Solids Retention Time (SRT), Sludge age

The sludge age SRT is probably the most important key figure in the activated sludge process. The SRT corresponds to the average residence time of the microorganisms in the activated sludge plant. The sludge mass in the activated sludge tank is set in relation to the daily removed sludge mass (surplus sludge or waste activated sludge WAS). The SRT thus describes the possible generation time of the bacteria. Put simply - only bacteria whose generation time is shorter than the sludge age can accumulate in the system.

The SRT characterizes the specific load of the system and thus the composition of the biocenosis. Carbon decomposition and denitrification can be achieved at a low SRT of only a few days. Nitrification is achieved, depending on temperature, only at SRT of about 10 d or more. At a very high SRT, the activated sludge is stabilized (SRT > 20 d).

The higher the SRT, the greater the risk of bulking sludge formation (poor settling behavior of the sludge). But advantageously, the higher the SRT, the higher the degree of biological degradation, since highly specialized bacteria can also accumulate in the system.

The SRT for the entire sludge is often set so high (or the amount in the aeration tank is kept so high) that not too many nitrifiers are flushed out even during shock loads, or this effect is compensated by a sufficient amount of sludge in the system.

This is not necessary if it is possible to keep the nitrifiers stable in the system. This is important, because it is usually design-relevant. So it has to be avoided to discharge them with the surplus sludge. This is achieved in the S::Select® process), where only the fast-growing bacteria are preferentially discharged. Thus, the typical SRT loses its significance.

Thus, based on the typical SRT, there is a significantly higher proportion of slow-growing (nitrifying) bacteria in the biomass, i. e. a significantly higher specific nitrifying performance, which is particularly evident during peak loads. Or vice versa - while in the classical design the generation time of the nitrifiers (plus safety margin) is the decisive design parameter, this parameter loses its importance in the S::Select® process due to the selective enrichment.

Specifically, this means that with the S::Select® process 2 SRT values are set in a biological system. The suspended biomass has a low SRT of 2-3 days, while the aerobic granular sludge held in the system with the nitrifiers represents a SRT of >>10 d. Thus, on the one hand, a high specific carbon degradation can be used (denitrification) and, on the other hand, nitrification can be carried out reliably. At the same time COD that is difficult to degrade is degraded by highly specialized bacteria with a high SRT.

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