Solids Retention Time (SRT) is directly related the growth rate of the biology we are relying on for treatment.  Different microorganisms grow at different rates.  Nitrifiers in particular are known to be slow-growers while those responsible for BOD oxidation and denitrification are known to be fast growers.

If we waste in a way that removes bugs faster than they grow, then we can’t sustain a population in the aeration basin. The minimum time required to sustain a microbial population is called the Washout SRT.  The figure above shows that the washout SRT varies by the type of bug and the operating conditions (especially temperature). SRT-based control is the recommended strategy for managing activated sludge process because it aligns wasting practices with the growth rate of the microorganisms. If a bug grows/reproduces every 5 days we need to keep that bug in the system for at least 5 days.  That’s SRT control in a nutshell.

It’s common to add in a safety factor when identifying an operating SRT, and it’s traditionally something like 2.0.  If the Washout SRT = 5 days; Operating SRT = 5 x 2 = 10 days. WWTPs use multiple communities of bugs simultaneously…one for BOD removal, one for ammonia removal, one for nitrate removal, etc.  Which community do you think dictates the operating SRT?