Department of Civil and Environmental Engineering, North Dakota State University, Fargo, ND 58108, USAHDR, Boise, ID, USADynamita, Ontario, CanadaDepartment of Civil and Environmental Engineering, North Dakota State University, Fargo, ND 58108, USA
With the rise in eutrophication cases which leads to fish kills, deteriorating aquatic environment and contamination of the drinking water supplies, strict regulations are being enforced for the wastewater treatment facilities (WWTF) to regulate the daily nutrient discharge limits. Therefore, to satisfy the regulatory requirements, it is important to understand the limits of different processes employed at WWTF. Activated sludge (AS) process is a common biological treatment process (after pre-treatment) because it effectively reduces carbon content, is cost effective and is eco-friendly compared to physiochemical processes. An AS process primarily removes soluble chemical oxygen demand (sCOD) and the effluents contain both, soluble organics that are not removed by the process and, organic material produced during the process. The produced organic material in the effluent includes a large fraction of soluble organic nitrogen (sON). Therefore, the aim of this study was to examine the limits of AS process by accounting for sON production at different solids retention times (SRTs) (2, 5, 10 and 20 days). To examine the sON production, four bench-scale chemostat reactors, operated in parallel, were fed with synthetic wastewater with no organic nitrogen. In addition, fractions of effluent COD and organic nitrogen i.e. particulate, colloidal and soluble (pCOD and pON, cCOD and cON, sCOD and sON) along with biodegradable fraction of effluent sON (bsON) were investigated at different SRTs. Effluent sON generated at each SRT was identified. At SRT of 2 d 4.56 ± 2.64 mg-N/L, at SRT of 5 d 3.19 ± 0.96 mg-N/L, at SRT of 10 d 2.50 ± 0.48 mg-N/L and at SRT of 20 d 28.59 ± 6.22 mg-N/L of sON was generated. Although effluent sON concentration decreased from SRT of 2 d to 10 d, a significant spike was observed at SRT of 20 d, which is suggested to be exerted by endogenous residues. At SRTs of 2, 5 and 10 d, >50% sON was biodegradable whereas only 35% sON was biodegradable at SRT of 20 d. Therefore, AS process besides removing sCOD, generates biologically-produced sON which can be significantly high at long SRTs.