Wastewater Terminology: What Is Sludge Age (Sludge Retention Time, SRT)?
Sludge age refers to the average residence time of microbial cells in the aeration tank. For the activated sludge process with return sludge, the sludge age is the time required for the sludge in the entire aeration tank to be completely renewed on average (expressed in days). A longer sludge age gives better treatment performance and less sludge; however, if it is too long, the sludge will become aged and its settling will be impaired. The sludge age of conventional activated sludge is generally 3–4 days, while for the high-load activated sludge process it is 0.2–0.4 days. The sludge age must not be shorter than the generation time of the microorganisms to be cultivated, so that those microorganisms can grow and proliferate in the aeration tank. It is generally calculated by dividing the total sludge mass in the tank under stable balanced operation (MLSS × aeration tank volume) by the daily amount of excess sludge wasted (or the daily sludge inflow).
Sludge age, generally denoted as SRT (Sludge Retention Time), refers to the average residence time of activated sludge in the whole system, i.e. the residence time of microorganisms within the activated sludge system. The SRT directly determines the age of the microorganisms in the activated sludge system. Generally, younger activated sludge has a stronger capability to catabolize organic pollutants but poorer coagulation and settling, whereas older activated sludge has weaker catabolic capability but better cohesion. Controlling sludge wastage by SRT is regarded as the most reliable and accurate method of sludge removal, by selecting an appropriate sludge age (SRT) as the target for sludge-wastage control. Generally, when higher treatment efficiency and better effluent quality are required, the SRT should be controlled at a higher level; when the temperature is higher, the SRT may be lower. The generation time of the vast majority of microorganisms that degrade organic pollutants is less than 3 days. The generation time of the nitrifying bacteria that oxidize NH3-N to NO3-N is 5 days.
Controlling the sludge age is a method of selecting the types of microorganisms in the activated sludge system. If the generation time of a certain microorganism is longer than that of the activated sludge system, that microorganism will be removed as excess activated sludge before it can reproduce the next generation, and it will never proliferate within the system. Conversely, if the generation time of a certain microorganism is shorter than the sludge age of the activated sludge system, it can reproduce the next generation before being removed as excess activated sludge; therefore, that microorganism can survive and proliferate in the activated sludge system and be used to treat wastewater.
