Effect of food : microorganism ratio in activated sludge foam control

Foaming is a common operational problem in activated sludge processes that often adversely affects the quality of the treated effluent. Overgrowth of the filamentous Nocardia spp. in the microbial ecosystem was previously ide ntified as the cause of foaming. In the present study, the specific growth rate of Nocardia amarae was found to be much higher than that of nonfilamen tous bacteria under food:microorganism (F:M) ratios lower than 0.5 mg of bi ological oxygen demand (BOD)/(mg of mixed liquor suspended solids [MLSS](.) d). This indicated that filamentous overgrowth may occur in normal activate d sludge processes that: are continually operated under the usual F:M range of 0.2-0.6 mg of BOD/(mg of MLSS(.)d). A novel two-component feast-fast op eration (FFO) that capitalized on the sensitivity of filamentous bacteria t o F:M ratio was designed to prevent and control foaming problems. The F:M r atio in the "feasting" aeration unit was 0.8 mg of BOD/(mg of MLSS(.)d) whe reas that in the "fasting" aeration unit was 0.2 mg of BOD/(mg of MLSS(.)d) . The FFO resulted in an overall process F:M ratio that still remained with in the normal range, while avoiding prolonged exposure of the activated slu dge ecosystem to an F:M ratio below 0.5 mg of BOD/ (mg of MLSS(.)d). The FF O suppressed the overgrowth of filamentous bacteria without adversely affec ting the organic treatment efficiency of the modified process.