A comparison of biological nutrient removal in intermittent cyclic and continuous activated sludge systems

Abstract

An assessment was made of biological phosphorus and nitrogen removal and related sludge bulking control in both intermittent cyclic and continuous activated sludge systems. Experimental results from a laboratory-scale investigation using fermented domestic sewage indicated the advantages of achieving high levels of P and N removal and good sludge-settling properties using the intermittent cyclic process, as compared to the continuous process. The modified intermittently fed and decanted system incorporating non-mixing sequences produced an effluent quality of NO 3-N < 5 mg L −1, PO 4-P < 1 mg L −1 and NH 3-N < 2 mg L −1, with a non-bulking sludge having a sludge volume index < 120 mL g −1, despite unfavourable influent characteristics (TCOD : TKN < 7, TKN > 60 mg L −1, TP-9 mg L −1 and RBCOD < 30 mg L −1). In contrast, the modified 4-stage Bardenpho process could not achieve effluent NO 3-N and PO 4-P concentrations of < 10 and < 1 mg L −1, respectively. The sludge volume index (SVI) values were always higher than 250 mL g −1 and occasionally reached as high as 422 mL g −1. The causative filaments were low F:M growers - Types 0041/0675.