Novel feast-fast technique for filamentous foam control in the activated sludge process – introducing a new foam-measurement method

Overgrowth of filamentous Nocardia spp. in the activated sludge microbial ecosystem was previously identified as the cause of foaming problems in Hong Kong. In this work, the specific growth rate of N. amarae was found to be significantly higher than that of non-filamentous bacteria under food-to-microorganism (F/M) ratios lower than 0.5 mgBOD/mgMLSS-day, and was used as the basis for the development of a novel Feast-Fast Operation (FFO) for the prevention and control of foaming problems. The F/M ratio in the “feasting” aeration unit was 0.8 mgBOD/mgMLSS-day while that in the “fasting” aeration unit was 0.2 mgBOD/mgMLSS-day, resulting in an overall process F/M ratio that remained within the normal range, while avoided prolonged exposure of the sludge ecosystem to an F/M ratio below 0.5 mgBOD/mgMLSS-day. The FFO suppressed filamentous overgrowth and foaming without adversely affecting the organic treatment efficiency of the modified process. A new and convenient measurement method was developed for quantifying the degree of foaming and foam stability in activated sludge.