Influence of composite fillers on sludge characteristics and extracellular polymeric substance composition and structure in the sequencing batch reactor

Residual sludge produced at sewage plants is high in water content and contains organic matter in large quantities. In addition, residual sludge is rich in pathogens and bacteria, and the processing cost is usually high. The humus-activated sludge process can improve sludge characteristics and reduce the processing cost for residual sludge in the subsequent stages. We prepared a novel type of composite fillers using the humus-activated sludge process. We comprehensively studied the influence of composite fillers on sludge settling and dewatering performance. The mechanism underlying composite fillers influence on extracellular polymeric substance (EPS) was revealed. Hence, the mechanism underlying composite fillers influence activated sludge settling, and dewatering performance was elucidated from the perspective of EPS. The results showed that as the amount of composite fillers added increased, the sludge volume index (SVI) and capillary suction time (CST) decreased. When the amount of composite fillers added was 100 g, the SVI and CST decreased to 37.5 mL/g and 43.9 s, respectively. Compared with the control group, the settling and dewatering performance increased by 35.3% and 31.8%, respectively. The composite fillers had an impact on the loosely bound extracellular polymeric substances (LB-EPSs), resulting in a significant reduction in polysaccharide (PS) and protein (PN) content and a significant increase in the PN-to-PS ratio. The content of hydroxyl and C–O radicals decreased dramatically. The sludge hydrophilicity decreased as well. The percentages of hydroxide, carbonyl, amido, and alkoxy radicals in protein increased significantly, increasing cation-binding sites. The present study provides a theoretical basis and technical support for improving sludge characteristics using the humus-activated sludge process and reducing the cost of residual sludge processing.