Effect of the mixed liquor parameters on sludge settling for a petrochemical activated sludge system including an aerobic selector

Abstract

Sludge settling is determined by the biomass quality, which in turn is dependent on the mixed liquor parameters. In order to assess their effect on SVI in an activated sludge system including an aerobic selector, experimental design arrangements were organized in a pilot-scale installation fed with petrochemical wastewater. Experiments on a CSTR revealed that while increasing DO (from <1 to 2–3 mg L−1), F/M ratio (from 0.2 to 0.5 g COD g−1 VSS day−1) and NH4+-N (from <1 to>2 mg L−1) and while reducing SRT (from >22 to <19 days), sludge settling presented a statistically significant improvement. When the aerobic selector was included in the activated sludge system, DO above 2 mg L−1 was the only requirement in the reactor to obtain SVI below 100 mL g−1. The critical parameters became those of the selector, where the flocculant biomass was formed, while degrading almost completely the BOD5. Despite the better quality of the biomass obtained with the selector, bench-scale, short-term experiments demonstrated that gravity settling principles, bioflocculation and water properties still played a significant role on sludge settling. With an increase in MLSS concentration (from 2.3 to 16 mg L−1) or in conductivity (from 20 to 60 mS cm−1), sludge settling was significantly worse. Increasing pH (from 8.0 to 9.0) or temperature (from 30 to 38 °C) enhanced sludge settling. Measurements of the sludge electrical particle charge with a Mütek system indicated an optimum value to obtain the minimum SVI, which could be attained by the addition of coagulants.