Process Sizing • Wastewater

Activated Sludge Sizing Calculator

An aeration basin is sized by loading, not by volume preference. The food-to-microorganism ratio fixes how much biomass a given reactor volume has to hold to treat a given load, and the sludge retention time fixes how long that biomass stays. Those three — load, MLVSS and F/M — close the volume; the yield closes the biomass; and the inventory over the production closes the age.

The reason this calculator spends effort on the audit rather than the answer is that the volume, the F/M, the SRT and the HRT are not independent. Give it a set of figures that came from different spreadsheets and the engine will derive each one from the rest and tell you which pair disagrees, instead of quietly returning whichever number you happened to type.

Design basis

Required: the flow, the two COD concentrations, the MLVSS, and one of the volume or the F/M. Required for the biomass loop: a yield — an observed yield, or a gross yield with k_d and an SRT. Nothing here is defaulted, including the yield.

Influent flow (m³/day) Influent COD (mg/L) Effluent COD (mg/L) MLVSS in the reactor (mg/L) F/M loading (1/day) — or give the volume instead Reactor volume (m³) — optional if F/M is given Observed yield (kg VSS/kg COD removed) Gross yield (kg VSS/kg COD) — alternative to the observed Decay coefficient k_d (1/day) — needed with a gross yield SRT (days) — optional; derived from the loop when omitted HRT (hours) — optional; audited when stated MLSS/MLVSS ratio — needed for the wasting rate in kg MLSS (mg/L) — needed for the OUR Influent ammonia (mg/L as N) — optional, but the oxygen changes a lot without it Effluent ammonia (mg/L as N) Temperature (°C) — optional; yield and k_d depend on it Dissolved oxygen (mg/L) — optional; sets the achievable OUR

Basis and limits

The F/M is applied on the influent load, not the removal. Using the removed load would under-size the reactor by the removal efficiency.
The yield is required and is never defaulted. A yield depends on substrate and temperature, and a built-in one would be one plant's number presented as everyone's.
A gross yield is not an observed yield. Net = gross − k_dθ_c, so mixing the two changes the wasting rate, and therefore the age the plant actually holds.
Air flow is not calculated. Oxygen demand is stoichiometry; turning it into air needs a transfer model — alpha factor, standard conditions, site depth — that this platform does not have.
Secondary clarification is not designed here. The solids leaving this reactor are a load for a settling stage, not a settling design.

Governing formulas

The volume loop and the biomass loop close separately, and the audit is what tests them against each other.

V = Q·S₀ ÷ (F/M × X) — the applied load divided by the loading times the solids concentration. X must be in kg/m³, which 3,000 mg/L is.
Px = Y_obs × Q × (S₀ − Se) ÷ 1000 — the yield multiplies the COD removed, not the load.
θ_c = V × X ÷ Px — the age the inventory and the production can actually hold.
Wasting = inventory ÷ θ_c — that identity is the definition of the age, not an assumption about it.

Engineering Disclaimer & Verification Notice

This calculator provides preliminary engineering estimates for informational and planning purposes. Actual reverse osmosis / engineering system performance depends on site conditions, feed-water chemistry, membrane characteristics, operating pressure, temperature, recovery limits, fouling/scaling potential, and system design. Verify results using project-specific data, manufacturer projections, and applicable engineering standards before final design or operation.

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