Activated sludge treatment of abattoir wastewater—II: Influence of dissolved oxygen concentration

Abstract

Performance of laboratory scale completely mixed activated sludge reactors fed with abattoir wastewater was measured at different dissolved oxygen (DO) concentrations. Degradation of fat present in the influent was inhibited at DO concentrations below about 0.5 mg l −1, leading to sludges with high fat content which settled poorly due to excessive numbers of filamentous microorganisms. Fat was degraded rapidly at higher DO concentrations (up to 4.0 mg l −1) and the sludge contained few filamentous microorganisms, a low fat content and settled readily. However, effluent quality was highest at low DO concentrations due to lower levels of soluble breakdown products from the fat. When wastewater was fed intermittently at constant aeration rate, sludge with a low fat content and good settleability resulted, even though the DO concentration was about 0.2 mg l −1 for more than 30% of the time. Effluent quality was also high. Thus it is concluded that for full-scale abattoir treatment plants where wastewater flow is intermittent, DO concentration may be low during periods of high loading without adversely affecting effluent quality or sludge settleability.