Activated Sludge Wastewater Treatment Simulator

Process Controls

Influent flow rate Q 80 m³/h

MLSS concentration X 3000 mg/L

Aeration intensity 60%

Sludge recycle ratio (RAS) 50%

Live Readouts

Effluent BOD— mg/L

Removal efficiency— %

F/M ratio— /day

Hydraulic retention θ— h

How it works

The aeration tank behaves as a continuously-stirred reactor (CSTR). At steady state a mass balance on soluble BOD (S) gives the effluent concentration directly from the influent load, the biomass concentration and the retention time:

(S0 − S) / θ = k · X · S
S = S0 / (1 + k · X · θ)

θ = V / Q                      (hydraulic retention time)
k = k_max · DO / (K_DO + DO)   (oxygen-limited rate constant)
F/M = (Q · S0) / (V · X)       (food-to-microorganism ratio, /day)

Q — influent flow. Higher Q shortens θ, giving pollutant molecules less contact time with the biomass flocs before they reach the clarifier.
X (MLSS) — mixed-liquor suspended solids, the concentration of active biomass flocs shown drifting through the tank. More biomass means faster substrate uptake.
Aeration — sets the dissolved-oxygen level the flocs need to metabolise organic matter; below roughly 1–2 mg/L the reaction rate collapses (visible as bubbles thinning and flocs going idle).
RAS ratio — the fraction of settled sludge pumped back from the clarifier hopper to the aeration inlet instead of wasted; this is what sustains X over time in a real plant.

Pollutant particles (amber) are consumed by nearby biomass flocs (brown) inside the turbulent aeration zone; whatever reaches the quiescent clarifier settles with the flocs or overflows the weir as treated effluent — visually mirroring the S0 → S reduction the formula predicts. V = 500 m³, S0 = 250 mg/L BOD (typical domestic wastewater) are held fixed so the four controls above map cleanly onto plant operating decisions.

Organic pollutant (BOD) Biomass floc (MLSS) Air (aeration)

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