Solids retention time is the average number of days solids stay in the system. It controls nitrification, floc quality and sludge production.
SRT and MCRT
Formula
SRT, days = (Aeration volume, MG x MLSS, mg/L x 8.34)
/ [(WAS flow, MGD x WAS conc, mg/L x 8.34) + (Effluent flow, MGD x Effluent TSS, mg/L x 8.34)]
MCRT adds the clarifier solids inventory to the numerator.
Inputs used
- Aeration basin volume (in service) (MG)
- MLSS (mg/L)
- WAS flow (gpd)
- WAS (RAS) concentration (mg/L)
- Effluent flow (MGD)
- Effluent TSS (mg/L)
- Clarifier volume (optional, for MCRT) (MG)
- Average clarifier solids (optional, for MCRT) (mg/L)
Worked example (White River WRRF, fictional)
- Inventory = 3.6 MG x 3,000 mg/L x 8.34 = 90,072 lb
- Wasted = 0.096 MGD x 9,000 mg/L x 8.34 = 7,206 lb/d
- Effluent loss = 8.0 MGD x 8 mg/L x 8.34 = 534 lb/d
- SRT = 90,072 / (7,206 + 534) = 11.6 days
Tips
- Use 7 day averages for MLSS, WAS and effluent TSS.
- Use the aeration volume in service. For aerobic SRT, leave out anoxic and anaerobic zones.
- Always include effluent solids; during storms they can be a large share of the total.
Results are estimates for operator use. Check units and inputs, and confirm decisions with plant data and procedures.