Studies on Aerobic Digestion of Sludge in Intermittently Aerated Activated Sludge Plants

In the past, activated sludge plants with intermittent aeration and simultaneous aerobic digestion were designed like activated sludge plants with continuous aera-tion, i.e. pauses in aeration were not considered. However, according to the amended ATV - Merkblatt M 210 -, only times when aera-tion or stirring do take place count as reaction time, which - depending on the idle period - requires a larger volume of the activated sludge tank. But since even during the idle period the microorganisms are supplied to some ex-tent with oxygen by the oxygen present as nitrate in the settling sludge, at least some of the idle period could be included as reaction time as well. To explore this in more detail, the degree of aerobic digestion of activated sludges was determined in a bench-scale test unit with continuous and intermittent aeration. The results showed that up to pauses in aeration of 15 hours per day there is hardly any difference in the degree of aerobic digestion of sludges.