A COMPARISON OF BIOLOGICAL NUTRIENT REMOVAL IN INTERMITTENT CYCLIC AND CONTINUOUS ACTIVATED-SLUDGE SYSTEMS

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 NO3-N < 5 mg L-1, PO4-P < 1 mg L-1 and NH3-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 NO3-N and PO4-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.