Composition and activity of activated sludge under starvation conditions

The response of activated sludge to starvation was studied in aerobic conditions during a 10 days starvation period. The 30% sludge mass decrease was related to the degradation of both proteins and polysaccharides as well as to the release of biopolymers in the water surrounding the biomass. After 8 days, organic carbon release as a result of the sludge degradation was relatively low (=2%) compared to the respired carbon (42%). The decrease of the total number of bacterial cells followed the degradation of the solids only after 4 days. The degradation of the sludge constituents occurred with a rapid decrease of the respiratory activity. After 11 days the sludge was still able to use an easily biodegradable substrate (yeast extract) contrary to the organic matter from primary wastewater. From our results, about 4 days of storage of the sludges may represent a limit to be considered for a high mass reduction and a low biochemical activity loss (allowing the recycling of a viable biomass).