Calculate how much sludge to waste each day to hold a target SRT.
WAS Rate for a Target SRT
Formula
WAS, lb/d = (Volume, MG x MLSS x 8.34 / Target SRT) - (Qe, MGD x TSSe x 8.34) WAS, gpd = WAS lb/d / (WAS conc, mg/L x 8.34) x 1,000,000
Inputs used
- Aeration basin volume (MG)
- MLSS (mg/L)
- Target SRT (days)
- Effluent flow (MGD)
- Effluent TSS (mg/L)
- WAS concentration (mg/L)
Worked example (White River WRRF, fictional)
- Total to remove = 3.6 x 3,000 x 8.34 / 11.6 = 7,765 lb/d
- Effluent loss = 8.0 x 8 x 8.34 = 533.76 lb/d
- WAS = 7,765 - 533.76 = 7,231 lb/d
- WAS flow = 7,231 / (9,000 x 8.34) x 1,000,000 = 96,337 gpd
Tips
- Change wasting by no more than about 10 to 15% per day.
- Verify the WAS flow meter and WAS concentration.
Results are estimates for operator use. Check units and inputs, and confirm decisions with plant data and procedures.