This is the 2D side cross-section counterpart to the 3D activated-sludge simulator — identical process engineering, viewed as a true side-on slice through the two-tank plant instead of a rendered 3D scene. 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)
This equation was checked numerically here across the full slider ranges of Q, X and aeration — S stays positive and bounded by S0, removal efficiency stays within 0–100%, and F/M stays positive throughout, so the 3D source's model needed no correction; this 2D build reuses it exactly.
- 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.
- Scroll to zoom, drag to pan the cross-section — the divider gap and clarifier hopper are easiest to follow zoomed in.
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.