💧 Wastewater Calculator
Enter flow, influent BOD, aeration basin volume, and solids data to compute food-to-microorganism ratio and sludge age.
How to Use This Calculator
Gather lab data
Collect MLSS/MLVSS, effluent TSS, and influent BOD results.
Enter flow and volume
Use average daily flow (MGD) and aeration tank volume (MG).
Input sludge flows
Add return (RAS) and waste (WAS) flow rates with sludge concentrations.
Formula
F/M = (Q × S0 × 8.34) ÷ (V × MLSS × 8.34)
SRT = (V × MLSS × 8.34) ÷ (QW × XW × 8.34 + Q × Xe × 8.34)
Q = influent flow (MGD), Sâ‚€ = influent BOD (mg/L), V = aeration volume (MG), MLSS = mixed liquor suspended solids (mg/L), QW = waste activated sludge flow (MGD), XW = WAS concentration (mg/L), Xe = effluent solids (mg/L).
Full Description
F/M ratio describes the balance between organic load and biomass, guiding aeration intensity and sludge age. Sludge retention time (SRT) indicates how long solids remain in the system, influencing nitrification, settling, and sludge characteristics.
Maintaining target F/M and SRT ranges stabilizes process performance, improves effluent quality, and optimizes energy use in activated sludge treatment.
Frequently Asked Questions
What are typical F/M targets?
Conventional activated sludge operates around 0.2–0.5 lb BOD/lb MLSS-day; extended aeration is lower.
How do I adjust SRT?
Change WAS flow or concentration; increasing wasting lowers SRT.
Why multiply by 8.34?
The factor converts mg/L and MGD to pounds (1 mg/L × 1 MGD × 8.34 = lb/day).
Should I use MLVSS instead of MLSS?
Yes for more accuracy. Replace MLSS with MLVSS in both formulas if volatile data is available.