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

Aerobic granular sludge (AGS) biotechnology has been extensively studied for wastewater treatment over the past two decades, gaining increased interest due to its enhanced treatment performance and potential for resource recovery. This research focused on optimizing curdlan biosynthesis in the aerobic granule matrix during wastewater treatment. Using Taguchi fractional factorial design, nine experimental runs were conducted to determine the effect of carbon-tonitrogen (C/N) ratio (carbon measured in form of chemical oxygen demand – COD), organic loading rate (OLR), and feeding strategy on curdlan biosynthesis in the aerobic matrix. These factors were tested at three levels: C/N ratios of 10, 20, and 30; OLRs of 0.8, 1.5, and 2.1 kg COD/m³∙d, and feeding strategies of 60 min feeding, 30 min feeding followed by 30 min resting phase, as well as 10 min pulse feeding followed by 50 min resting phase. Results indicate that C/N ratio had no effect on curdlan yield. Pearson correlation analysis revealed no correlation (r = -0.07) between C/N ratio and curdlan yield. This result was not statistically significant at 95% confidence level (p=0.516). The OLR was identified as the most influential factor. Curdlan yield increased with increasing OLR, attaining an optimum yield of 74 ± 6 mg curdlan/g biomass. Findings show a significant positive correlation (r = 0.82) between OLR and curdlan yield; and this was statistically significant at 95% confidence level (p=0.007). The feeding strategy showed minimal effect on curdlan yield with a weak negative Pearson correlation (r = -0.25) between feeding strategy and curdlan yield. However, this result was not statistically significant at 95% confidence level (p = 0.852. From mean effect analysis, OLR of 2.1 kg COD/m³∙d, C/N ratio of 10, and feeding strategy of 30 min feeding followed by 30 min resting phase were optimal for curdlan production. The AGS system achieved stability throughout the duration of the experiments as both SVI5 and SVI30 values in all the experimental runs were in the range 16 ± 2 – 43 ± 1 mL/g, and the SVI30/SVI5 ratio was consistently between 0.9 and 1.0. Additionally, the AGS system achieved organic matter, ammonia-nitrogen, and phosphorus removal efficiencies reaching 99.6 ± 0.6%, 97 ± 1%, and 91 ± 6%, respectively. These findings look promising to enhance the sustainability of AGS-based wastewater treatment plants contributing to the attainment of circular economy in the wastewater management industry.