Equalization Basin Volume Calculator
Dampen diurnal flow swings before downstream treatment. Enter your average flow, peak factor, and how long flows run above average to size the basin.
Equalization Volume
| Item | Value |
|---|
How the calculation works
Formulas (constant-outflow method)
Peak flow = peak ratio × average flow
Surplus rate during peak block = (peak ratio − 1) × average flow
Storage (MG) = surplus rate (MGD) × peak hours ÷ 24
Storage (% of daily flow) = storage (MG) ÷ average daily flow (MG) × 100
Off-peak flow = (24 × average − peak hours × peak flow) ÷ (24 − peak hours)
Detention at average flow (hr) = storage (gal) ÷ average flow (gph)
Peak flow = peak ratio × average flow
Surplus rate during peak block = (peak ratio − 1) × average flow
Storage (MG) = surplus rate (MGD) × peak hours ÷ 24
Storage (% of daily flow) = storage (MG) ÷ average daily flow (MG) × 100
Off-peak flow = (24 × average − peak hours × peak flow) ÷ (24 − peak hours)
Detention at average flow (hr) = storage (gal) ÷ average flow (gph)
| Plant size | Typical peak / average ratio |
|---|---|
| Small (< 1 MGD) | 2.5 – 3.0 |
| Medium (1 – 10 MGD) | 2.0 – 2.5 |
| Large (> 10 MGD) | 1.5 – 2.0 |
Notes & limitations
This is the simplified constant-outflow (cumulative surplus) method described in Metcalf & Eddy. Final design should use the full diurnal hydrograph with a mass-curve analysis, and include freeboard plus a mixing/aeration system to keep solids suspended.
- The peak-hours × peak-ratio combination must balance over 24 hours — the tool rejects combinations where the implied off-peak flow goes negative.
- Equalization ahead of biological treatment also dampens load swings (BOD), not just flow — size for the governing case.
- Provide mixing (typically 0.5–1.0 hp per 1,000 ft3 or diffused air) so the basin does not turn septic.
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