Effects of short-time aerobic digestion on extracellular polymeric substances and sludge features of waste activated sludge

Abstract

Aerobic digestion is an important stabilization process for waste activated sludge. However, extended aerobic digestion consumes substantial oxygen and increases the running cost (oxygen demand and sludge production) and CO2 emission. To tackle these issues, this study investigated the effects of short-time aerobic digestion (STAD) on microbial extracellular polymeric substances (EPS), sludge features, and their correlations. The levels of proteins and polysaccharides in EPS consistently increased as the aerobic digestion time increased. Nucleic acid in EPS increased only within the first 4h and then decreased. With an extended digestion time, sludge floc size (Dv50) increased by 4.67%, and the sludge volume index (SVI) increased by 5.06%, indicative of a deteriorating settleability. The sludge after STAD exhibited better flocculability and dewaterability than that after the prolonged aerobic digestion. Proteins and polysaccharides in the EPS significantly correlated with Dv50 (R2=0.97, P=0.00 and R2=0.92, P=0.00, respectively), SVI (R2=0.88, P=0.00 and R2=0.93, P=0.00, respectively), effluent suspended solids (ESS, R2=0.68, P=0.01 and R2=0.82, P=0.00, respectively) and the normalized capillary suction time (CST, R2=0.66, P=0.01 and R2=0.78, P=0.00, respectively). No significant correlation was found between nucleic acid and these sludge features. These results indicated that proteins and polysaccharides in EPS may govern sludge floc size, settleability, flocculability and dewaterability. Interestingly, good sludge flocculability and dewaterability were observed when the proteins and polysaccharides in EPS were lower than 21.55 and 12.27mg/gVSS, respectively.