Sludge settling enhancement in a pilot scale activated sludge process treating petrochemical wastewater by implementing aerobic or anoxic selectors

Industrial Continuous Stirred Tank aerobic biological Reactors (CSTR) often suffer from low F/M filamentous bulking, which results in deficient sludge settling. In order to find solutions to this issue, a pilot-scale Activated Sludge System treating petrochemical wastewater was operated under three different conditions: i) Increasing the F/M supplied to the CSTR ii) Including an anoxic selector iii) Including an aerobic selector. The initial sludge quality, characterized by DSVI values of 500 mL g(-1), open floc structure and bridging filaments was slightly improved by increasing the F/M from 0.10 up to 0.40 g COD g(-1)VSS day(-1). However, the Diluted Sludge Volumetric Index (DSVI) didn't reach values lower than 100 mL g(-1) until a selector was implemented. Anoxic and aerobic selectors were effective to improve sludge settling, reducing DSVI to average values of 80 and 45 mL g(-1), respectively. However, whereas the anoxic selector produced lots of short filaments, the aerobic selector reduced significantly the number of filaments, obtained a more compact floc structure and leaded to a more reliable operation. Biomass and nitrogen balances suggested that production of new biomass cells and substrate uptake for storage occurred in both selectors. The aerobic selector system, compared to the CSTR, also leaded to a slightly higher soluble COD removal efficiency, as well as lower observed ammonia assimilation and lower observed volatile suspended solid production per unit of COD, in the main reactor. The last factor indicated that including a selector resulted in an older sludge, susceptible of experiencing nitrification issues.