Untreated flow enters at the left carrying a contaminant load (grit, suspended solids, dissolved organics and pathogens). It moves through three sequential stages, each stripping out a different pollutant class, and the particle's colour shifts from murky brown toward clear as its load falls. Disabling a stage routes flow straight past it — the particle keeps whatever load it entered with.
load_out = load_in · (1 − η_stage)
η_mech = 0.30 (fixed: screens + grit removal, unaffected by dosing)
η_bio = 0.75 · saturate(aeration/60) · dwell_factor(flow)
η_chem = 0.65 · saturate(coagulant/60)
- Mechanical — bar screens and grit chambers pull out rags, sand and coarse solids by settling and straining alone; no dosing needed, so its removal is constant whenever it's on.
- Biological — an aeration basin where aerobic bacteria oxidise dissolved organic matter (the process behind BOD removal); too little aeration starves the bacteria of oxygen, and too little contact time (high flow rate) doesn't let them finish, so both hurt removal.
- Chemical — coagulant addition clumps remaining fine particles and pathogens into settleable flocs for a final clarifier; more dose removes more, with diminishing returns past the coagulant demand of the water.
- Flow rate — faster throughput means less time in each tank (shorter hydraulic retention time), which mainly hurts the biological stage.
Real plants run exactly this sequence — preliminary/primary (mechanical), secondary (biological) and tertiary (chemical/disinfection) treatment — because each stage targets pollutants the others can't touch efficiently.