Work out mlss and srt instantly with clear inputs, formula shown and shareable results.
Solids retention time is the biomass inventory in the system divided by the mass wasted each day. It is the master control variable of activated sludge: below about 4-6 days nitrifiers wash out, 8-15 days gives stable nitrification, and above 20 days gives extended aeration with well-mineralised sludge.
Solids inventory
Inventory (kg) = volume (m3) x MLSS (mg/L) / 1000
Solids retention time
SRT (days) = inventory / (waste flow x waste concentration / 1000)
MLSS is the consequence of SRT and loading, not an independent variable. Controlling the plant by wasting to a target SRT gives stable performance; chasing an MLSS target does not.
Excessive endogenous decay, pin floc, poor settling and unnecessary aeration energy. Nitrate build-up can also cause rising sludge in the clarifier through denitrification.