 1993). Pathogen
inactivation is dependant on factors such as temperature, retention period, reactor
configuration, microbial competition, pH, and chemical interactions (Smith et al., 2005).
Weed seeds found in manure or biomass wastes are also inactivated by anaerobic
digestion. Generally, the longer the retention time of the sludge the greater the
inactivation of weed seeds (Jeyanayagam et al., 1984).
v. Anaerobic digestion has a positive ("clean") energy balance. When compared to
aerobic digestion, anaerobic digestion has a net positive energy balance (Khanal, 2008).
The energy requirements for the aeration of aerobic systems can account for up to 75% of
the total wastewater treatment plant energy costs (Reardon, 1995). Excess methane
20

produced from the AD process can be used as a fuel. Most often biogas is used in a
combined heat and power (CHP) system to offset the energy requirements of waste
treatment. Waste heat can be used for heating the anaerobic digesters and other accessory
buildings. Excess electricity produced can be sold to the grid. The most commonly used
CHP systems are the four stroke engine, gas-diesel engine, Stirling engine, gas turbine,
and fuel cell (Deublin and Steinhauser, 2008). Biogas can also be upgraded and used as a
vehicle fuel or sold to the natural gas grid. The biogas produced is considered carbonneutral because carbon dioxide released during its combustion was captured from the
atmosphere during previous biomass growth (Muradov and Veziroglu, 2008). Energy
produced from the anaerobic digestion of wastes may even be considered carbon negative
since methane (approximately 20 times more potent greenhouse gas than carbon dioxide)
that may have otherwise been generated uncontrolled in a landfill is combusted to form
carbon dioxide and water.
vi. Anaerobic digestion produces a nitrogen and phosphorous rich fertilizer.
Dewatered, anaerobically digested sludge, often termed biosolids, can contain up to 6%
total N and 3% phosphorous by weight as well a many trace nutrients and heavy metals
(Deublin and Steinhauser, 2008). Application of biosolids has been shown to act a
fertilizer improving soil fertility and plant growth (Singh and Agrawal, 2008; Prescott
and Belvins, 2005; Cogger et al., 2001). Application of biosolids to farm land can be seen
as closing the nutrient loop (Figure 1.2).

21

Consumption

^

•Bp

Wast*
Treatment

land Application
Of Biowlidi

Figure 1.2 Land application of biosolids and the nutrient loop.
Care must be taken when applying biosolids to land to ensure that trace elements, organic
pollutants, and excess N and P do not cause detrimental effects. Many countries have
strict regulations regarding the land application of biosolids in order to minimize loss of
nutrients from the loop and their possible negative effects on waterways, plants, and
animals (Iranpour et al., 2004).
1.6 Disadvantages and limitations of anaerobic digestion
i. Anaerobic digestion has significant capital and operational costs. Often, the capital
and operational costs of running an anaerobic digestion program solely for energy
production make the process uneconomical. AD must be considered as part of an entire
waste treatment system with other benefits besides energy production such as; volume
reduction, odour reduction, fertilizer production, pollution control, and waste heat/carbon
dioxide utilization.
ii. Anaerobic digestion has a slow initial start-up time. Due to the low biomass volume
(sludge volume) produced anaerobic digestion requires a long start up time in order to
achieve optimum biomass concentrations in the digester. The start-up phase of an
anaerobic digester can last 2 to 4 months (Deublin and Steinhauser, 2008).

22

ii. Anaerobic digestion is vulnerable to disturbances and recovery times can be long.
Methanogenic organisms are very sensitive to environmental factors and the recovery of
a digester from failure can take a long time. For this reason AD requires considerable
operator attention.
1.7 Anaerobic digestion, global warming, and renewable energy
Global demand for energy is on the rise and is expected to increase 49%, from
522 to 780 quadrillion kJ by the year 2035 (IEA, 2010). The majority of the energy
consumed in the next 15 years will be supplied by coal, natural gas, and liquid fossil fuels
(IEA, 2010). The intergovernmental panel on climate change (IPCC) has reported that
fossil fuel use, since the 1850's, has lead to a rapid increase in atmospheric carbon
dioxide emissions (IPPC, 2011). Increased carbon dioxide emissions have contributed to
a rise in the average global temperature (IPCC, 2011). An increase in average global
temperature may negatively affect human populations by raising sea levels, increasing the
risk of flooding in some areas, increasing the chance of extreme weather events, by
placing stress on water resources in certain places, increasing the rate of plant and animal
species extinction, and by decreasing crop production in seasonally dry and tropical
regions (IPCC, 2007). The use of renewable energy, such as anaerobic digestion, for
electrical, thermal, and transport energy has the potential to help mitigate climate change
by reducing net greenhouse gas emissions (IPCC, 2011). Thus, the use of anaerobic
digestion as a renewable energy source has the ability to both reduce the amount of
pollution from municipal and industrial sectors while simultaneously producing a
consistent, renewable, carbon neutral energy source that can help reduce carbon dioxide
emissions and the risks associated with rising global temperatures.

23

1.8 Anaerobic digestion and the municipal sector
It is estimated that municipal wastewater contains approximately 10 times the
energy required to treat it (Shizas and Bagley, 2004). Surprisingly, almost all wastewater
treatment plants in North America are net energy users. In the United States, wastewater
treatment consumes 21 billion kWh of electricity per year (Kalogo and Monteith, 2008).
The majority of the energy content of wastewater is contained in the organic solids, or
sludge portion. Energy can be recovered from sewage sludge by gasification,
incineration, pyrolysis, and/or anaerobic digestion (Kalogo and Monteith, 2008). A
combination of processes is preferred in order to extract as much energy from the sludge
as possible.
During the last twenty years the government of Canada has been continuously
improving the level of wastewater treatment in Canada (Environment Canada, 2010).
This has been achieved by building new wastewater treatment plants where wastewater
treatment was not used, by upgrading existing wastewater treatment facilities to higher
levels of treatment, and through the introduction of new treatment technologies. Since
1983, Canada has increased the percentage of the population with access to secondary
water treatment from 40% to 70% while at the same time reducing the amount of people
with no treatment from 30% to 2% (Environment Canada, 2010). Increased secondary
treatment and increased access to wastewater treatment have consequently increased the
production of sewage sludge. Approximately 389,000 dry tones of sewage sludge are
produced each year in Canada (CH2Mill, 2000). Approximately 43% of this sludge is
applied to land, 47% of it is incinerated, and 4% is sent to landfills (Apedaile, 2001). A
small amount is also used for reclamation purposes. Land application and incineration

24

have garnered considerable negative public opinion due to concern over environmental
contamination and health risks (Nazareth et al., 2003). Economically, sewage sludge
disposal can account for more than 50% of the total cost of wastewater treatment
(Rulkens, 2004). Therefore, the benefits of using anaerobic digestion to treat waste are
twofold; waste volume reduction and energy production.
1.9 Food waste and co-digestion
It is estimated that 38% of all food available for retail sale is wasted in Canada
(Statistics Canada, 2008a). The Canadian food system (farm production, food processing,
and preparation) accounts for about 11% of Canada's total energy demand (CAEEDAC,
2000). Not accounting for food transport, or waste disposal, food waste compromises
approximately 4% of Canada's total energy demand. Considering how energy intensive
the food system is, it is surprising that little or no energy is recovered from wasted food.
Some energy is recovered through incineration along with municipal solid waste;
however, the high water content limits the amount of energy that can be recovered from
food waste. The majority of food waste is disposed in landfills where it is allowed to
decompose uncontrolled, contributing potent greenhouse gases to the atmosphere such as
methane.
The high water content and high nutritional value of food waste makes it an ideal
substrate for anaerobic digestion (Zhang et al., 2007). However, fruit and vegetable waste
has been shown to be too easily biodegradable on its own (Bouallagui et al., 2005).
Therefore, anaerobic digestion of food waste must be performed with a co-substrate with
a high buffering capacity in order to prevent VFA acidification. The anaerobic digestion
of two or more different waste streams is termed co-digestion.

25

The benefits of co-digestion include: dilution of potential toxic compounds,
improved nutrient balance, synergistic effects of microorganisms, increased load of
biodegradable matter, and ultimately increased biogas yield (Sosnowski et a!., 2003).
Often, the C/N ratio of wastes is not ideal for anaerobic digestion. The ratio should be in
the range of 16:1 to 25:1 (Deublin and Steinhauser, 2008). Addition of a high C substrate
such as vegetable waste or straw can raise the carbon\nitrogen ratio of a carbon deficient
substrate. Addition of a highly nitrogenous substrate, such as manure or slaughterhouse
waste, can decrease the carbon to nitrogen ratio of a high carbon substrate. Addition of
co-digestate provides excess nutrients which maximize microbial activity and subsequent
biogas production. Zitomer et al. (2008) have shown that brewing waste yeast extract can
increase the biogas production in an operational wastewater treatment plant much more
than theoretically expected by added COD only. The authors hypothesize that the
addition of trace nutrients such as Co, Fe, Ni may have produced synergistic effects.
As well as improving the yield of biogas, co-digestion can improve the
dewaterability and degree of stabilization of the final biosolids (Habiba et al., 2009).
Often, the biogas output of existing facilities treating a single waste stream can be greatly
improved with little or no capital investment by the implementation of co-digestion
(Zupancic et al., 2008). Economically, combined use of wastewater treatment equipment
for treating liquid and solid waste can be advantageous (Hamzawi et al., 1998). Codigestion at facilities treating municipal wastewater sludge are of particular interest
because of the low solids content, low C/N ratio, and low nutritional value of municipal
sludge. These parameters can be optimized by co-substrate addition. There has been great
interest on co-digestion in the last few years. Research papers with the word, "co-

26

digestion" in the title have quadrupled in the last four years (Mata-Alvarez et al., 2011).
Table 1.7 outlines a number of full scale co-digestion applications that have been
reported in the literature. Table 1.8 outlines a number of co-digestion studies at the lab
scale.

27

Table 1.7 Full scale co-digestion studies utilizing organic waste that have been reported in the literature since the year 2000.
(OFMSW-Organic fraction of municipal solid waste. SS- sewage sludge. OLR- organic loading rate. FVW-fruit and vegetable waste.
FW-Food waste. BW- Brewery waste. AR- Agricultural residues. NR-Not reported)
Substrate

Increase in

Increase in Biogas

Comments

Reference

_______________
OFMSW
SS

0.2 kg m"3 d"1
25% increase

0.2-0.5 m3 kg"1 VSS fed
80% increase

- Increased VSS degradation by 10%
- No increase in VSS of effluent

Zupancic et al., 2008

OFMSW
SS

0.2 kg VS m"3 d"1
20% increase

0.54 m3 kg'1 VS fed
50% increase

- Methane content of biogas increased

Bolzonella et al., 2006

OFMSW
SS

0.32 kg VS m"3 d"1
40% increase

0.77 m3 kg"1 VS fed
500% increase

- 8-9 tons of OFSW per 80-90m3 sludge
- Digester was co-digesting for 2 years

Bolzonella et al., 2006

FW
SS

3-6 kg VS m"3 d"1
23% increase

0.6-0.79 m3 m"3 d"1
43%

- No negative impact of digester performance
- HRT decreased from 15.4-7.5

Kubler et al., 2000

FVW
SS

NR
20%

0.049 m3 d"1
27%

- Utilized supermarket FVW
- Small digester volume (240m3)

Edelmann et al., 2000

Manure
AR

2.14 kg VS m"3 d"1

1.41 m3 m"3d"'

- Co-digested agricultural residues
- Electrical output of the plant doubled

Lindorfer et al., 2008

FW
SS

NR

Average of 755 m3 d"1

- 7% of plants electrical requirement
- 42% volatile solids destruction (VSD), 60% CH4

BW/FW
SS

NR

70% increase

-extra 16,000kwhd"1, worth $200,000
- only 1% increase in COD

Yoneyama and Takeno,
2002
Zitomer and Adhikari,
2005

28

Table 1.8 Lab scale co-digestion studies utilizing organic waste that have reported in the literature since the year 2000.
Operation

Substrate

Increase in
OLR

Increase in Biogas
Production

Comments

Reference

Batch

FVW
SS

NR

0.18-0.732 Lg1 VS fed

• Majority of 54 FVW had yields > 0.3 L g added
• Lemon seeds exhibited highest 0.732L /g VS added

Nallathambi Gunaseelan,
2004

Batch

FVW
SS

NR

0.68 L g1 VS fed
1.04 L g"' VS removed

- Co-digestion performed better than individual
substrates alone.

Anhuradha et al., 2007

Batch

FW
SS

1-4 g VS L"1

0.116-0.257 LC^g"1 VS
removed

- 40% food waste by VS was optimum
- increased gas output was due to additional C

Kim et al., 2003

Batch

FW
SS

4 g VS L

0.239 L g" VS removed

- Industrial FW (pig slaughter and FVW screening)
- Optimum C:N was 15:1

Siddiqui et al., 2011

Batch

FW
SS

197 g VS L

0.49 L CH, g"'VS

- 20% TS 30% SS
- 30-50% VSD

Forster-Carneiro et al., 2008

Batch

FW
SS

0.5 g VS L"1 d"1

0.29 L CH4 g'1 VS fed

- 60% VSD

Altinbas et al., 2010

Batch

FW
SS

25-45 g VS l/1

0.43 L CH, g"1 VS fed

- BMP tests
- 90% FW by VS produced maximum methane

Heo et al., 2003

Batch

FVW/FW

NR

0.69-0.79 L g 1 VS fed

- Low food to microorganism ratio

Scaglione et. al., 2008

Batch

FW
SS

3.1 g VS L"1

0.43 L g'1 VS

- Varying F/M ratio
- 50% VSD, 25 days HRT

Liu et al., 2009

29

Table 1.8 Continued.
Operation

Substrate

Increase in
OLR

Batch & Semicontinuous

OFMSW
SS

0.392-3 g VS L"1 d"1

0.4-0.6 L g"1 VSS fed

Semi continuous was two stage and more effective
Slower biogas production at high OLR

Sosnowski et al., 2003

Pilot Scale

OFMSW

2.8 g VS L"1

0.3-0.4 L g1 VS

No statistical differences between housing types,
area, kitchen wrapping materials, or sorting

Davidsson et al., 2007

Batch
(pilot)

FVW
SS

160 gVS L"!

0.09Lg' VS fed

Room temperature, Low COD removal (20%) poor
results. No mixing, high OLR.

Rizk et al., 2007

Batch
(pilot)

OFMSW
SS

24.5 g VSS L'1

0.439 L CH, g_1 VSS d"1

OFMSW inhibited due to VFA accumulation
Stable production due to buffering capacity of SS

Sosnowski et al., 2008

Batch
(pilot)

FVW/KW
SS

0.77 to1.13 g L"1 d'1
0-23% increase

0.15 to 0.45 m3 L"1 d"1

24-33% VSD
% CH, decreased from 63 to 58%

Caffaz et al., 2008

Batch &
Continuous

FVW
SS

0.29 L CH, kg"1 VS
0.63 L g"1 VS removed

30 day BMP
VSD 67%

Lin et al., 2010

3.07 gVSL"1

Batch &
Continuous

FW
SS

1-3 g VS L"1
0.5 -1.0 g VS L"1 d"1

0.53-0.75 L g1 VS
0.3 L g"1 VS

Increasing F/M ratio increased gas production.
NaOH required to control pH. 80% VSD

Chen et al„ 2010

Batch &
Continuous

FW
SS

NR

NR

0.288 L g1 VS fed

80% VSD
70-80% VSD

Mohan and Bindhu,

1.5-5.5 gL"1 d"'

Continuous

FVW
SS
FVW
SW & M

2.5-3.5 g VS d"1

0.6-0.8 L g'1 VS removed

Low mixing was found to be most favorable

Gomez et al., 2006

1.1-1.3 g VS L"1 d"'

0.002- 0.35 L CH, g"1
VS fed

FVW, manure, and SW in various mixtures. Performed
better than substrates alone except FVW SW (1:1)

Alvarez and Liden,
2008

Semicontinuous

Comments

Increase in Biogas
Production

Reference

2008

30

Table 1.8 Continued.
Operation

Substrate

Increase in
OLR

Increase in Biogas
Production

Comments

Reference

Semi-continuous
(pilot)
Semi-continuous

FW
SS
FVW
SS

19.4 gVSd"'

0.5-0.53 Lg-1 VS

- Source sorted supermarket waste

1.5- 5.9 g VS L-1 d-1

0.6 L g-1 VS

- Process overload at 4.4, 5.3, and 5.9 g VS L"1 d"1

Corti and Lombardi,
2007
Murto et al., 2004

Semi-continuous

FVW
SS

NR

0.0072 L g-1 VS fed

- 68% VSD over 30 days, yield was very low

Saev et al., 2009

Semi-continuous

OFMSW
SS

7.4-8.4 g VS L-l d-1

0.41-0.51 L g-1 VS fed

- Improvement of VSD over each individual
substrate
- 6- 22% SS, 22% SS was ideal

Garcia et al., 2010

Sequencing Batch

FVW
SS

2.51 g VSL-1 d-1

0.58 L g-1 VS removed
0.49 L g-1 VS fed

- 43% increase in biogas due to better C/N ratio
(22-25)
- WAS improved buffering capacity of FVW

Bouallagui et al, 2009

Sequencing Batch

FVW

0.3 -3.5 g VS L-l d-1

0.29-0.57 L g-1 VS removed

• Improved filterability of final sludge

Habiba et al., 2009

31

There have been very few full scale co-digestion examples reported in the
literature, especially at wastewater treatment plants (Mata-Alverez et al., 2011). Chapter
two of this thesis represents a six week study co-digesting supermarket waste in the
anaerobic digester of a fully functioning wastewater treatment plant.
Chapter three of this thesis was determined after the completion of the full-scale
study to determine the optimum digester (first or second stage) in which to add codigestate.
1.10 Anaerobic digestion in the pulp and paper industry
In 2007, over 5 billion cubic meters of water were used industrially in Canada
(Statistics Canada, 2007). The pulp and paper industry takes in the most water of any
Canadian industry accounting for almost 38% of the total (Statistics Canada, 2007). This
water is used to dissolve wood fibers and produce pulp. It is estimated that the pulp and
paper industry accounts for almost 50% of all waste discharged into Canadian waters
(Sinclair, 1990). Sludge production from the aerobic treatment of pulp and paper
effluents is estimated at almost 1.5 million dry tons annually (Marche et al., 2003).
Anaerobic digestion is not widely used in the pulp and paper industry due to the long
retention times required to treat secondary sludge and the low degradability of pulp
primary sludge. However, recent advances in the pre-treatment of secondary wastewater
sludges prior to anaerobic digestion have renewed interest in the use anaerobic digestion
to treat pulp mill sludge.
1.11 Pre-treatment of puip sludge
Microbial cell walls are composed of peptidoglycan; glycan strands cross-linked
with peptide chains. Peptidoglycan provides strength to the cell wall and resistance to

32

degradation. Due to the resistant nature of bacterial cells, hydrolysis is considered the rate
limiting step when dealing with biological sludges (Eastman and Ferguson, 1981;
Shimizu et al., 2004). The pre-treatment of sludge can increase the hydrolysis rate of
secondary sludge by using mechanical, chemical, biological, or thermal forces to break
apart sludge floes and rupture cell walls. Pre-treatment solubilizes cell components
making them more easily consumed by anaerobic organisms. Disintegrating floe structure
and destroying cell walls also provides easier access to the hydrolytic enzymes that are
released during anaerobic digestion promoting breakdown of the sludge. The advantages
of pre-treating sludge are listed below (Deublin and Steinhauser, 2008).
i. Increase in the yield of biogas: Organic matter found inside large floe structures or
protected behind cell walls is not easily digestible. Pre-treatment increases the proportion
of easily accessible organic matter to anaerobic organisms shortening the hydrolysis
phase. Therefore biogas is produced more quickly reducing the HRT.
ii. Increase in the degree of sludge decomposition: Increased access to internal cell
components, and increased biogas production, ultimately lead to a greater VS destruction
rate. An increased VS destruction rate leads to less sludge.
iii. Sludge viscosity is lowered: Increasing the degradation rate of the sludge lowers the
viscosity due to the lower solids content of the sludge. Pumping, heat transfer, and
mixing require less energy.
iv. Reduction in foaming and floating sludge: Pre-treatment can destroy filamentous
organisms that cause foaming. Also, large floes that trap gas bubbles are destroyed
reducing the amount of floating sludge.

33

Some of the disadvantages of employing sludge pre-treatment are explained below
(Deublin and Steinhauser, 2008):
i. Dewaterability can decrease: As sludge floes are made smaller they become harder to
dewater due to their larger surface area.
ii. Increase in nutrient back load to wastewater treatment plant: Rupturing cell walls
releases N, P, and C into the wastewater. Nitrogen and carbon backload can increase by
up to 30% and 40% due to pre-treatment, respectively (Deublin and Steinhauser, 2008).
In a municipal wastewater treatment plant the minimization of N and P are crucial to the
wastewater treatment plant maintaining appropriate effluent regulations. Thus, pretreatment can be detrimental. In an industrial setting, such as pulp and paper wastewater
treatment, wastewater requires nutrient addition to facilitate microorganism growth.
Increased back load of N and P can be helpful by reducing the amount of external
inorganic fertilizer required for aerobic treatment.
ii. Power consumption of the treatment plant is increased: Pre-treatment of sludge
requires energy. The higher the level of disintegration achieved the greater the amount of
energy required. Careful consideration must be taken to ensure that the benefits of sludge
pre-treatment outweigh the energy costs associated with pre-treatment.
The effectiveness of sludge pretreatment is often measured by the proportion of
soluble COD compared total COD. Increasing the pre-treatment energy will increase the
ratio of sCOD/tCOD to a point. Sometimes physical characteristics are used to evaluate
sludge disintegration such as particle size distribution and microscopic evaluation (Figure
1.3).

34

c)

MW 50 (WAS)

MW100 (WAS)

US IS {WAS)

MW175 (WAS)

US 90 (WAS)

MW 50
(Mixed sludge)

US 15 (Mixed sludge)

MW 100
(Mixed sludge)

MW 175
(Mixed sludge)

US 90 (Mixed sludge)

Figure 1.3 Effects of disintegration on pulp mill waste activated sludge and mixed
primary and secondary waste activated sludge particle size distribution after a) no
treatment b) various intensities of microwave pretreatment and c) various intensities of
ultrasound pretreatment (Saha et al., 2011).
1.12 Alkali pre-treatment
Alkali pre-treatment leads to the saponification of cell walls and internal cell
components making them soluble and therefore more easily hydrolysed (Deublin and
Steinhauser, 2008). Alkaline pre-treatment has been shown to solubilize sludge on its
own, but it is more often combined with other pre-treatment methods to improve their
effectiveness (Saha et al., 2011; Wood et al., 2009; Chiu et al., 1997). Alkaline pre­
treatment has been shown to be especially good at degrading complex wastes such as
lignocelluloses (Hendriks and Zeeman, 2009). Lignin is often found in high quantities in
pulp mill sludge (20-58%) and cannot be digested easily without pre-treatment (Kyllonen
et al., 1988; Deublin and Steinhauser, 2008; Khanal, 2008). Thus, the use of alkaline pre-

35

treatment on pulp sludge is advantageous for solubilizing both microbial cell walls and
recalcitrant lignocellulosic components of the pulp sludge.
1.13 Ultrasound pre-treatment
Ultrasound pre-treatment involves subjugating the sludge to high frequency sound
waves (usually at 20 or 40 kHz). When an ultrasound wave propagates through the sludge
it compresses and expands, pushing and pulling the molecules together and apart. When
the molecules are pulled apart cavitation bubbles form due to large negative pressure. The
cavitation bubbles grow and expand until they collapse and produce very high localized
temperatures and pressures (Pilli et al., 2011). Therefore, ultrasonication produces
mechanical shear forces through cavitation bubbles which rupture cell walls releasing
cellular matter into solution (Elliot and Mahmood, 2007).
A£OU$tiC
Pressure

Growth
Implosion

'Creation

0

Too

aSo

3<5o

aoo

a5o

eoo

Time in ps

Figure 1.4 Development and collapse of the cavitation bubble (Sonotronic
Technologies).
In addition to mechanical shear forces, sonochemical reactions occur in the sludge that
produce highly reactive free radicals such as OH-, HCV, and H- (Khanal, 2008). The
oxidizing effect of the hydroxyl radical also contributes to sludge disintegration (Pilli et

36

al, 2011). However, the effect of the hydroxyl radical oxidation has been shown to be
quite small compared to the effect of cavitation (Pilli et al., 2011; Wang et al., 2005).
Thickening of sludge prior to ultrasonication can have a considerable effect on the
level of sludge disintegration. Increasing the solids content up to 3-4% improves sCOD
release; a TS content of over 4% was found to be detrimental to pre-treatment
effectiveness (Mao et al., 2004; Show et al., 2007; Nies et al., 2000). Sludge viscosity
increases with increasing TS content thus improving energy adsorption and increasing
pre-treatment effectiveness. However, if the solids concentration is too high cavitation
formation and propagation can be hampered (Show et al., 2007).
One negative consequence of ultrasound treatment is its effect on dewaterability.
At low energies (without considerable cell lysis) dewaterability is improved, but at higher
energies dewaterability decreases with increasing energy input (Pilli et al., 2011). As cells
are lysed smaller particles are released and formed which produce a larger surface area
for holding water and are thus more difficult to dewater (Chu et al., 2001). Table 1.9
outlines various studies using pre-treatment to increase the biogas production of pulp and
paper secondary sludge.

37

Table 1.9 Studies anaerobically digesting pre-treated pulp mill sludge since the year 2000 (PPS- Primary pulp sludge. SPS- Secondary
pulp sludge. SM- Sulphite mill. KM- Kraft mill. NS- Not significant. N/A- Not applicable.).
Operation

Pre-treatment

Substrate

Batch &
Continuous

Enzymatic
Ultrasound

SPS

Batch

None

SPS

Biogas
Production
0.1-0.2 LCH,g' VS

0.2 L CH4 g1 VS fed

Comments

Reference

50% @ 8d
44% @ 4d,
NS @ lOOd

- ultrasound & enzymatic pre-treatment didn't improve
yield
- ultrasound improved initial rate of gas production

Karlsson et al..

2011

N/A

- co-digested with monosodium glutamate waste liquor

Lin et al.,

Biogas
Jhngrovement

2010

N/A

Alkaline

SPS

N/A

N/A

• sCOD/COD improved from 7.4% to 32%
• pre-treatment reduced VSS (21% at 60 meq / L)

Navia et al.,
2002

Batch

Microwave
Ultasound
Microsludge

PPS &
SPS

0.12 Lg"1 COD
0.11 Lg-1 COD
0.1 Lg"1 COD

63%

Saha et al.,
2011

34%

• microwave and ultrasound were not economical
• microsludge treated waste lost 23% VS during
pre-treatment

Batch

Thermal
Thermochemical
Sonication

SPS

SM 0.24 KM 0.19 Lg-1 COD
SM0.3 KM0.2 Lg1 COD
SM 0.18 KM0.06 Lg"' COD

KM:50% SM:280%
KM:300% SM:18%
KM: NS SM: NS

• sludge from KM and SM
- fraction of methane in biogas unchanged
• SM produced more biogas than KM

Wood et al.,
2009

Batch

Biological

SPS

0.23 L CH, g'1 VS fed

132%

- co-digested with monosodium glutamate liquor
- pre-treated with mushroom compost extract

Lin et al.,
2010

Batch

Alkaline

PPS &
SPS

0.32 L CH, g"' VS removed

184%

• 8g NaOH/lOOg TS was optimal
• sCOD increased 83%

Lin et al.,
2009

Batch

None

PPS

0.13-0.42 LCHtg VS

N/A

• sludges originated from 3 different plants

Parkarinen et
al., 2010

51%

38

Pre-treatment of sludges produced in the municipal sector has been well
documented in the literature. Municipal sludge pre-treatment technologies have been
demonstrated in many full scale applications (Hogan et al., 2004; Kepp et al., 2000;
Stephenson et al., 2005; Froud et al., 2005; Seivers et al., 2004). These full-scale pretreatment units are available commercially from a range of suppliers. Very little research
has been performed on the pre-treatment of pulp sludge (Elliot and Mahmood, 2007).
Even fewer reports have been published on the combination of multiple pre-treatment
methods for pulp sludge (Table 1.9). The objective of chapter four of this thesis was to
determine the effectiveness of combining alkaline and ultrasonic pre-treatment to
solubilize thickened and non-thickened secondary sludge obtained from Quesnel River
Pulp, B.C., for improved subsequent anaerobic digestion. COD solubilization, biogas
production characteristics, and sludge degradation were used to evaluate pre-treatment
effectiveness.

39

Preface
Chapter 2 of this thesis is an extended version of the published article:
Park, N.D., Thring, R.W., Garton, R.P., Rutherford, M.P., Helle, S.S. 2011. Increased
biogas production in a wastewater treatment plant by anaerobic co-digestion of
fruit and vegetable waste and sewer sludge- A full scale study. Water Science and
Technology 64(9):1851-56.

40

CHAPTER 2: Increased biogas production in a wastewater treatment plant by
anaerobic co-digestion of fruit and vegetable waste and sewer sludge A full-scale study
2.1 Abstract
Anaerobic digestion is a well-established technology for the reduction of organic matter
and stabilization of wastewater. Biogas, a mixture of methane and carbon dioxide, is
produced as a useful by-product of the process. Current solid waste management at the
city of Prince George is focused on disposal and not on energy recovery. Co-digestion of
fresh fruit and vegetable waste (FVW) with sewage sludge can improve biogas yield by
increasing the load of biodegradable material. A six-week full-scale project co-digesting
almost 15,000 kg's of supermarket waste was completed. Average daily biogas
production was found to be significantly higher than in previous years. Digester operation
remained stable over the course of the study as indicated by the consistently low total
volatile acidity-to-alkalinity ratio. Undigested organic material was visible in centrifuged
sludge suggesting that the FVW should have been added to the first stage digester to
prevent "short-circuiting", and to increase the hydraulic retention time of the FVW.
2.2 Introduction
An estimated 38% of solid food available for retail sale was wasted by Canadians
in 2007 (Statistics Canada, 2008a). The majority of this organic waste was disposed of in
landfills. Issues with landfill space, groundwater contamination, resource sustainability,
and greenhouse gas emissions have sparked interest in diversion of waste from landfills.
Composting is the most common form of organic waste diversion from landfills in
Canada. Approximately 1.7 million tonnes of organic waste were composted in
centralized facilities in Canada in 2002 (Statistics Canada, 2008b). Composting produces
41

a valuable resource as a soil amendment, but is an energy intensive process. Also, recent
concerns over energy sustainability, energy security, and anthropogenic climate change
have sparked great interest in producing energy from waste. Incineration is the most
common form of energy generation from waste. However, the high water content in food
waste limits its energy production potential.
Anaerobic digestion (AD) is a process in which bacteria consume organic matter,
in the absence of oxygen, to produce methane gas and carbon dioxide. Biogas, as the gas
mixture is termed, can be used as fuel like conventional natural gas. The remaining
stabilized solids, or biosolids, are beneficial as a nitrogen and phosphorous rich soil
amendment. The AD process has been in use for decades to stabilize and reduce the
organic solids found in wastewaters. The biogas produced can be used to supplement the
energy demands of the wastewater treatment facility. Most municipal AD systems are
constructed with excess capacity to account for future population growth and have low
organic loading rates due to the low solids content of incoming raw sludge. Total solids
content of the digester sludge at the Lansdowne wastewater treatment plant (LWWTP) is
approximately 1.5% and could be increased to the 3 - 4% range without significant plant
modifications (Garton, 2010). Biogas yield could be improved, with minimal capital
investment, by adding biodegradable matter to increase the digester loading. This process
is also known as co-digestion.
Anaerobic digestion of organic wastes is a viable waste disposal method to reduce
greenhouse gas emissions (Haight, 2005). Due to the sealed and controlled nature of AD,
volatile gases are trapped and combusted to form carbon dioxide. Aerobic treatment of
wastes, on the other hand, produces large and uncontrolled emissions of volatile

42

compounds, such as ketones, aldehydes, ammonia and methane (Mata-Alverez et al.,
2000). Methane is of significant importance due to the large amount formed during the
decomposition of organic wastes and its potent greenhouse gas warming potential (almost
20 times that of carbon dioxide). Anaerobic digestion is therefore advantageous from an
emissions standpoint by producing methane in a controlled manner. This methane can
then be burnt to produce carbon neutral CO2 and offset emissions from energy production
that may have otherwise come from fossil fuels (Ward et al., 2008).
The benefits of co-digestion include: the dilution of toxic compounds, improved
balance of nutrients, synergistic effects of organisms, increased load of biodegradable
matter, and ultimately an increase in biogas yield (Sosnowski et al., 2003). Much research
has been conducted in the laboratory on the co-digestion of various organic wastes (Ward
et al., 2008; Alatriste-Mondragon et al., 2006). Lab scale experiments by Zhang et al.
(2007) have shown that food waste is a highly desirable substrate for anaerobic digestion
due to its high biodegradability and methane yield. Full scale anaerobic co-digestion
projects are less widely reported (Mata-Alvarez et al., 2011).
Zupancic et al. (2008) increased the organic loading rate (OLR) of digester
influent by 25% using the organic fraction of municipal solid waste (OFMSW) in a fully
operational wastewater treatment plant. Biogas quantity increased by 80%, electrical
energy production increased by 130%, and heat production increased by 55%. The
increased volatile solids load also improved the volatile solids destruction by 10% and
there was no significant increase in digester effluent volatile solids content. Bolzonella et
al. (2006) have also increased the OLR of a fully operational WWTP by as much as 20%
with the addition OFMSW and subsequently increased gas production by 50%.

43

Edelmann et al. (2000) found an increase in biogas production of 27% when the organic
loading was increased by co-digesting supermarket waste in a relatively small sewage
treatment plant.
The full scale study presented here was undertaken to assess the feasibility of
using local fruit and vegetable waste to improve biogas production in the anaerobic
digesters of the LWWTP in Prince George, British Columbia, Canada. FVW was added
to the second stage digester in order to increase the loading of easily biodegradable
matter and potentially promote digestion of recalcitrant sludge. The waste was collected
from six supermarkets in town; hand sorted, characterized, shredded, and pumped into the
second stage digester. Overall digester performance was evaluated during the course of
the study.
2.3 Materials and Methods
2.3.1 The Lansdowne wastewater treatment plant (LWWTP) process
Discharge to
Fraser River

-ATrickling
Filter
Wastewater
Influent

/

Primary
Sedimirrtation

Disinfection (If required)

Solids
Contact

Clanfier

Skimming. Grit and Screenings

Primary Digester

Secondary Digester

Centrifuge
Oewatering

ESosoiids for Land
Application

Figure 2.0 The LWWTP plant process.
The LWWTP (Figure 2.0) is a secondary wastewater treatment plant that treats
wastewater coming from the municipality of Prince George, British Columbia, Canada.

44

The plant is sized for 115,000 population equivalents (PE). Digesters are fed
approximately 120 m3 d"1 of crude sludge (approximately 3.5-4% TS) from two primary
clarifiers. The anaerobic digestion system is composed of two digesters, operated in
series, each with a maximum volume of 2986 m3. The digesters are operated at
mesophilic (36 ± 2°C) temperatures. The combined hydraulic retention time of both
digesters is approximately 35-40 days. After the sludge has been stabilized it is dewatered
by centrifugation to about 25% dry matter. Methane gas is used in a boiler to produce
heat, or cleaned of hydrogen sulphide and siloxanes and combusted by as set of
microturbines to produce electricity (Figure 2.1). The microturbines are used mainly in
the summertime when the heating requirements of the LWWTP are low.
All in-process measurements were performed by the City of Prince George
laboratory staff. Combined digester biogas production was measured continuously and
totaled daily. Digester gas carbon dioxide concentration is measured 2-3 times per week
using a Fyrite™ gas meter. Methane gas concentration is assumed to make up the
remaining balance. Total volatile acidity and alkalinity of the digester sludge were
determined twice per week by the method outlined by Dilallo and Alberton (1961).
Digester sludge pH was measured by standard methods (APHA, 1998).

45

Micro turbine
Electricity
"Waste Heat"
Heat

Graphite Filter
Gas Flare

Wastewater
Treatment
Plant
Boiler

t
/
^—*lion S >onge

L_!_Jt

Biosolids for Land Application
Centrifuge

Crude Sludge From Primaiy
Sedimentation Tank

Supermarket Waste

Water
Primary Digester

Secondary Digester

Organic Waste Blender

Figure 2.1 Set-up for digester and biogas end use. Biogas is either utilized in boiler (1) or
micro-turbines (2), both with excess biogas flared.
2.3.2 Fruit and vegetable waste (FVW) collection and addition
FVW was collected from Save-On Foods (four locations), Shoppers Wholesale,
and Old Town Country Market in Prince George; B.C. Waste was collected every
weekday morning (excluding statutory holidays). FVW was then spread out on a clean
concrete pad and visible impurities were removed by hand. The "clean" FVW was then
shoveled into a Vaughn™ "Veggie Chopper" pump, mixed with water, and shredded for
2-4 minutes. The shredded waste was then pumped into the sludge heating recirculation
line of the second stage digester.
2.3.3 FVW laboratory analysis
Five to ten kilograms of FVW were selected by random shovelfuls and combined.
Approximately 1 kg of roughly chopped FVW was blended to a thick consistency (2-3
minutes) using a Krups™ household blender. Blended waste was then stored at 4 °C and
analyzed within 48 hrs. Samples of waste were frozen for COD analysis at the end of the
study. The mass of FVW waste inserted was recorded daily. TS, VS, COD, pH, total
46

volatile acidity, and alkalinity were analyzed in duplicate 3 times per week (randomly).
TS, VS, and pH were determined by standard methods (APHA, 1998). Sewage sludge
(SS) and FVW were diluted 5:1 and 25:1, respectively, with deionised water for COD
measurements. COD was determined by the HACH™ colorimetric method (Hach, 2010).
Total volatile acidity and alkalinity were determined by the method outlined Dilallo and
Alberton (1961).
2.3.4 Statistical data treatment
Historical mean daily biogas production over the same six weeks was compared
to that of the study period using analysis of variance (ANOVA). When p was significant
(< 0.05), statistical differences among treatment means were determined using TukeyHSD test. JMP 8 statistical software (SAS, NC, USA 2010) was used to perform
statistical calculations. Microsoft Excel was used to produce the graphs.
2.4 Results and discussion
2.4.1 Fruit and vegetable waste addition
The total mass of FVW inserted weekly is shown in Table 2.0. Impurities were
mainly plastic food wrapping, and other garbage. It was removed by hand and the wet
mass was recorded daily.

47

Table 2.0 Characteristics of FVW added to the second stage digester.
Week

FVW
(kg)

TS
(%)

July 12-16
July 19-23
July 26-30
Aug 3-6
Aug 9-13
Aug 16-20
Total
Mean
SD

1990
2166
2801
2391
2215
3036
14599
2433
404

12.5
12.2
9.6
11.6
13.0
9.7
NA
11.4
1.4

TS

VS Added
(kg)

Impurities
(kg)

Impurities

M

VS
(%TS)

249
264
270
277
288
296
1644
274
17

90.0
89.0
79.9
86.7
89.7
81.7
NA
86.2
4.3

224
234
215
240
259
242
1416
236
1

5.1
1.6
2.5
6.3
3.6
6.1
25.2
4.2
2

0.26
0.07
0.09
0.26
0.16
0.20
NA
0.17
0.08

(%)

Supermarket waste proved to be a reliable and consistent waste stream. One cubic meter
of fresh waste weighed approximately 460 kg. The TS and VS of the collected waste
remained stable over the course of the experiment (Table 2.0). Source sorted supermarket
FVW was found to be extremely low in impurities when compared to similar waste
streams. Kubler et al. (2000) found food waste originating from canteens to be
considerably higher in impurities, 5% by mass respectively.
2.4.2 FVW and SSS characteristics
A comparison of the FVW and second stage digester sludge (SSS) characteristics are
shown in Table 2.1.

48

Table 2.1 Characteristics of FVW and SSS.
Parameter

FVW

SSS*

TS (%)
VS (%TS)
pH
Total Volatile Acidity (mg L" )
Total Alkalinity (mg CaC03 L"1)
COD (g O2 L"1)

11.5 ±2.5
86.2 ± 7.5
4.6 ± 0.4
2477 ± 445
979 ± 359
139 ± 39

1.42 ±0.21
65.8 ± 1.4
7.12 ±0.07
171 ±4
2713 ±52
12.8 ±0.7*

a) Average values from July 1- Aug 31, 2009 (^average COD determined from 1
sampling of SS)
The high moisture content (MC), approximately 89.6%, of the FVW suggests
sufficient water for AD. The high VS content (86%TS) imply a high level of
biodegradability. TS and VS (Table 2.1) of the supermarket waste were found to be
similar to waste collected from similar sources. Bouallagui et al. (2005) reviewed the
characteristics of a number of mixed fruit and vegetable wastes and found it them to have
a TS content between 8-10%, a VS content of about 90%, and total COD around 104 g O2
L"1. More variable waste streams such as OFMSW were found to be much higher in TS
content, approximately 23-35%, and lower in VS, 57-70% when compared to FVW
(Kubler et al., 2000).The VS content of the FVW was approximately 20% higher than the
SSS. This suggests that the FVW is more degradable than the sewage sludge. The total
volatile acidity concentration of the FVW was quite high suggesting that the FVW may
not have had the buffering capacity to maintain optimal pH if anaerobically digested
alone (Ward et al., 2008). Co-digestion with a high buffering substrate such as sewage
sludge was therefore necessary.
2.4.3 Digester performance and response
Average daily biogas production for the study period under investigation (July 12Aug 20, 2010) was compared to the historical biogas production during the same period
49

(Figure 2.2) and was found to be significantly greater than 2008, 2007, and 2006, but not
2009. More stable gas production was observed during addition of FVW than in previous
years as seen by the lower standard deviation. Addition of co-digestate may have helped
to stabilize gas production by providing easily digestible matter and possibly other trace
nutrients that may have been lacking in the second stage sewage sludge.
2700 -J

•f

^ 2500 o

o

"§ 2300 CL
in

8> 2100 o

m

it 1900 -

O
v

2 1700 >

<

1500 2010

2009

2008

2007

2006

Year

Figure 2.2 Average daily biogas production during the addition of FVW (July 12- Aug
20, 2010) compared to previous years without addition (Means with the same letter are
not significant at p < 0.05).
The average weekly biogas concentrations for 2010 were 8-17% higher than the
historical weekly average (Figure 2.2). Not enough FVW could have been added to
account for all of the increased biogas production. Other factors may have contributed to
the increase in biogas. Sownoski et al. (2006) suggest that besides added biodegradable
matter co-digestion can improve biogas yield by dilution of toxic compounds and
improving the balance of nutrients inside the digester. The most important parameter that
contributes to biogas production is the volume of crude sludge that flows into the
digesters. Sludge volume flow to the digesters, over the last year, was directly
50

proportional to an increase in biogas production (data not shown). The average daily
crude flow of sludge into the digesters was not significantly higher in 2010, during the
study period, than any of the previous years (Figure 2.3). Therefore, the increase in
biogas production in 2010 cannot simply be accounted for by an increase in flow of crude
sludge alone.

Figure 2.3 Average daily flow of crude sewage sludge to the first stage digester during
the addition of FVW (July 12- Aug 20, 2010) compared to previous years without
addition (Means with the same letter are not significant at p < 0.05).
Digester pH remained relatively stable over the course of the study (Figure 2.4).
The second stage digester's pH declined slightly when compared to the first stage
digester. This was most likely due to the addition of fresh organic matter. Highly
biodegradable matter such as FVW can cause acidification of the digester due to rapid
hydrolysis, acidogenesis, and subsequent increase in the volatile acids concentration of
the sludge (Ward et al., 2008). However, only a small amount FVW was added to the

51

digester and the strong buffering capacity of the SSS would have prevented any large
shifts in pH.

7.2
7.18
7.16 ^
Primary

I 7.14

y = 0.0004x +7.1039

Q.

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0)
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y = -0.001X + 7.0946

7.06
°

O

Rz = 0.1446

7.02 7
10

15

20

25

30

35

40

Day of Study (July 12 to August 20th, 2010)

Figure 2.4 First and second stage digester sludge pH over the course the study. • Second
stage digester pH. 0 First stage digester pH.
As seen in Figure 2.5, there is an increase in the second stage digester carbon
dioxide concentration after addition of FVW to the digester. This increase may have been
due to the increased hydrolysis and acidogenesis occurring because of the addition of
fresh waste. Addition of FVW occurred between Monday and Friday; thus the drop in
carbon dioxide concentrations on the weekends. It should be noted that the carbon
dioxide concentration of the second stage digester biogas is quite variable.

52

1400

43

_ 1200

%

800
39 Si ™

c 600
o

400

20

30

40

Day of Study (July 5 to September 1 2010)

Figure 2.5 Second stage digester biogas carbon dioxide concentration over the course the
study (July 12 - August 20, 2010). The dashed line represents carbon dioxide
concentration. The solid line indicates the mass of FVW inserted.
The total volatile acidity of the second stage digester generally followed the trend
of the primary, but was overall lower (Figure 2.6). Alkalinity remained stable. The lower
VS content of the second stage digester is due to decreased sludge activity and formation
of volatile acids when compared to the primary digester. Digester health is often
determined by the ratio of total volatile acidity to total alkalinity. Ratios of 0.3-0.4 are
indicative of digester upset (Water Environment Federation, 2008). The volatile acids to
alkalinity ratio of both digesters remained stable (0.06-0.08) throughout the study due to
the high alkalinity and buffering capacity of the sewage sludge (Table 2.1). This suggests
that more co-digestate could have been added without subsequent digester acidification.

53

0.080
Primary Dioestef

0.075

£
c
5 0.070

To
•g
"o
<

® 0.065

••o
TO

O
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0.060
Secondary Digester

0.055
10

15

20

25

30

35

40

Day of Study (July 12 to August 20,2010)

Figure 2.6 First and second stage digester sludge volatile acids/alkalinity ratio.
2.4.4 Operation and maintenance
The only mechanical problem that arose was the clogging of the hose that ran
from the chopper pump to the sludge recirculation line. FVW often plugged this line. It
then had to be manually cleared. Increasing the pump chopping time, decreasing the
amount of waste added per batch, as well as increasing the rinse cycle may have
decreased clogging. Overall there were no other mechanical problems with pumps or
clogging downstream from the waste addition. Edelmann et al. (2000) also found no
negative impact on mechanical operation of anaerobic digesters at a sewage treatment
plant treating similar amounts of supermarket waste. The authors did however notice
accumulation of fibrous scum build-up near the floating roof of the digester. This was not
noticeable at the LWWTP, most likely because the digester in this study was much larger
in volume (2986 m3 compared to 240 m3) and the study period was much shorter (6
weeks compared to 14 months).

54

2.4.5 Biosolids quality

Figure 2.7 Left- Pre experiment biosolids Right- Biosolids centrifuged during the course
of the experiment.
Biosolids collected during the study period were visibly higher in impurities such
as elastic bands, "twist ties", and other plastic (Figure 2.7). The British Columbia
Ministry of Environment Land Application Guidelines for the Organic Matter Recycling
Regulation and Soil Amendment Code of Practice (2008) suggest no more than 1%
foreign material in biosolids used for land application. Increased addition of FVW may
cause regulatory problems with land application. A more thorough pre-treatment process
may be necessary to reduce the impurities. Visible undigested organic matter, possibly
corn husks, was also found in dewatered sludge. This undigested matter suggests that
some of the added waste passed through the second stage digester without fully
decomposing. Addition of waste to the first stage digester may have improved the
digestion of more difficult to digest components by increasing the hydraulic retention
time and preventing "short-circuiting" of waste when sludge was removed for
centrifugation.
2.5 Conclusions
•

FVW is a consistent, reliable, and clean organic waste stream.

55

•

FVW can be disposed of in a fully operational WWTP with no digester
disruption.

•

FVW contributed to an increase in biogas production.

•

Co-digestion may have been more complete if FVW was inserted into the first
stage digester instead of the second.

•

Anaerobic co-digestion of FVW with sewage sludge is a possible alternative to
landfilling.

2.6 Acknowledgements
The authors wish to recognize the City of Prince George for their support. The
following grocery stores, namely; Save-On Foods, Shoppers Wholesale Cash and Carry,
Old Town Country Market were very generous for supplying the fruit and vegetable
waste. The authors would also like to thank to Fikre Debela, Stacey Shantz, Gord Everitt,
Tyler Olsen, and Joanne Logie for their assistance. This research was funded by the
National Science and Engineering Research Council (NSERC) of Canada.

56

Preface
Chapter 3 of this thesis is a version of the manuscript that has been accepted for
publication:
Park, N.D. Thring, R.W. Helle, S.S. 2011. Comparison of methane production codigesting fruit and vegetable waste with first stage and second stage anaerobic
digester sludge from a two stage digester. Accepted for publication in: Water
Science and Technology. Manuscript number WST-WSTWS-EM111060

57

CHAPTER 3: Comparison of methane production co-digesting fruit and vegetable
waste with first stage and second stage anaerobic digester sludge
from a two stage digester.
3.1 Abstract
Fruit and vegetable waste (FVW) was co-digested with first stage sludge (FSS)
and second stage anaerobic digester sludge (SSS) separately, over the course of ten days,
in batch reactors. Addition of FVW significantly increased the methane production in
both sludges. After 10 days of digestion FSS + FVW produced 514 ± 57 L CH4 kgVS"
'added compared to 392 ± 16 L CH4 kgVS"1added for the SSS + FVW. The increased
methane yield was most likely due to the higher inoculum substrate ratio of the FSS. The
final VS and COD content of the sewage sludge and FVW mixtures was not significantly
different from the control values, suggesting that all of the FVW added was degraded
within 10 days. It is recommended that FVW be added to the first stage of the anaerobic
digester in order to maximize methane generation.
3.2 Introduction
Anaerobic digestion is the decomposition of organic waste, by microorganisms, in
an anoxic environment. A mixture of gases, containing mainly carbon dioxide and
methane, are produced and collectively termed biogas. Often, wastewater treatment plants
utilize anaerobic digestion to reduce the volume of sludge solids. As an added benefit,
biogas produced can be consumed as fuel to offset the energy requirements of the
wastewater treatment plant. Some of the advantages of using anaerobic digestion to treat
the organic solids in wastewater include:
•

low sludge volume compared to aerobic digestion

•

low odour emissions

58

•

stabilized digested matter can be used as a N and P rich fertilizer

•

biogas produced is a renewable, carbon neutral source of energy

Due to the low solids content of municipal wastewaters, anaerobic digestion of
low solids primary sludge often produces low volumes of biogas per unit of wastewater
sludge treated. Addition of another organic waste stream, or co-digestion, can increase
biogas production from low solids sludge. Co-digestion can improve biogas yield by:
diluting toxic compounds, improving the balance of nutrients, increasing the synergistic
effects of different microbial populations, and increasing the load of biodegradable matter
in the digester (Sosnowski et al., 2003). Co-digestion of organic waste with sewage
sludge in a fully functional wastewater treatment plant has been shown to improve biogas
yields from anaerobic digesters (Zupancic et al., 2008; Bolzonella et al., 2006; Edelmann
et al., 2000). Biogas production can be increased substantially, with little or no impact on
the wastewater treatment plant effluent quality, and with low initial capital investment
(Zupancic et al., 2008).
Food waste is a particularly promising substrate for anaerobic digestion due to its
high nutrient content, high water content, and rapid degradability (Zhang et al., 2007).
Food waste is also available in consistently large quantities. Statistics Canada (2009)
estimates that 40% of the food that is produced in Canada ends up composted or in the
landfill. This amounts to approximately 27 billion dollars of waste, or by comparison,
slightly below the value of all Canadian agricultural and agri-food exports in 2007
(Gooch et al., 2010). The majority of this waste was disposed in landfills where it was
allowed to decompose uncontrolled. Preserving landfill space, preventing groundwater
contamination, and reducing greenhouse gas emissions can be achieved by diverting

59

organic waste from landfills. To circumvent the need for extensive sorting of residential
waste, and to improve collection efficiency, fruit and vegetable waste (FVW) can be
collected from direct sources such as supermarkets, wholesale distributors, and food
processors. FVW often has a higher methane yield than other sources of organic waste
such as yard wastes, paper, and mechanically sorted municipal solid waste due to its high
VS content and water content (Ward et al., 2008). Specifically, Zhang et al. (2007) have
shown mixed food wastes produce 435 L CH4 kgVS"'removed- Gunaseelan (2004) has also
extensively studied the methane production potential of over 54 various fruit and
vegetable wastes using batch tests. Methane production ranged from 190-400 L CH4
kgVS addedThere are often two anaerobic digesters in series at a wastewater treatment plant.
As the first stage digester is filled with incoming sludge it drains into a second digester.
Sludge is then periodically pumped from the second stage digester for dewatering.
Having two digesters in series minimizes the shock associated with dewatering sludge,
reduces short circuiting of fresh sludge, and allows for a backup digester in case of
mechanical problems. Due to the longer retention time of the sludge in the second stage
digester it is often lower in TS and VS than the primary stage sludge. Very little waste
decomposition and biogas production occur in the second stage digester compared to the
first (Appels et al., 2008). When co-digesting organic waste, waste can be pumped
directly into either the first or second stage of the anaerobic sludge digester.
In a recent full scale study, Park et al. (2011) demonstrated increased biogas
production when co-digesting FVW in the second stage of an anaerobic digester in a fully
operational wastewater treatment plant.

60

In the present study, co-digestion of FVW with first stage digester sludge (FSS)
and second stage digester sludge (SSS) was investigated to determine the appropriate
digester for co-digesting FVW with sewage sludge. First stage and second stage sludge
were collected from the Lansdowne wastewater treatment plant (LWWTP), in Prince
George, British Columbia. The ability of each type of sludge to digest FVW was based on
methane production, solids reduction, and chemical oxygen demand (COD) reduction.
3.3 Materials and Methods
3.3.1 Lansdowne Wastewater Treatment Plant
Please refer to section 2.3.1 for a detailed description of the LWWTP.
3.3.2 Food waste
Food waste used in this study was sampled from a 240 kg load of supermarket
FVW collected from local supermarket outlets. Five to ten kilograms of FVW were
selected by random shovelfuls and combined. Approximately 1 kg of roughly chopped
FVW was blended to a thick consistency (2-3 minutes) using a Krups™ household
blender. 100 g was frozen until use in batch digestion experiments. The FVW was tested
for TS and VS by standard methods (APHA, 1998). pH was determined by centrifuging
food waste at 6,000 r.p.m. for 10 minutes and measuring the pH of centrate using a
Thermo Scientific Orion 3 Star™ pH meter. Food waste was also diluted 25:1 with
deionised water, blended using a household blender for 10 minutes, and analyzed for
chemical oxygen demand (COD) by the Hach™ method 8000 (Hach, 2010). Total
volatile acidity and alkalinity were determined on the centrate of fresh waste, after
centrifuging at 6,000 r.p.m. for 10 minutes, in duplicate, by the method outlined by
Dilallo and Alberton (1961).

61

3.3.3 Sewage sludge
Approximately 2 L of FSS and SSS were sampled by wastewater treatment plant
personnel for use in batch experiments. TS and VS of first and second stage sludge were
determined in duplicate by standard methods (APHA, 1998). First and second stage
digester sludge were diluted 2:1 with deionised water and analyzed for COD by the Hach
method 8000 (Hach, 2010). Average plant values for pH, total volatile acidity, and
alkalinity of sewage sludge were determined on non-centrifuged sludge by laboratory
personnel as described in, "Food waste" above.
Table 3.0 Initial characteristics of FVW and sewage sludge collected from the first (FSS)
and second stage (SSS) digester used in batch experiments.
Parameter
TS (%)
VS (%)
FS (%)
MC (%)
VS/TS (%)
COD (g 02 L1)
pH

Total Volatile Acidity (mg L )
Total Alkalinity (mg CaC03 L"1)

FVW

FSS

SSS

9.16(0.63)
8.13(0.26)
1.04 (0.66)
90.9 (0.6)
89(6)
80 (6)
4.6 (0.01)
1770(42)
770(183)

1.63 (0.01)
1.12(0.01)
0.52 (0.00)
98.4 (0.01)
68 (0.4)
12.56 (0.03)
7.1 (0.04)
207(20)
3000(9)

0.98 (0.01)
0.66 (0.01)
0.33 (0.00)
99.0 (0.01)
67 (0.5)
7.94 (0.48)
7.1 (0.04)
191 (14)
3021(23)

3.3.4 Batch digestions
250 mL amber glass bottles were filled with 150 mL of either first or second stage
digester sludge. Digester sludge received approximately 1 mL of deionised water as a
control and co-digested sludges each received 1.0 g of FVW. Bottles were capped with
rubber septa and the headspace was purged with nitrogen for 2 minutes. Bottle headspace
pressure was equalized using an airtight syringe. Samples were then placed in a dark,
reciprocating water bath at 36 °C, and shaked at 80 r.p.m. All samples were repeated in
triplicate. Biogas volumes were measured by inserting an airtight syringe and allowing
62

the plunger to equilibrate. Daily room temperature and pressure at the time of gas
sampling were recorded and gas volumes were corrected to STP (0 °C and 1 atm). A
syringe was used to sample 0.1 mL of biogas to determine methane and carbon dioxide
concentrations by gas chromatography. At the end of the experiment, samples of sludge
were analyzed for TS and VS by standard methods (APHA, 1998) and COD by the Hach
method 8000 (Hach, 2010).
3.3.5 Biogas analysis
0.1 mL samples of biogas were manually injected and analyzed for carbon dioxide
and methane content using a SRI 8610C gas chromatograph equipped with a Haysep-D
column and thermal conductivity detector (TCD). Helium at 15 mL min"1 was used as a
carrier gas. Column oven temperature was set at 65 °C and the TCD oven at 104°C.
Samples were injected at room temperature. A standard gas sample of methane and
carbon dioxide was used to identify retention factors of component gases (Appendix B).
Peak areas were corrected for differences in TCD response as outlined by Dietz (1967).
Corrected peak areas were normalized to calculate gas volume in percent (Dietz, 1967).
3.3.6 Statistical data treatment
Average final VS, COD, biogas production, and methane production were
analyzed by analysis of variance (ANOVA). When p was significant (< 0.05), statistical
differences between treatment means were determined using the Tukey-HSD test. JMP 8
statistical software (SAS, NC, USA 2010) was used to perform statistical calculations.
Microsoft Excel was used to produce graphs.

63

3.4 Results and Discussion
3.4.1 Initial characteristics of sewage sludge and FVW
The high water content and the high VS/TS ratio of FVW, shown in Table 3.0,
suggest that it is well suited for anaerobic digestion (Zhang et al., 2007). TS, VS, and
COD content of the FSS were considerably higher than the SSS as seen in Table 3.0. The
FVW had a much higher TS, VS, and COD content than both of the sludges due to its
higher solids content and relative freshness. FVW characteristics were consistent with
those reported in the literature (Bouallagui et al., 2005). The neutral pH of both sludges is
due to the high alkalinity and subsequent buffering capacity of the sludge. The high total
volatile acidity of the FVW, and lack of buffering capacity, causes the pH to be lower
than that of the sludges.

64

3.4.2 Biogas Generation

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Figure 3.0 Cumulative biogas production at STP. • FSS + FVW. o FSS control. A SSS +
FVW. 0 SSS control. Error bars represent the standard deviation of three measurements
of the daily biogas generation.
Cumulative biogas generation was higher for the FSS than the SSS due to the
higher initial VS content of the FSS (Figure 3.0). Average biogas yield per kg of
VSconsumed was about the same for the FSS + FVW when compared to the SSS + FVW,
483 ± 33 and 492 ± 63 L kgVS"1 consumed (Table 3.1), respectively. Initial rates of biogas
and methane production were determined by calculating the slope of a straight line fit to
the first four data points (Table 3.1). The data fit a straight line well as seen by the R2
values close to 1. Addition of FVW almost doubled the initial rate of biogas production in
both cases. Initial rates of biogas production increased more in the FSS than the SSS

65

when compared to the controls. At the end of the digestion period the amount of VS
consumed was greater for the FSS than the SSS. The SSS has been in the digesters for a
longer period of time and therefore only the more difficult to digest components remain.
Thus, VS destruction is less than what is seen in the FSS (Table 3.1). The final VS
content of both sludges with FVW added was found to be statistically the same as the
controls suggesting that all of the FVW was consumed within 10 days.
Table 3.1 Biogas and methane production rates between the SSS and PSS. Standard
deviations are in parenthesis. Means with different letters indicate significant differences
(p < 0.05).
Parameter

FSS

FSS + FVW

SSS

SSS + FVW

VS FVW added (g)
Initial VS (g)
Final VS (g)
VSconsumed (g)
Biogas Produced (mL)
BiogaS kgVS consumed (L)
Initial Rate (mL d"1)
R2
CH4 Produced (mL)
CH4 kgVS consumed (L)
Net CH4 (mL)
CH4 kgVS"'added (L)
Initial Rate (mL d'1)
R2

0
1.661 (0.018)
1.230 (0.023)a
0.432 (0.023)a
181 (5)a
420 (23)a
105
0.97
105 (4)a
244 (15)ab
NA

0.081 (0.001)a
1.741 (0.001)
1.231 (0.015)a
0.510 (0.015)b
246 (16)b
483 (33)a
179
0.99
147 (9)b
289 (18)a
42 (4)a
514 (57)a
97
0.99

0
0.981 (0.012)
0.816 (0.008)b
0.164 (0.09 l)c
47 (l)c
287 (11 )b
42
0.86
14.2 (0.4)c
87 (4)c
NA

0.082 (0.002)a
1.063 (0.002)
0.855 (0.03)b
0.208 (0.027)d
101 (2)d
492 (63)a
82
0.98
47 (l)d
226 (27)b
32 (l)b
392 (16)b
24
0.99

47
0.99

3.8
0.97

66

3.4.3 Methane generation
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Digestion Time (Days)

Figure 3.1 Cumulative methane production for: • FSS + FVW. o FSS control. A SSS +
FVW. 0 SSS control. Error bars represent the standard deviation of three measurements
of the daily biogas generation. All gas volumes were corrected to STP.
Cumulative methane generation was statistically higher when FVW was added to
both the FSS and SSS as seen in Figure 3.1. Average net methane production was
considerably higher (approximately 30%) in FSS (42 ± 4 mL) than in SSS (32 ± 1 mL).
As seen in Table 3.1, the average volume of methane produced per gram of VSadded was
significantly higher in the FSS than the SSS, 541 ± 57 L compared to 392 ± 16 L. These
values where higher than the values reported by Zhang et al. (2007) who determined a
methane generation rate of 348 L kgVS"'added, after 10 days, when digesting food waste.
The FVW used in this study had a slightly higher VS/TS ratio. The added high quality VS
of the FVW can account for the increase in methane production over the controls.

67

Methane production per kg of VSCOnsumed of the FSS + FVW was significantly greater than
that of the SSS + FVW (Table 3.1). In the second stage digester, of a two stage anaerobic
digestion system, very little solids reduction and gas production occur (Appels et al.,
2008). Thus, the microbial populations and subsequent inoculum substrate ratios (ISR's)
are much lower in the second stage of the digester than the first (approximately 21:1 for
FSS and 13:1 for SSS on a VS basis). It should be noted that these ratios do not represent
true ISR's because inoculum sludge has not been completely "degassed" and therefore
some of the VS may be in the form of primary wastewater solids. Increasing the ISR, to a
point, has been shown to improve the specific methane yield in batch tests (Kameswari et
al., 2011; Raposo et al., 2009). When the loading rate is increased by the addition of
FVW the acidogenic bacteria increase carbon dioxide, acetate, and hydrogen production.
The high carbon dioxide concentration of the SSS + FVW biogas (Figure 3.2) when
compared to the FSS +FVW is a result of this. The methanogenic bacteria cannot
increase their activity as quickly as the acidogens due to their long regeneration rate (1516 days) when compared to the faster growing acidogens (< 24-90 hrs) (Deublin and
Steinhauser, 2008). Therefore, when FVW is co-digested with sewage sludge, methane
generation is dependant on the growth rate of the Methanogens (Bouallagui et. al., 2005).

68

3.4.4 Methane content of biogas

C

a>
c
o 40

•*-'

a
0)
c
<0

4)
2
10n
u>
o
m

0.0

2.0

4.0

6.0

8.0

10.0

Time (Days)

Figure 3.2 Biogas methane concentration of: • PSS + FVW. o PSS control. A SSS +
FVW. 0 SSS control. Error bars represent the standard deviation of three measurements.
Methane content of the biogas increased as digestion time increased (Figure 3.2).
Maximum methane content of the biogas was approximately 60-65%. Chen et al. (2010)
found biogas produced from cafeteria waste and commercial kitchen waste to have
similar methane content, 55 and 60% respectively. FSS reached maximum methane
potential much faster than SSS. Final biogas methane concentrations were increased in
both FVW amended sludges.

69

20.0

18.0

16.0
_i 14.0
£

c

0 12.0
1

—{|)—

I 10.0
a
01

I
tu 8.0

SI

aj

5 6.0
4.0
2.0
0.0

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

Digestion Time (Days)

Figure 3.3 Daily methane production rate of sludges. • FSS + FVW. o FSS control. A
SSS + FVW. 0 SSS control. Error bars represent the standard deviation of three
measurements of the daily methane generation.
Daily methane generation reached a sharp maximum by approximately day 1 of
the digestion (Figure 3.3). It then decreased as more difficult to degrade components were
hydrolysed. Methane generation started to decline after day seven. Addition of FVW
increased the daily methane production in both sludge's when compared to the controls.
Initial daily methane generation was increased dramatically in both sludges amended with
FVW suggesting that the majority of the FVW was very easily digested within the first 24
-48 h.

70

3.4.5 COD reduction
Table 3.2 COD reduction of sewage sludge over 10 days of digestion. Standard
deviations of three measurements are in parenthesis. Means with different letters indicate
significant differences (p < 0.05).
Parameter

FSS
(g 02 L1)

FSS + FVW
(g 02 L1)

SSS
(g Oz L1)

SSS + FVW
(g 02 L1)

Day 1 COD
Day 10 COD
CH4 COD '(L g02"')

12.6 (0.3)a
10.9 (0.1)a
65 (l)a

12.6 (0.00)a
10.7 (0.2)a
78 (12)a

7.9 (0.5)b
7.0 (0.3)b
17 (5)b

8.0 (0.5)b
6.6 (0.2)b
32 (5)b

Final COD values (Table 3.2) for the FSS were not significantly different than
FSS + FVW. This suggests that all of the FVW added was removed within 10 days. SSS
+ FVW showed a slight decrease in the final COD values when compared to the control
suggesting that FVW may have stimulated the digestion of sludge. Methane production
per kg of CODCOnsumed was higher in both of the sludges with co-digestate added. As seen
in Table 3.2, FSS produced more than twice the amount of methane per kg of CODconsumed
than SSS.
3.5 Conclusions
The addition of FVW significantly increased the methane production of first and
second stage digester sludge. However, the specific methane production of the FSS (514
± 57 L CH4 kgVS"1added) was significantly greater than that of the SSS (392 ± 16 L CH4
kgVS"1 added) most likely due to the higher ISR of the FSS. Post digestion COD and VS
levels concluded that all of the FVW added was consumed within 10 days. When codigesting fruit and vegetable waste with sewage sludge co-digestate should be added to
first stage of the anaerobic sludge digester in order to maximize methane generation.

71

3.6 Acknowledgements
The authors wish to recognize the City of Prince George for their support. The
authors would also like to thank Joanne Logie and Randy Garton, from the City of Prince
George, for their assistance. This research was funded by the National Science and
Engineering Research Council (NSERC) of Canada.

72

Preface
Chapter 4 of this thesis is a version of the manuscript under review:
Park, N.D., Thring, R.W., Helle, S.S., 2011. Combined alkaline and ultrasound pretreatment of thickened pulp mill waste activated sludge for improved anaerobic
digestion Submitted to Biomass and Bioenergy.

73

CHAPTER 4: Combined alkaline and ultrasound pre-treatment of
thickened pulp mill waste activated sludge for improved anaerobic
digestion
4.1 Abstract
Samples of pulp mill waste activated sludge were thickened by centrifugation,
from 2.5 to 6.5%, and subjected to combined alkaline (20.6 and 26.lg NaOH lOOgTS"1,
respectively) and ultrasonic (39560 and 16822 kJ kg"1, respectively) pre-treatment in
order to improve methane production and shorten sludge retention time. Pre-treatment
increased the soluble TS, VS, and COD of the pulp sludge 3-14 times over non-treated
sludge. Batch anaerobic digestions, over 28 d, were used to compare the effects of sludge
pre-treatment. Pre-treatment did not significantly improve biogas production over 28 d,
but did increase the VS destruction (21 ± 3 to 30 ± 0.8% for 2.5% TS sludge and 23 ± 0.5
to 27 ± 0.7% for 6.5% TS sludge). Pre-treatment increased the initial rate of methane
production. 80% of the total methane production was reached 5-6 days earlier when pretreated. Overall methane production was found to be 404 ± 205 and 222 ± 123 L kgVS"
'consumed for 2.5% TS sludge non-treated and pre-treated sludge and 384 ± 183 and 354 ±

93 for 6.5% TS non-treated and pre-treated sludge, respectively. Pre-treatment reduced
the dewaterability of both sludges. Thickening by centrifugation reduced the total S
content of the sludge. Overall, biogas production from pulp sludge was unstable and
inconsistent. Further research is required to determine the cause of inhibition.
4.2 Introduction
The production of wood pulp for paper manufacturing uses large amounts of
water. Depending on the type of paper being made anywhere from 20 - 60 m of water
may be used to produce one tonne of paper (Thompson et al., 2001). Pulp and paper

74

effluents are mostly treated at the primary and secondary treatment levels. Primary
treatment involves the sedimentation of fines, fibers and fillers that remain in the water
due to poor separation during the pulping process (Mahmood and Elliot, 2006). Wood
fiber found in the primary sludge is often recycled within the pulping process to increase
yields, reduce costs, and reduce sludge production. Secondary sludge consists mainly of
cell mass produced by the biological conversion of the soluble organic fraction of
wastewater into carbon dioxide and water. Disposal of secondary sludge is more costly
due to its poor dewaterability compared to primary sludge (Mahmood and Elliot, 2006).
The production of secondary sludge has been on the rise in recent years as pulp
production increases and as environmental regulations pertaining to effluent quality have
become more stringent (Mahmood and Elliot, 2006). Sludge disposal and management
can amount to 60% of the total cost of pulp mill wastewater treatment; for this reason the
minimization of sludge production is important (Canales et al., 1994).
One method that is often used to reduce the volume of municipal wastewater
sludge is anaerobic digestion (AD). Anaerobic digestion is the biological breakdown of
organic waste in the absence of oxygen. The process of AD is often broken down into
four general steps; hydrolysis, acidogenesis, acetogenesis, and methanogenesis. During
the hydrolysis stage large, insoluble polymers are broken down into soluble monomers by
extra-cellular enzymes. These monomers are then converted into short chain organic
acids by the acidogenic bacteria. Next, the organic acids are then converted to acetate by
the acetogenic bacteria. Lastly, methanogenic microorganisms convert acetate, carbon
dioxide, and methyl type substrates into methane. Some of the benefits of anaerobic
digestion include; reduced sludge volume, sludge stabilization, sludge disinfection, and

75

energy recovery in the form of methane. Compared to the use of AD in municipal
wastewater sludge treatment, the utilization of AD in the pulp and paper industry is less
widespread. There have only been four anaerobic treatment system utilized in Canada for
the treatment of pulp and paper sludge (Elliot and Mahmood, 2007). Of these four
installations only two are currently operational (Elliot and Mahmood, 2007). Sludge
produced from the pulp and paper industry is mainly composed of microbial cell mass
that forms during secondary treatment and lignin and chemical residuals from the pulping
process (Kyllonen et al., 1988). This type of waste is less amenable to degradation than
the primary wastewater solids that are anaerobically digested in the municipal sector.
Consequently, the AD of pulp sludge requires long retention times, has a low degradation
efficiency, and poor economics. Therefore, AD is less widely used in the pulp and paper
industry (Elliot and Mahmood, 2007; Lin et al., 2009).
Increasing sludge digestion through sludge disintegration can increase methane
production, decrease sludge volume, reduce sludge retention time, and thereby improve
the overall economics of the process. Disintegration helps to speed up the hydrolysis
stage by mechanically, chemically, thermally, or biologically aiding in the breakdown
and solubilization of the sludge by reducing floe particle size and rupturing microbial cell
walls.
Alkaline addition has been shown to increase solubilization of pulp and paper
sludge and improve biogas yield (Lin, L., 2009; Heo et al., 2003). Sodium hydroxide is
most often used over other chemicals such as KOH and Ca(OH)2 due to its high rate of
COD solubilization (Kim et al., 2003). Hydroxy anions weaken cell walls and break a
part floe structure rendering the sludge amenable to further treatment. Alkaline pre-

76

treatment has been combined with ultrasound, microwave, thermal, high pressure
gradients to further sludge breakdown (Yiying, et al., 2009; Dogan and Sanin, 2009;
Neyens and Baeyens, 2003; Daniel et al., 2007).
Ultrasound pre-treatment of municipal wastewater sludge has been studied at
length due to its ease of operation and effectiveness (Khanal et al., 2007). Ultrasound
treatment involves using high frequency sound waves generated by a vibrating probe. As
the sound waves travel through the liquid, gas bubbles are formed and collapse violently
producing localized high temperatures and pressures. Cell walls and floe structures are
destroyed making their contents' susceptible to anaerobic digestion. Combinations of
both ultrasound and alkali pre-treatment have been shown to improve sludge digestion
over either treatment alone (Chiu et al., 1997; Yiying et al., 2009). Improvement of
sludge sonication efficiency has been improved by increasing the total solids content of
the sludge (Wang et al, 2005; Khanal et al., 2006).
Much of the research conducted on the pre-treatment of secondary sludges has
been done using municipal wastewater sludges. Much less has been reported on the
treatment of sludges produced from pulp mill wastewater treatment (Elliot and
Mahmood, 2007). Some studies pre-treating pulp sludge have been reported in the
literature, however; few have reported combined treatment methods, or have pre-treated
thickened pulp sludge (Yunquin et al., 2010; Lin et al., 2009; Wood et al., 2009).
The objective of this experiment was to determine the methane potential of
secondary sludge produced by the Quesnel River Pulp (QRP) mill. Thickened (6.5% TS)
and non- thickened sludges (2.4% TS) were subject to alkaline pre-treatment followed by
ultrasonication in order to improve sludge AD efficiency and compared to non pre-treated

77

sludge. Pre-treatment effectiveness was analyzed by comparing the soluble COD
(sCOD), soluble TS (sTS), and soluble VS (sVS) fraction in treated and non-treated
sludges. AD efficiency was based on methane production and VS reduction. Hydrogen
sulphide production was also calculated by comparing sludge sulphur content before and
after digestion. QRP sludge is known to be high in S due to sulphite addition during the
pulping process.
4.3 Materials and Methods
4.3.1 Pulp sludge
Secondary (WAS) sludge was collected from the Quesnel River Pulp mill, in
Quesnel, B.C. Canada. The QRP mill produces bleached chemi-thermo-mechanical pulp
(BCTMP) and thermo mechanical pulp (TMP). Wastewater is composed of
approximately 2:1 BCTMP: TMP effluent. The plant typically uses 60% white spruce and
40% lodgepole pine. Wastewater is treated using a moving bed biological reactor
followed by activated sludge treatment. Pulp mill sludge (PMS) samples were stored at 4
°C and used within 24 hrs of sampling.
4.3.2 Pre-treatment
Sludge was used at initial solids content and was thickened to approximately
6.5% TS by centrifugation at 6,000 rpm for 5 min. Non-thickened sludge was dosed with
NaOH at 500 mg L"1 (20.6g NaOH lOOg"1 TS) and thickened pulp sludge was dosed with
NaOH at 1700 mg L'1 (26.lg NaOH lOOg"1 TS). Samples were then mixed for 10 min
using a Hamilton Beach™ handheld mixer, and allowed to sit for two hours. 400 mL
portions of treated sludge were then sonicated using a 40 kHz Fisher Scientific™ model
150T ultrasonic dismembrator (150W maximum output). Sludge samples were packed in

78

ice to prevent overheating and sonicated for 2 hours, 10 s on 5 s off, for a total of 1.5 h, at
60% amplitude, with a 1/8" microtip titanium probe. Specific energy was defined as,
Es = (P)(t)/(v)(TS),
Where, P = ultrasonic power (W), t= ultrasonic time (s), v= sample volume (L), TS= total
solids concentration (kg L"1). Values were calculated to be 39560 kJ kg"'TS"' for the 2.5%
TS sample and 16822 kJ kg^TS"1 for the 6.5% TS sample.
4.3.3 Sludge characterization
Approximately 15 L of pulp sludge was sampled for use in batch experiments.
Sludge samples were characterized for TS and VS in duplicate by standard methods
(APHA, 1998). COD was determined in triplicate on diluted sludge samples by the closed
reflux method outlined by the Hach method 8000 (Hach, 2010). Sludge samples were
centrifuged at 20,000 rpm for 10 min and decanted to determine the sTS, sVS, and sCOD.
Supernant pH was determined using an Orion pH probe. Pulp sludge samples where dried
at 104 °C for 48 h and analysed for total C, N, and S content using < 100-mesh samples
(ground in a Model MM200 ball mill; Retsch, Haan, Germany) via dry combustion
method in a Model 1500 Elemental Analyzer (Fisons, Milan, Italy).
4.3.4 Inoculum sludge
Inoculum sludge was collected from the first stage anaerobic digester at the
Lansdowne wastewater treatment plant (LWWTP) in Prince George, B.C., Canada, for
use as inoculum. Anaerobic digesters are fed semi-continuously with sludge from two
primary clarifiers. The primary digester is operated at mesophilic (36 ± 1 °C)
temperature. More information on the wastewater treatment process at the Lansdowne
Wastewater Treatment Plant can be found in section 2.3.
4.3.5 Batch digestions
79

250 mL amber glass bottles were filled with 75 g of sludge inoculum and 75 g of
pulp mill sludge. Control inoculum received 75 mL of deionised water. The experimental
units are as follows; control, 6.5% non-treated PMS, 6.5% pre-treated PMS, 2.4% nontreated PMS, 2.4% pre-treated PMS. All digestions were repeated in triplicate. Bottles
were capped with rubber septa and the headspace was purged with nitrogen for 2 min.
Bottle headspace pressure was equalized using an airtight syringe. Samples were then
placed in a dark, reciprocating water bath at 36 °C, and shaked at 80 r.p.m. Biogas
volumes were measured by inserting an airtight syringe and allowing the plunger to
equilibrate. Daily room temperature and pressure at the time of gas sampling were
recorded and gas volumes corrected to STP (0 °C and 1 atm). A syringe was used to
sample 0.1-0.2 mL of biogas to determine methane and carbon dioxide concentrations by
gas chromatography. At the end of the experiment samples of sludge were analyzed for
TS and VS by standard methods (APHA, 1998).
4.3.6 Biogas analysis
0.1-0.2 mL samples of biogas were manually injected and analyzed for carbon
dioxide and methane content using a SRI 8610C gas chromatograph equipped with a
Haysep-D column and thermal conductivity detector (TCD). Helium at 15 mL min"1 was
used as a carrier gas. Column oven temperature was set at 65 °C and the TCD oven at 104
°C. Samples were injected at room temperature. A standard gas sample of methane and
carbon dioxide was used to identify retention factors of component gases. Peak areas
were corrected for differences in TCD response as outlined by Dietz (1967). Corrected
peak areas were normalized to calculate gas volume in percent (Dietz, 1967).
4.3.7 Specific resistance to filtration

80

Specific resistance to filtration test (R) has been widely used to asses the
filterability and the dewaterability of sludges. The test involves the use of a Buchner
funnel, filter paper, and pressure or vacuum. The volume of filtrate is recorded with
respect to time. In order to make the measured filterability of the sludge independent
from as many variables as possible Coackley and Jones (1956) described a mathematical
model that takes into account the sludge volume filtered, area of filter surface, the solids
content of the sludge, and the filtration vacuum pressure. 100 mL of sludge, at 10 °C, was
filtered using an 11 cm in diameter, Whatman #2 filter paper, at a constant vacuum
pressure of 0.69 bar. Sludge was filtered for 32 minutes and the volume of filtrate
collected was recorded with respect to time. R was calculated from a plot of filtration
time/filtrate volume vs. filtrate volume as described in detail by Habiba et al. (2009).
4.3.8 Statistical data treatment
Average final TS, VS, biogas production, and methane production were analyzed
by analysis of variance (ANOVA). When p was significant (< 0.05), statistical
differences between treatment means were determined using the Tukey-HSD test. JMP 8
statistical software (SAS, NC, USA 2010) was used to perform statistical calculations.
Microsoft Excel was used to produce graphs.

81

4.4 Results and Discussion
4.4.1 Effect of sludge pre-treatment
Table 4.0 Characteristics of thickened (6.5% TS), non-thickened (2.5%TS), and
inoculum sludge pre-treated with NaOH and ultrasound. Numbers in brackets represent
standard deviations, n=2).
Parameter

2.5% TS
Non-treated

2.5% TS
Pre-treated

6.5% TS
Non-treated

6.5% TS
Pre-treated

Inoculum

TS (%)
VS (%)
VS/TS (%)
FS (%)
MC (%)
COD (g02L-1)
sCOD/tCOD (%)
C:N Ratio
pH

2.42 (0.01)
1.87 (0.01)
77.0 (0.2)
0.56 (0.003)
97.58(0.01)
29.8 (0.7)
6(1)
10.5:1
7.3

2.73 (0.02)
2.01 (0.01)
73.8 (0.08)
0.71 (0.01)
97.27 (0.02)
27.4(1.6)
19(2)
10.3:1
8.4

6.52 (0.06)
5.52 (0.07)
84.7 (0.2)
0.99 (0.004)
93.48 (0.06)
87.8 (0.8)
3.6 (0.2)
9.8:1
7.2

6.42 (0.01)
5.32 (0.01)
82.8 (0.3)
1.1 (0.02)
93.58(0.01)
75.0(1.3)
17.9 (0.5)
10.2:1
8.8

1.67 (0.03)
1.18(0.03)
70.7 (0.76)
0.49 (0.01)
98.33 (0.03)
16.9 (0.4)
7.9 (0.3)
8.1:1
7.3

As seen in Table 4.0, the VS/TS content of the pulp sludge's is high, even larger
than the municipal primary sludge used as inoculum, suggesting that it is ideal for
anaerobic digestion (Zhang et al., 2007). FS content of the pre-treated sludge increased
due to the added NaOH during the pre-treatment process as well as a small amount of
possible sludge mineralization caused by pre-treatment. Pre-treatment reduced the COD
value of both sludges. This may have been due to loss of volatile components in the
sludge during ultrasonication (Santos et al., 2009). Sludge solubilization (sCOD/tCOD)
increased 3 times in the pre-treated 2.5%TS sludge and almost 5 times in the pre-treated
6.5% TS sludge. The pH of the pre-treated sludge supernant increased by just over 1 unit
in both sludges due to the addition of NaOH. Saha et al. (2011) thoroughly analyzed
secondary sludge from the QRP mill in 2010 and found the TS, VS, COD, and sCOD to
be similar to what is reported in Table 4.0.

82

18000 -I
16000 14000 -

• sCOD

g> 12000 <oc
2 10000 2
C
| 8000 o
o

| 6000 -

wo
4000 -

2000 -

0Non Treated 2.5%

Pre-treated 2.5%

Non-treated 6.5%

Pre-treated 6.5%

Figure 4.0 Difference in soluble TS, VS, and COD between pre-treated and non-treated
sludge. Error bars represent the standard deviation of 2 (TS and VS), or 3 (COD)
measurements.
Figure 4.0 shows the difference in the soluble TS, VS, and COD between the pretreated and non-treated sludge. In the 2.5% TS PMS pre-treatment increased the sTS,
sVS, and sCOD by approximately 3, 9, and 3 times, respectively. Pre-treatment of 6.5%
TS PS increased the sTS, sVS, and sCOD by approximately 4, 13, and 4 times,
respectively. The 6.5% TS sludge was dewatered by centrifugation prior to use. This
would have lowered the initial soluble organic content of the sludge making effects of
pre-treatment seem larger. Also, the 6.5% TS sludge was dosed with a higher
concentration of NaOH (initial sludge tests indicated a TS content of 8.2% when
centrifuged at 6000 rpm for 5 min, but the subsequent batch of sludge used in this test
was more difficult to dewater). Pre-treatment of 6.5% TS sludge may have also improved

83

due to the increased ultrasound efficiency at higher TS contents (Khanal et al., 2006;
Wang et al., 2006).
Wood et al. (2009) treated sulfite mill secondary sludge and Kraft mill secondary
sludge with ultrasound at 20 kHz. COD solubilisation was low compared to other pretreatment methods. Similarly to this study, the sulfite mill secondary sludge increased
from 11% sCOD to 23% sCOD and the Kraft mill sludge increased from 1% to
approximately 5% sCOD after pre-treatment. Saha et al. (2011) have also treated pulp
mill secondary sludge originating from the QRP with ultrasound (at 20 kHz) alone and
found sCOD increased with increasing treatment time. The sCOD/tCOD values were
slightly higher than reported in this study. This may have been due to the higher
ultrasound intensity at 20 kHz. Values increased from 5- 7.8 times over the control
depending on the treatment time. Yunquin et al. (2009) treated a combination of primary
and secondary pulp and paper sludge with lower dose (16gNaOH lOOgTS"1) of NaOH for
a longer time (6 hours). Soluble COD increased approximately 12 times when compared
to the control. Soluble COD increases may have been higher due to the solubilization of
primary sludge that was high in lignocellulosic residues.

84

4.4.2 Biogas Production
700.0

(0

600.0

5

A
a

o
o

o
o

0

E 500.0

•6

400.0

<0

!? 300.0

<D
® 200.0

3 100.0

0.0

3.0

88

i

8

6.0

9.0

12.0

8

8

15.0
18.0
Time (days)

O
0

21.0

24.0

27.0

30.0

Figure 4.1 Net cumulative biogas generation, a 6.5% pre-treated A 6.5% non-treated o
2.5% pre-treated 0 2.5% non-treated. Error bars represent the standard deviation of the
daily biogas production from three replicates. Biogas generated from inoculum only
blanks was subtracted from the data.
Initially, biogas production increased in the thickened sludge when compared to
the non pre-treated thickened sludge (Figure 4.1). Around day 20 for the 6.5% TS sludge
the cumulative biogas volume of the non-treated sludge surpassed the pre-treated sludge.
After 28 d both of the non-treated sludges had produced more biogas than the pre-treated
sludge (Figure 4.1). Statistically, pre-treatment of sludge did not increase biogas
production over 28 days. Wood et al. (2009) also reported that pre-treatment with
sonication did not significantly increase biogas yield in either Kraft or sulphite pulp mill
sludges, yet an initial improvement in the rate of gas production was observed. 80% of
the total biogas production was reached by day 8.5 for the pre-treated 2.5% TS sludge
compared to day 14.5 for the non pre-treated 2.5% TS sludge (data not shown). This is an
85

increase of 6 days. For the 6.5% TS sludge, 80% of total biogas production was reached
by day 8 for the pre-treated sludge compared to day 13 for the non pre-treated sludge
(data not shown). This was an improvement of 5 days.

4.4.3 Methane Production
400.0

£
350.0

i

300.0

i

tS 250.0

® 200.0

»

5 150.0

> 100.0
=

3

o
o

o
0

6

50.0

9.0

12.0

15.0
18.0
Time (days)

21.0

24.0

27.0

30.0

Figure 4.2 Net cumulative methane generation. • 6.5% pre-treated A 6.5% non-treated o
2.5% pre-treated 0 2.5% non-treated. Error bars represent the standard deviation of the
daily methane production from three replicates. Methane generated from inoculum only
blanks was subtracted from the data.
Initially, methane production increased in the thickened sludge compared to the
non-treated thickened sludge (Figure 4.2). On approximately day 23 for the 6.5% TS
sludge the cumulative methane production of the non pre-treated sludge surpassed the
pre-treated sludge. After 28 days both of the non-treated sludges had produced more
methane than the pre-treated sludge (Figure 4.2). Pre-treatment did not significantly
improve methane production. 80% of the total methane production was reached by

86

approximately day 7.5 for the pre-treated 2.5% TS sludge compared to day 14 for the non
pre-treated 2.5% TS sludge (data not shown), an increase of 6.5 days. For the 6.5% TS
sludge, 80% of total biogas production was reached by day 7.5 for the pre-treated sludge
compared to day 13 for the non pre-treated sludge (data not shown). This was an
improvement of 5.5 days. Karlsson et al. (2011) also reported very similar trends in
methane production after pre-treating WAS, originating from two different pulp mills,
with ultrasound. Initial methane production rates increased, but the effects were
diminished over time (Karlsson et al., 2011).
Overall methane and biogas production decreased slightly when sludge was pretreated. The results obtained by Wood et al. (2009) suggested that addition of NaOH may
solubilize recalcitrant compounds, or generate inhibitory compounds thus lowering the
methane yield. Mineralization of sludge components may have caused the slightly lower
total methane output from the pre-treated sludge. As seen in Table 4.0, the FS content of
both sludges increased, and the VS/TS ratio decreased after pre-treatment.
4.4.4 Post digestion sludge characteristics
Table 4.1 Sludge characteristics post anaerobic digestion. Values in brackets represent
standard deviations, n=3.
2.5% TS
2.5% TS
6.5% TS
6.5% TS
Parameter
Non-treated
Pre-treated
Non-treated
Pre-treated
Initial VS (g)
Final VS (g)
VS Consumed (g)
VS reduction (%)
Net Biogas (ml)
Biogas kgVS Consumed (L)
Net Methane (ml)
Methane kgVS"1 consumed (L)
% CH4 in Biogas
Final pH
R (m kg"1)

1.397 (0.044)
1.108 (0.042) a
0.290 (0.042) a
21 (3)a
180 (99)a
603 (299)a
122 (69)a
404(205)a
64-70
7.4
5.61E+14

1.511 (0.046)
1.057 (0.012) a
0.454 (0.012) b
30 (1) b
143 (90) a
313(195)a
102(58)a
222 (123)a
66-70
7.5
9.61E+14

4.142 (0.080)
3.189 (0.019) b
0.952 (0.019) c
23(1) ab
582 (280) a
610 (290)a
366 (176)a
384 (183)a
62-66
7.5
7.91E+14

3.990 (0.044)
2.932 (0.029) c
1.058 (0.029) d
27(1) be
549 (153)a
518 (137) a
375 (105) a
354 (93)a
65-69
7.6
14.6E+14
87

The net VSconsumed for pre-treated sludge's was significantly greater than nontreated sludge's (Table 4.1). This corresponded to a greater VS reduction efficiency in
pre-treated sludge (Table 4.1). Improvements were similar to those reported by Saha et al.
(2011) who at after 90 min of sonication, at 20 kHz, and 45 days of AD, using QRP
sludge, improved VS reduction from 23 ± 3% to 30 ± 4%. Net production of biogas was
statistically the same whether or not sludge was pre-treated (Table 4.1). Due to the higher
VS destruction efficiency, biogas produced per g of VS was lower in the pre-treated
sludge. Biogas production was 3-4 times greater in the 6.5% TS sludge than the 2.5%
TS sludge due to the increased VS content of the thickened sludge. Net methane
production was also the same and did not change with pre-treatment. Methane content of
the biogas was slightly higher for pre-treated sludges. Methane production was consistent
with the values reported (320 L CH4 kgVS"1 consumed) for batch digestions of pulp mill
sludge (PMS), pre-treated with NaOH, by Lin et al. (2009). Puhakka (1992) reported
similar values for biogas generation, 570 L biogas kgVS"1COnsumed, using a pilot scale
reactor to digest Kraft pulp mill WAS.
It should be noted that overall biogas production was unstable (Appendix C). At
least one batch digestion out of three (in some cases two) had a significantly lower
biogas/methane production compared to the other two samples. This can bee seen in the
high standard deviations reported for net methane and biogas production. More research
is required to determine the cause of inhibition. One possibility is that sulphur reducing
bacteria (SRB) may have out competed methane producing bacteria (MPB) in some of
the batches. This would explain the increased VSD without an increase in methane
production. Pre-treatment seemed to increase the amount of inhibition as seen in
88

Appendix C. Therefore, in the case of high sulphur waste streams pre-treatment not only
benefits the MPB, but also the SRB. More research is needed to determine the conditions
which favor MPB.
Final pH of supernant from digested sludge was found to be near the optimum
range of 6.7-7.5 for methane-forming bacteria (Deublein and Steinhauser, 2008). This
suggests that addition of sodium hydroxide did not adversely affect the AD process, nor
was acidification by hydrogen sulphide a major concern.
Pre-treatment almost doubled the specific resistance to filtration of both the
digested sludges as seen by the larger R values reported in Table 4.1. Saha et al. (2011)
also reported a reduction of pre-digestion sludge dewaterability after ultrasonication.
4.4.5 Sulphur Reduction
Table 4.2 Sludge sulphur reduction.
Parameter

2.4% TS
Non-treated

2.4% TS
Pre-treated

6.5% TS
Non-treated

6.5% TS
Pre-treated

Inoculum

%S (total)
Initial S(mg batch'1)
Final S (mg batch'1)
Net Diff(mg batch'1)
H2S Max (%)

1.97(0.02)
52.7(0.6)
40.5 (0.6)
10.8(1.2)
4.0(2.2)

1.98(0.03)
48.3(0.7)
30.3(0.7)
16.6(1.3)
7.7(4.9)

1.39(0.02)
79.0(1.1)
67.4 (1.2)
10.2(1.8)
1.2(0.6)

1.37(0.02)
79.1 (1.0)
68.8(0.4)
8.9(1.3)
1.1(0.3)

0.99(0.02)
12.3(0.3)
10.8(0.7)
1.46(0.8)
0.7(0.4)

Initial total sulphur content was lower in the thickened sludge than the nonthickened. This suggests that the majority of the sulphur is in soluble forms (most likely
S042") and dewatering by centrifugation decreases the sulphur content considerably. Pretreatment had no discernable effect on the sulphur concentration of the sludge. Nonthickened sludge had increased sulphur consumption (Table 4.2) when compared to
thickened sludge due to the overall higher soluble sulphur component. Maximum H2S
content was calculated by assuming that all net sulphur lost (initial S mg batch"1 - final S
89

mg batch"1 - inoculum sulphur lost mg batch"1) was in the form of H2S. This was not the
case as some sulphur may have been liberated as other reduced sulphur compounds such
as methylsulphide and dimethlysulphide. Therefore the values stated in Table 4.2 are high
estimates. Standard deviations are also high due to the variability in biogas production.
Hydrogen sulphide production from the thickened sludge was in the necessary range for
proper operation, 0.5-6% (Chynoweth and Isaacson, 1987). Dufresne et al. (2001)
reported excellent biogas production, and COD removal, anaerobically digesting Kraft
pulp mill condensates at a biogas hydrogen sulphide concentration close to 4%. Nonthickened sludge was near the upper limit for H2S headspace concentration suggesting
that sludge thickening may be necessary to reduce the soluble sulphur component, and
lower the H2S content of the biogas in order to prevent problems with excessive H2S
production and possible inhibition.
4.4.6 Economics
Pre-treatment using a lab scale ultrasound device requires approximately 297
kWh m"3 for the 2.5% TS and 6.5% TS sludge each. At $0.07 kWh"1 it would cost
approximately $21.00 m'3 for ultrasonic pre-treatment. Sodium hydroxide addition at
$350 Mg'1 would cost $1.75 for the 2.5% TS sludge and $5.95 for the 6.5% TS sludge per
m3 making the overall cost for pre-treatment $22.75 m"3 (2.5% TS) and $26.95 m"3 (6.5%
TS). Capital investment, ultrasound probe replacement (every 1.5-2 yrs), operational,
and maintenance costs would also have to be considered. Also, decreased dewaterability
would increase costs associated with dewatering and disposal. With no significant
increase in methane production the cost of pre-treatment would not be economical. One
possible benefit would be the increased VS reduction, and a shorter digestion time. This

90

could save money by allowing for a smaller digester to be constructed, or perhaps
postpone the construction of a new digester if the current digester was near capacity.
Theoretically, even if the VSD rate had been increased 2 fold (from 23% to 46%) and
biogas was sold at $10.00 GJ total income would amount to an extra $1.40 m"3 of sludge
treated at 6.5% solids. Estimating disposal costs at $250 Mg"1 TS, savings due to sludge
pre-treatment would be $3.10 m"3. This is a total savings of $4.50 m"3 of sludge treated
due to increased biogas production and decreased sludge disposal costs. Muller et al.
(2004) estimate that for a municipal wastewater treatment plant theoretically using pretreatment to improve VSD by 20%, at 5% TS, 250,000 PE sized plant, with much lower
pre-treatment energy demand (0.3 kWh MgTS"1), pre-treatment would only be
economical if sludge disposal costs were greater than $670 Mg"*TS. Specific data for
achieved level of disintegration, costs for sludge disposal, and size of wastewater
treatment plant must be examined thoroughly to decide if the pre-treatment is worthwhile
in each individual setting (Muller et al., 2004).

4.5 Conclusions
•

Biogas and methane production (total average of pre-treated and non-treated)
from QRJP sludge produced 0.51 ± 0.24 and 0.34 ± 0.15 m3 of biogas kgVS"
'consumed, respectively.

•

Pre-treatment increased volatile solids removal in both the thickened (by 17%)
and non-thickened sludge (by 43%).

•

Pre-treatment did not increase the overall methane yield, but did increase the
initial rate of methane production.

91

•

80% of total methane/biogas production was reached 5.5-6.5 days faster when
pre-treated; therefore reducing the hydraulic retention time.

•

Pre-treatment decreased the dewaterability of the sludge.

•

Thickening of sludge reduced the total S content of the sludge and subsequently
reduced the amount of sulphur liberated during anaerobic digestion.

•

Overall, biogas production from pulp sludge was unstable and inconsistent,
possibly due to competition between SRB and MPB. Further research is required.

4.6 Acknowledgements
The authors of this paper wish to thank West Fraser, the Quesnel River Pulp Mill,
Nick Finch, and Anna Rankin for supplying sludge samples. The City of Prince George
helped to supply sludge inoculum samples. Also, we would like to thank Clive Dawson at
the British Columbia Ministry of Forests laboratory in Victoria, B.C. for completing
some of the analysis. This research was funded by the National Sciences and Engineering
Research Council of Canada.

92

CHAPTER 5: Conclusions and Recommendations
Supermarket fruit and vegetable waste was found to be a relatively clean and
reliable source of organic waste. Over 15,000 kg of FVW were co-digested in the
anaerobic digester of the Lansdowne Wastewater Treatment Plant over six weeks.
Digester operation remained stable, as confirmed by the low total volatile
acidity/alkalinity ratio observed; suggesting that more waste could have been added
without digester upset. No negative operational consequences were observed downstream
from the anaerobic digesters. Co-digestion contributed to an increase in biogas
production from the anaerobic digesters. Average daily biogas production during the
study period was significantly higher than what was observed historically. Undigested
organic matter was found in biosolids produced during the study period suggesting that
organic matter should be added to first stage digester in order to prevent "short
circuiting" and to increase the hydraulic retention time of the organic waste.
Lab results confirmed that the first stage digester is more favorable over the
second for co-digestate addition. First stage digester sludge produced significantly more
methane when FVW was added when compared to second stage digester sludge. The
increased methane production was most likely due to higher inoculum substrate ratio and
increased methanogenic populations in the first stage digester. All of the FVW added was
consumed within 10 days suggesting that FVW is an easily digestible source of
biodegradable matter for anaerobic digestion.
The high water content of the FVW made the collection and transportation of
FVW a limiting factor. Approximately 90% of the FVW was water by mass. The
transport of co-digestion substrates with high water content is inefficient. Dewatering of

93

FVW prior to transportation will improve transportation and handling efficiency as well
as the amount of biogas produced per kg of FVW added. More research is needed in this
area to improve the economics of the co-digestion of FVW and sewage sludge. Another
avenue of research that would benefit the City of Prince George would be the possibility
of using other sources of organic waste within the City of Prince George waste stream
that are higher in TS than FVW. The organic fraction of municipal solid waste, waste
paper, grass clippings, and food waste are all produced within the City of Prince George's
operations and could be possible candidates for co-digestion. The large size and low
solids content of the digesters allows for significantly higher volumes of co-digestate to
be added in the future. Research into the optimum ratios of different types of waste, and
changes in seasonal waste variability would be useful for any further full scale studies.
Lastly, a longer study period would be useful to highlight any long term consequences of
co-digestate addition such as: increased\decreased biosolids output, increased\decreased
dewaterability of sludge, foaming and scum, buildup of difficult to degrade components
of co-digestate in digesters, increased COD, N, and P output in plant effluent, and
downstream mechanical problems due to co-digestate addition.
Overall, addition of organic waste to the first stage anaerobic digester at the
Lansdowne wastewater treatment plant is a viable alternative to landfilling for the City of
Prince George. The benefits include; increased energy output in the form of biogas,
improved sewage sludge digestion, reduced landfill tipping fees, and reduced greenhouse
gas emissions.
Combined alkaline and ultrasonic pre-treatment of pulp mill waste activated
sludge was shown to also improve anaerobic digestion performance. Pre-treatment

94

greatly increased the sTS, sVS, and sCOD of both thickened and non-thickened sludge.
The increased soluble component of the sludge led to increased rates of methane
production. Eighty percent of the total methane was produced 5-6 days faster which
would significantly reduce the hydraulic retention time in a full scale digester. Over 28
days the effects of pre-treatment diminished and methane production was not
significantly greater than without pre-treatment. Pre-treatment of pulp sludge improved
the VS reduction, but decreased the sludge dewaterability. Thickening of sludge greatly
reduced the amount of sulphur found in the sludge and may be a viable method to reduce
the hydrogen sulphide content of the biogas, and prevent possible inhibition. More
research is required to directly measure H2S concentrations in the biogas produced from
QRP sludge. Results suggest that SRB may be in competition with MPB causing reduced
methane production. If the H2S concentrations of the biogas were determined it may
show that SRB were out competing MPB in the batches with reduced methane
production. Optimization of conditions to favor MPB would be useful. More research
could also be undertaken to optimize the pre-treatment process on QRP sludge. Sodium
hydroxide dosage, reaction time, and ultrasound application should all be tailored to
produce maximum soluble COD and minimal cost for the QRP sludge. Optimization
would greatly reduce the cost associated with pre-treatment.
The high cost of pre-treatment and the reduction in sludge dewaterability make
pre-treatment using ultrasound uneconomical. Other less energy intense pre-treatment
methods, such as Microsludge™, or Biogest Crown Disintegration™ may prove to be
better methods of pre-treatment. Overall, even with a considerable increase in VS
destruction the energy balance may not prove to be worthwhile. Sludge disposal is

95

relatively cheap at the QRP mill. Pre-treatment has only been shown to be cost effective
when sludge disposal costs are high.
Biogas production was unstable and inconsistent from the pulp sludge. A serious
problem with the anaerobic digestion of pulp and paper sludge is inhibition. Historically
anaerobic digestion was used at the QRP mill, however; due to problems with
inconsistent gas production it is no longer used in the wastewater treatment process.
Inputs into the pulp mill change depending on the type of pulp produced, species of wood
used, and various chemical additives. Methanogenic organisms are very sensitive to
environmental changes and react negatively. More research is required to pinpoint
potential causes of inhibition at the QRP mill and prevent them. A more thorough
investigation into the inhibitory compounds found in the effluent stream is necessary to
determine possible causes of toxicity. Experimentation with pilot scale high SRT
digesters may prove more resilient to changes in incoming pulp mill wastewater. Often a
lack of trace nutrients can cause inhibition during anaerobic digestion. Further research
into trace metal concentrations in the QRP effluent may provide further insight into AD
optimization.
Lastly, more information about the forms and concentrations of N and P after
sludge pre-treatment would be useful to plant operations because pre-treatment could be
used to reduce the upstream nutrient dosing requirements for aerobic treatment.
To increase biogas production from pulp sludge co-digestion with an easily
degradable substrate would provide more biogas; likely require less energy, and lower
initial capital investment than sludge pre-treatment. Co-digestion may also help to dilute

96

any toxicity found in the pulp mill effluent and improve the final sludge dewaterability.
More research in this area is needed.

97

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110

Appendix A- Sample calculations
The following sample calculations were performed with data from 10:00 AM, November
12, 2010 (primary digester sludge control, replicate # 1).
Correction of Gas Volume to STP (0°C and 1 atm)
The temperature of the biogas in the bottle headspace was determined to be 31 °C by
inserting a thermometer through the septum.
Atmospheric pressure was determined by the University of Northern British Columbia
atmospheric sciences roof top weather station located 7 m above the research lab building
roof. Data can be found a, "http://cirrus.unbc.ca/wx/".
14 mL of Biogas were collected. Following from the equation,
PV = nRT
PV
PV
nRT ~ nRT
1 (atm) = 101.325 (kPa)
101.325(kPa)xV(mL) 92.900(kPa)xl4(mL)
nRx 273.15(K)
~
nRx 304.\5(K)
V= 11.5 (mL)
Correction of Gas Chromatography Response (Peak Areas) for differences in TCD
responses between methane and carbon dioxide.
Dietz (1962) published a series of correction factors to compensate for the differences in
thermal conductivities between different compounds. These correction factors are
specific to thermal conductivity detectors and independent of column type, oven
temperature, carrier gas, flow rate, and concentration. They have a precision of
approximately ± 3%.
The peak areas must be divided by the appropriate correction factor to obtain the true
peak area,
(Peak Area) / (Thermal Response Factor) = Corrected Peak Area
For Methane
33.368/35.7 = 0.9347

111

For Carbon Dioxide
27.812/48 = 0.5794
Once the peak areas have been corrected for differences in TCD response the percent
volume of the gas can be determined by normalization. It is assumed that the
concentrations of trace gases found in the biogas are very low. The presence of trace
gases will lower the overall methane and carbon dioxide concentration.
Normalization of Corrected Peak Areas to Calculate Methane Gas Volume in Percent
% Volume CH4 =

[(Corrected Peak Area for CH4) / (Corrected Peak Area for CH4) +
(Corrected Peak Area for CO2)] x 100 %
0.9347
0.9347 + 0.5794
0.6173x 100%

=

61.7 % CH4 by volume

Normalization of Corrected Peak Areas to Calculate Carbon Dioxide Gas Volume in
Percent
% Volume CO2 =

[(Corrected Peak Area for CO2) / (Corrected Peak Area for CH4) +
(Corrected Peak Area for CO2)] x 100 %
0.5794
0.9347 + 0.5794
0.3827x 100%

=

38.3 % C02 by volume

Determination of Volume of Methane in Biogas Samples
Volume of CH4 = (Percent volume of CRO x (Corrected Volume of Biogas)
100%
= (0.6173) x (11.5 mL)
= 7.1 mL of CH4 (at 0 °C and 1 atm)

112

Determination of ultrasound specific energy (Es) in kJ kg'1 TS>' for 2.4% TS sludge

Where, P = ultrasonic power (W), t = ultrasonic time (s), v = sample volume (L), TS
total solids concentration (kg/L).
150W maximum output at 60% amplitude is 90W
9 0 ( J / s ) x 4800(5)
5 ~ 0.4(1) x 0.0273(kgTS/L)

Es = 39560(kJ/kgTS)
Calculation of cost of ultrasound pre-treatment for 2.4% TS sludge
Cost = 39560 kJ/kgTS x 27 kgTS/m3
Cost = (1068120 kJ/m3) / 4800 s
Cost = 223 kW/m3x 1.333 h
Cost = 297 kWh/m3 x $0.07 per kWh
Cost = $21/m3
Calculation of cost of NaOH addition for 2.4% TS sludge
Dosage of NaOH = 20.6 g NaOH / lOOg TS
20.6(gNaQH)
x(gNaOH)
\00(gTS) "24200( g T S / m 3 )
Dosage of NaOH = 5 kg NaOH / m3
$350
$x
lOOO(kgNaOH) ~~ 5(kgNaOH)
Cost of NaOH = $1.75 / m3
Total pre-treatment cost per m3 = Cost of ultrasound + Cost of NaOH
Total pre-treatment cost per m3 = $21 + $1.75
Total cost per m3 = $22.75

Determination of specific resistance to filtration
Specific resistance to filtration (R) was determined using a plot of filtration time/filtrate
volume (t/V) vs. filtrate volume (V).
120000000
y = 4972763219227.06x - 3875823.05
x
R2 = 0.96

100000000

oINOC

I

y = 2748629508172.01X - 4179373.26
R2 = 0.96

80000000

s

• 2.5 Non

s
y = 1250015597134.1 Ox + 9070139.69
R2 = 0.98

60000000
u.

a

y = 988968655917.50x + 76974.37
JB
R2 = 1.00

E

o

A 6.5 non

40000000

X 2.5 TR

y = 532872746758.73x + 2269254.57
R2 = 0.98
X 6.5 TR

S

u.
20000000

0

0.00001

0.00002

0.00003 0.00004
Filtrate volume (m3)

0.00005

0.00006

0.00007

Figure A Plot of filtration time/filtrate volume (t/V) vs. filtrate volume (V) for inoculum,
2.5% TS non-treated, 2.5%TS pre-treated, 6.5%TS non-treated, 6.5%TS pre-treated pulp
mill sludge post anaerobic digestion.
Using the slope of the line, R was calculated using the following equation,
t/V = (hRw/2A2P)V + uRm/AP
Where,
"R" is specific resistance to filtration (m kg"1)
"P" is the pressure of filtration (bar)
"n" is the viscosity of the filtrate (Pa s)
"V" is the volume of the filtrate (m3)
"t" is the filtration time (s)
"w" is the weight of the dry solids per volume of filtrate (kg m"3)
"A" is the area of the filter paper (m2)
"Rm" is the resistance on the medium (m"1)

114

"Rm" is very small for compressible sludge and can be ignored. Therefore the equation is
reduced to,
t\V = (jiRw/2A2P)V
Using the slope (b) of lines of best fit from Figure A,
R - (2A2P/nw)b
For 6.5% TS pre-treated it follows,
R = [(2 x (0.00950 m2)2 x 69000 Pa) / (0.001307 Pa s x 32.4 kg m"3)] x 4.9xl012 (s m6)
R = [12.45 m4 Pa / 0.0423 Pa s kg m'3] x 4.9x1012 (s m"6)
R = [294.3 m7 s"' kg1] x 4.9xl012 (s m"6)
R= 14.6xl014 m kg"1
Determination of estimated H2S concentration of biogas
Pre-treated 6.5%:
Total initial S content of dried 6.5% TS sludge = 1.366 ± 0.018%
TS 6.5% (g/batch)

= (6.52%/l 00) x 75 g/batch
= 4.890 ± 0.01 (g/batch)

Standard deviation

= (0.013%/l 00) x 75 g/batch
= 0.01 g/batch

S (g/batch)

= (1.366%/!00) x 4.890 g/batch
= 0.067 ± 0.001 g/batch

Standard deviation

= 0.001 g/batch
Total S content of dried inoculum = 0.986 ± 0.022%
TS inoculum (g/batch)

= (1.67%/100)*75 g/batch
115

1.2525 ± 0.019 g/batch
S (g/batch)

(0.986%/100) x 1.2525 g/batch
0.0123 ± 0.0003 g/batch

Total initial S per batch

S 6.5% TS sludge + S inoculum
79.1 ± 1.0 mg/ batch

Final S per batch

68.77 mg/ batch

S lost during digestion

Initial S - Final S
79.1 -68.77
8.91 ± 1.32 mg/ batch

Net Biogas produced

0.549 ±0.153 L

Concentration of S in gas

mg S lost / L of biogas produced
8.91 (mg/batch) / 0.549 (L Biogas/ batch)
(16.23 ± 5.1 mg/L) x 1000 L/ m3
16233 ±5121 mg/m3

Concentration of H2S

[16233 (mg/ m3) x 22.4 (L/mol)]/ 34 g/mol
(10694 ± 3374 ppm) /10,000 (ppm/l%vol)
1.07 ±0.34% vol H2S

Appendix B- Sample chromatograms.

Replicate 2

Replicate 1
1

Replicate 3

r

Oxygen & Nitrogen

Methane

Carbon Dioxide
/

Figure Bl- Chromatogram of methane standard and human breath. Gas chromatography conditions are reported in section 4.3.6.

117

I

Biogas Sample 1

!

,
,

,

Biogas Sample 2

Biogas Sample 3
,

,

1

Oxygen & Nitrogen

Methane

Carbon Dioxide

Figure B2- Chromatogram of biogas. Inoculum replicates 1, 2 and 3, 9:00 am, June 19th, 2011. Gas chromatography conditions are
reported in section 4.3.6.

118

Appendix C- Chapter four methane generation raw data
100.0 -

90.0 -

eo.o -

10.0 -

0.0

2.0

4.0

6.0

8.0

10.0

1 2.0

14.0 16.0 18.0
Time (days)

20.0

22.0

24.0

26.0

28.0

30.0

Figure CI Cumulative methane generation for inoculum at standard temperature and
jressure. A Replicate 1. • Replicate 2. 0 Replicate 3.
300.0

250.0

200.0

150.0

100.0

AAAAAA A A

0.0

2.0

4.0

6.0

A

8.0

10.0

12.0

14.0

16.0

18.0

20.0

22.0

24.0

26.0

28.0

30.0

Time (days)

Figure C2 Cumulative methane generation for 2.5%TS, not pre-treated, at standard
temperature and pressure. A Replicate 1. • Replicate 2. 0 Replicate 3.

119

250.0

200.0

o

3o 150.0
100.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

22.0

24.0

26.0

28.0

30.0

Time (days)

Figure C3 Cumulative methane generation for 2.5%TS, pre-treated, at standard
temperature and pressure. A Replicate 1. a Replicate 2. 0 Replicate 3.
600.0
EL

OT 500.0

c 400.0

O. 300.0

I 200.0

E 100.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0 16.0
Time (days)

18.0

20.0

22.0

24.0

26.0

28.0

30.0

Figure C4 Cumulative methane generation for 6.5%TS, not pre-treated, at standard
temperature and pressure. A Replicate 1. • Replicate 2. 0 Replicate 3.

120

700.0 -

600.0

500.0
400.0

BA A A
300.0

200.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

22.0

24.0

26.0

28.0

30.0

Time (days)

Figure C5 Cumulative methane generation for 6.5%TS, pre-treated, at standard
temperature and pressure. A Replicate 1. • Replicate 2. 0 Replicate 3.

121