Solids Retention Time (SRT) is the average time the biomass stays in the activated sludge system…the time in days a particle moves around in the aeration basin and clarifier before being wasted out of the system. SRT is calculated by taking the total pounds of MLSS in the system and dividing it by the total pounds leaving the system each day.  See the equation below:

 SRT = Aeration basin pounds / (WAS pounds + Effluent pounds)

Effluent pounds are often ignored in the calculation because that value is typically very small compared to the other parts of the equation and won’t have a big impact on the math.  Typical SRTs range from 5 to 30 days. The SRT you need to operate at is dependent on site-specific conditions (e.g. temperature) and permit requirements. The operating SRT has a direct impact on the mixed liquor suspended solids (MLSS) concentration in the reactor and overall treatment performance. It is a valuable number to know because it dictates a lot of things tied to performance, process control, and troubleshooting.  What is the SRT your WWTP typically operates at?