SRT Calculator (Solids Retention Time)
Solids retention time (SRT), also called sludge age or MCRT, estimates how many days solids stay in the aeration basin. It compares the solids inventory with the solids leaving each day through wasting and the effluent.
Result
Solids retention time
12 days
Solids in aeration basin
25,020 lb
Solids wasted
2,002 lb/day
Solids lost in effluent
83 lb/day
Your SRT is 12 days, meaning solids stay in the aeration basin about 12 days on average before they are wasted or leave in the effluent. Compare it with your plant's own target and recent trend.
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The formula
SRT (days) = (V × MLSS) ÷ (Q_WAS × TSS_WAS + Q_eff × TSS_eff)- V
- Aeration basin volume (MG)
- MLSS
- Mixed liquor suspended solids (mg/L)
- Q_WAS, TSS_WAS
- Waste activated sludge flow (MGD) and its solids (mg/L)
- Q_eff, TSS_eff
- Effluent flow (MGD) and effluent TSS (mg/L)
Good to know
- Use the same volume and flow units throughout. This version uses the aeration basin as the process boundary.
- The right SRT target is facility-specific. It depends on temperature, DO, pH, influent characteristics and process configuration.
Read the full guide: Solids Retention Time in Wastewater: How to Calculate SRT
Frequently asked questions
Is SRT the same as sludge age or MCRT?
They are closely related and often used interchangeably. All describe how long solids stay in the system. Check which definition and process boundary your plant uses, and use the same one every time.
What is a good SRT?
There is no single right number. The right SRT target is facility-specific and depends on temperature, DO, pH, influent characteristics and process configuration.
Do I include effluent solids in the SRT formula?
This calculator does: solids leave through wasting (WAS) and through the effluent. If your effluent TSS is very low, it changes the result only slightly.
Stop running this math by hand
ComplyFlo for wastewater plants can calculate and trend SRT, F/M, HRT, RAS, SVI and removal efficiency from your daily readings, next to your plant dashboard and DMR-ready reports.
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