The role of extracellular polymeric substances on carbon capture in a high rate activated sludge A-stage system

Abstract

This paper quantifies the effect of varying solids retention time (SRT), hydraulic retention time (HRT) and dissolved oxygen (DO) concentrations on extracellular polymeric substances (EPS) production and subsequently effluent quality, carbon capture (bioflocculation) and carbon redirection (settling) in a high rate activated sludge A-stage system treating domestic wastewater. Two pilot-scale A-stage reactors were set-up with HRTs of 30 and 60min. Cascade DO control was used to maintain 3 DO set-points of 0.5, 1.0 and 1.5mg/L. A mixed liquor suspended solids (MLSS) concentration of 3000mg/L was maintained and the waste activated sludge (WAS) flow was varied to achieve SRTs of 0.28 and 0.56day. EPS fractions and the protein and polysaccharide concentrations of the mixed liquor were measured. Operation at the 0.56day SRT and 1.0mg/L DO resulted in the highest total suspended solids (TSS), total COD (tCOD), particulate COD (pCOD), and colloidal (cCOD) removal. The best overall performance in terms of bioflocculation (cCOD removal) and carbon capture (percent COD in the WAS) occurred at the 0.56day SRT and coincided with decreasing total EPS concentrations but the settling characteristics of the sludge were better at the 0.28day SRT. Overall, low correlations were found between EPS production and system performance. It is likely that at the high loading rate of the A-stage system, EPS production did not play a major role compared to the influence of operating parameters on effluent quality, carbon capture and redirection.