Optimization of xanthan biosynthesis in the granule matrix in aerobic granular sludge wastewater treatment systems

The aerobic granular sludge (AGS) biotechnology provides the opportunity to recover resources such as phosphorus, alginate-like exopolysaccharides, polyhydroxyalkanoates, tryptophan, etc. from wastewater. Recently, xanthan, a versatile biopolymer with applications in the food, oil, geotechnical, and biomedical industries, has been identified in the aerobic granule matrix. The production of xanthan from waste is encouraged because conventional processes for its production are costly and require specialized laboratories. This study aimed at determining the effect of organic loading rate (OLR), chemical oxygen demand (COD) to nitrogen (COD/N) ratio, and feeding strategy on xanthan biosynthesis in the aerobic granule matrix during wastewater treatment in AGS-based wastewater treatment systems. Using Taguchi fractional factorial design, nine experimental runs were conducted to evaluate the effects of these parameters on xanthan production, treatment performance, and AGS system stability. Results indicated that OLR showed a critical influence on xanthan production. Xanthan yield increased with increasing OLR, attaining an optimum yield of 41 ± 7 mg xanthan/g biomass. A significant positive correlation (r = 0.831) was found between OLR and xanthan yield (p = 0.006). A negative correlation (r = -0.512) was obtained between COD/N ratio and xanthan yield, which suggests that increasing the COD/N ratio results in decreased xanthan yield. This result was not statistically significant at 95% confidence level (p=0.158). The feeding strategy had a very weak positive correlation (r = 0.042) with xanthan yield. This result was not statistically significant at 95% confidence level (p = 0.915), implying that variations in the feeding regime has minimal effect on xanthan production. Taguchi mean effect analysis showed that OLR of 2.1 kg COD/m³∙d and C/N ratio of 10 were optimal for xanthan production (41 ± 7 mg xanthan/g biomass) in the aerobic granule matrix. Additionally, the AGS system achieved COD, ammonia-nitrogen, and phosphorus removal efficiencies of 95 ± 5%, 73 ± 23%, and 72 ± 18%, respectively. System stability was also maintained throughout the experimental period as both 5-min sludge volume index (SVI5) and SVI30 values ranged from 20 ± 2 – 30 ± 2 mL/g and the SVI30/SVI5 ratio was consistently between 0.9 and 1.0. These findings contribute to optimizing AGS systems for sustainable xanthan recovery from wastewater, offering an efficient alternative for biopolymer production along with efficient wastewater treatment.