Effect of extracellular polymeric substances removal and re-addition on the denitrification performance of activated sludge: Carbon source metabolism, electron transfer and enzyme activity

Extracellular polymeric substances (EPS) play an important role in the metabolic activities of activated sludge. The effect of EPS removal (pellet group) and re-addition (pellet + EPS group) on the denitrification of activated sludge were analyzed in this study. The results indicated that EPS removal deteriorated the denitrification performance of the activated sludge. The denitrification rates in the pellet group with EPS extraction degrees of 7.77%, 12.37%, 35.89%, and 100% were only 82.6%, 75.7%, 64.1%, and 26.9%, respectively, of the control group. However, the denitrification rate of the pellet increased by 19–34% by re-adding the extracted EPS. Moreover, when the EPS extraction degree was 7.77%, the denitrification rate of the pellet + EPS group was 10.4% higher than that of the control group. The EPS contain many redox mediators. The electron transport system activity (ETSA) and key enzymes in biological denitrification were improved by re-adding EPS. The ETSA in the pellet + EPS group were 1.66-fold that in control group. The pellet + EPS group had a lower reaction activation energy, which was 42.9% lower with that of the control group. In addition, EPS re-addition promoted the activities of enzymes related to glycolysis by 9.60–18.08%, and nicotinamide adenine dinucleotide yield increased by 6.5%, which enhanced the denitrification rate of activated sludge. This study provides a novel method to enhance the denitrification performance of activated sludge by in situ appropriate desorption of EPS as endogenous redox mediator.