Observed Biomass Yield Calculator
Not all the BOD you remove becomes sludge — endogenous decay consumes part of it. Enter yield, decay rate, SRT, and BOD removed to estimate net biomass production for wasting decisions.
Net Biomass Production
| Item | Value |
|---|
How the calculation works
Formulas
BOD removed (lb/day) = 8.34 × flow (MGD) × (influent BOD − effluent BOD)
Observed yield Yobs (lb VSS / lb BOD) = Y ÷ (1 + kd × SRT)
Net biomass production Px (lb VSS/day) = Yobs × BOD removed
Longer SRT means more decay, so Yobs falls as SRT rises — extended-aeration plants make less sludge per pound of BOD.
BOD removed (lb/day) = 8.34 × flow (MGD) × (influent BOD − effluent BOD)
Observed yield Yobs (lb VSS / lb BOD) = Y ÷ (1 + kd × SRT)
Net biomass production Px (lb VSS/day) = Yobs × BOD removed
Longer SRT means more decay, so Yobs falls as SRT rises — extended-aeration plants make less sludge per pound of BOD.
| Coefficient | Typical value |
|---|---|
| True yield Y (heterotrophs) | 0.4–0.8 lb VSS/lb BOD |
| Decay rate kd at 20°C | 0.06 /day |
Notes & limitations
Yield and decay coefficients follow Metcalf & Eddy, Wastewater Engineering (Y = 0.4–0.8 g VSS/g BOD, kd = 0.06 d-1 at 20°C for heterotrophic biomass). This is the simplified net-yield form used in operator training.
- The full Metcalf & Eddy sludge-production equation adds endogenous residue and influent inert solids; this tool gives the biomass-growth term only, which dominates at typical SRTs.
- Results are lb VSS per day. Mixed liquor is roughly 70–85% volatile — divide by ~0.8 for an approximate lb TSS/day wasting target, and confirm with your plant’s VSS/TSS data.
- kd drops in cold wastewater, raising net yield in winter. Adjust if your plant runs much colder than 20°C.
- Use this with your SRT target to set wasting: waste the net production each day to hold SRT steady.
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