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Wastewater Treatment Sizing Calculator

Size key components of an activated sludge wastewater treatment plant: aeration, clarifiers, aeration power, and sludge production.

About this calculator

This calculator sizes the core components of a conventional activated-sludge treatment train from four numbers: design flow, influent BOD, target effluent BOD, and influent TSS. It first converts flow and concentration into a mass loading rate using the standard 8.34 conversion factor (pounds per million gallons per mg/L), then works out the required removal efficiency by comparing influent and target effluent BOD. From there it sizes the aeration basin using a fixed food-to-microorganism ratio of 0.3 and a mixed liquor suspended solids concentration of 3,000 mg/L — both typical design assumptions for conventional activated sludge, not values you can adjust here, so a plant designed for extended aeration or high-rate systems will need different numbers.

Aerator horsepower follows from an assumed oxygen demand of 1.5 lb O2 per lb of BOD removed and a standard aeration efficiency of 2 lb O2 per horsepower-hour. Clarifier area uses a fixed 800 gallons-per-day-per-square-foot peak overflow rate, and sludge production combines an assumed 0.5 lb of solids generated per lb of BOD removed with 70% TSS capture in primary treatment. Because several of the underlying coefficients (F/M, MLSS, oxygen demand, overflow rate, sludge yield) are fixed industry rules of thumb rather than inputs, treat the results as a preliminary sizing check against EPA's secondary treatment regulation, 40 CFR Part 133, which sets the 30 mg/L monthly-average (45 mg/L weekly-average) BOD5 effluent ceiling this calculator uses as its default target — a real design still needs a process engineer's review and site-specific pilot or bench data.

Inputs

MGD
mg/L

EPA 40 CFR 133: domestic influent 200–300 mg/L; secondary effluent limit ≤30 mg/L

mg/L

EPA 40 CFR 133: secondary standard 30 mg/L monthly avg; advanced treatment <10 mg/L; reuse <5 mg/L

mg/L

Results

BOD Loading

2,085 lb/day

Aeration Power

60 HP

≈ 6 EV chargers

Required Efficiency92%
Clarifier Area1,250 ft²
Sludge Production2,419 lb/day
Aeration Volume277,778 gal

Figures current as of 2026. Source: U.S. Environmental Protection Agency, 40 CFR Part 133 (Secondary Treatment Regulation), § 133.102

How to Use This Calculator
  1. Enter population served and per-capita daily flow (gpd).
  2. Set infiltration and inflow allowance and target effluent quality.
  3. Review design flow, treatment unit sizing, and required hydraulic retention time.

How the result changes with Design Flow

Design FlowBOD LoadingAeration Power
0.51,043 lb/day30 HP
0.751,564 lb/day45 HP
1.53,128 lb/day90 HP
2.55,213 lb/day150 HP

What each input means

Design Flow
Average daily wastewater flow in million gallons per day. Small community ≈ 0.1-1; medium city ≈ 5-20 MGD.
Influent BOD₅
Biochemical oxygen demand of raw wastewater per EPA secondary treatment standards (40 CFR Part 133). Typical domestic wastewater 200–300 mg/L. Industrial wastewater can range 500–2,000+ mg/L.
Effluent BOD₅ Target
Required effluent BOD concentration per NPDES discharge permit (40 CFR Part 133). EPA secondary treatment standard: 30 mg/L monthly avg, 45 mg/L weekly avg. Advanced treatment (nutrient removal): <10 mg/L.
Influent TSS
Total suspended solids concentration in raw wastewater. Typical domestic is 200-350 mg/L.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Design Flow = 1, Influent BOD₅ = 250, Effluent BOD₅ Target = 20, Influent TSS = 250 = 4 input(s) provided
  2. Calculate BOD Loading
    BOD Loading
    2085 = 2085
  3. Calculate Aeration Power
    Aeration Power
    60 = 60
  4. Calculate Required Efficiency
    Required Efficiency
    92 = 92
  5. Calculate Clarifier Area
    Clarifier Area
    1250 = 1250

Figures and sources

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Why are the F/M ratio and MLSS concentration fixed at 0.3 and 3,000 mg/L instead of being inputs?

Those two values define the aeration basin volume formula (V = Q × BOD / (F/M × MLSS × 8.34)) and are typical design constants for conventional activated sludge, so the calculator uses them as fixed assumptions rather than user inputs. An extended-aeration plant typically runs a lower F/M (around 0.05-0.15) and would need a larger basin than this tool reports, while a high-rate system runs higher F/M and a smaller one — so treat the aeration volume output as specific to conventional activated sludge design, not a universal answer.

What does the 'Required Efficiency' output actually mean?

It's the percent BOD removal needed to go from your influent BOD to your effluent BOD target, computed as (influentBOD − effluentBODTarget) / influentBOD × 100. It isn't a capability check — the calculator doesn't verify whether the sized aeration basin can actually hit that efficiency, it just reports what removal percentage the treatment train needs to achieve.

Why does raising the effluent BOD target lower the aerator horsepower requirement?

Aerator horsepower is driven by oxygen demand, and oxygen demand (o2Required) is calculated as BOD loading times 1.5 lb O2 per lb removed times the required treatment efficiency fraction. Loosening the effluent target lowers that required efficiency, which means less BOD mass has to be removed and therefore less oxygen — and less horsepower — is needed to get there.

Does influent TSS or BOD concentration change the clarifier area result?

No — clarifier area is calculated purely from design flow (in gallons per day) divided by a fixed 800 gpd/ft² peak overflow rate, so it scales only with flow, not with BOD or TSS concentration. Sludge production, by contrast, does depend on both: it sums a BOD-removal-based component and a separate TSS-based component (assuming 70% capture in primary treatment).

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