SRT and F/M Ratio
Solids retention time and food-to-microorganism ratio with conventional-activated-sludge check.
Example
You enter
- Aeration volume (gal) 1000000
- MLSS (mg/L) 2500
- MLVSS (mg/L) 2000
- WAS flow (MGD) 0.05
- WAS TSS (mg/L) 8000
- Effluent TSS (mg/L) 10
- Effluent flow (MGD) 0.95
- BOD load (lb/day) 5000
You get
- MLSS in tank 20,850.0 lb
- SRT 6.11 d
Checked against published worked examples from: WEF MOP 11 + Metcalf & Eddy activated-sludge identity.
Details, formula, and sources
Formula or tableSRT (solids retention time, days) = (MLSS_lb in tank) / (TSS_lb wasted per day + TSS_lb lost in effluent per day). F/M ratio = BOD_lb_per_day / MLVSS_lb_in_tank. Conventional activated sludge typical: SRT 4-15 d, F/M 0.2-0.5.
Source and editionMetcalf & Eddy (Wastewater Engineering: Treatment and Resource Recovery), 5th edition by name. Water Environment Federation MOP-8 (Design of Municipal Wastewater Treatment Plants) by name.
Free accessMetcalf & Eddy / WEF MOP-8 licensed; principles free in EPA wastewater operator-training materials at epa.gov.
What governsEstimate. Operator of record and primacy agency govern.
API field namesaeration_volume_gal, mlss_mg_l, mlvss_mg_l, was_flow_mgd, was_tss_mg_l, effluent_tss_mg_l, effluent_flow_mgd, bod_load_lb_day, mlss_lb, srt_days
- Conversion factor 1 MGD × 1 mg/L × 8.34 = 1 lb/day (mass-balance constant)physical fact
- CAS range SRT 4-15 d, F/M 0.2-0.5 typicalMetcalf & Eddy 5th ed.