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.
This calculator also appears on ActivatedSludge.com, our consulting site. Results are for training and operator use; always follow your plant’s procedures.