Increased biogas production by anaerobic co-digestion of wastewater sludge with fruit and vegetable waste, and by sludge pre-treatment.

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⁻¹ added) than SSS (392 ± 16 L CH₄ kgVS⁻¹ 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.