Solids Retention Time – SRT
Solids retention time is the ratio of the mass of solids in the aeration basin divided by the solids exiting the activated sludge system per day. Exiting solids are equal to the mass of solids wasted from the system plus the mass of solids in the plant effluent.
Ensuring an adequate SRT is very critical to the SBR biological nutrient-removal design process. The design SRT for nitrifying systems should be based on the aeration time during the cycle, not the entire cycle time.