Compartmentalizing anaerobic zone improves sludge settling property of enhanced biological phosphorus removal system under low sludge retention time

The study investigated the effect of compartmentalization of anaerobic zone on sludge settling property in an enhanced biological phosphorus removal (EBPR) system. Continuous-flow pilot-plant experiments with synthetic influent were performed at varied anaerobic-oxic volume ratios (V_an: V_o, 1: 9 and 2: 8), involving 5, 10 and 15 days of sludge retention times (SRTs). Experimental results indicated that, regardless of V_an: V_o and SRT, satisfactory removal of COD and phosphate were achieved during all test runs. However, extremely high sludge volume index (SVI) values were observed when there were 5 days of SRT. Further experiments with compartmentalization of anaerobic zone were therefore conducted to confirm the necessity of incorporating the kinetic selection mechanism of bulking control into an EBPR process operated at a low SRT. These results revealed that the two factors, V_an: V_o and anaerobic compartmentalization, had interaction with respect to sludge settling property in terms of SVI. When anaerobic retention time increased from 0.72hr to 1.36hr, compartmentalizing anaerobic zone significantly decreased the SVI value because of generating high COD concentration gradient and phosphate release in the anaerobic zone. This result implies that supplementing kinetic selection pressure of bulking control exhibits a critical influence on sludge settling when the EBPR process operates at a lower SRT.