WASH R&D Centre · Study series

A wastewater treatment works, unit by unit

An illustrated atlas of the municipal treatment works: the liquid train from screens to reuse, the internal recycles that make biological nutrient removal work, and the sludge line where half the real operating effort lives. Each unit is drawn as an exploded or x-ray view with its component schedule — the hardware a plant walk actually inspects. A companion to the WESS study guide, at the scale a municipality runs.

The waste in the water

Domestic wastewater is about 99.9% water. The works exists for the last gram per litre, and that gram is not one substance but half a dozen groups, each harmful in its own way and each removed by a different part of the plant. Knowing the groups is knowing why the works has the shape it has.

The gram in the litre, made visible. Raw sewage magnified: rag and grit sinking, a fat globule floating, dissolved organics, ammonium and phosphate ions, a bacterium, a virus, a helminth egg, suspended particles and salt ions — each labelled with its typical raw strength. Everything the works exists to separate from the 99.9%.

Group	Typical compounds	The harm	Removed by
Gross solids, rags, grit (highly variable)	Cellulose (paper, rags), plastics (PET, polypropylene), silica sand SiO₂	Mechanical: wrapped pump shafts, blocked pipes, abraded impellers, digesters filling with sand	Screens, grit chamber (§4)
Fats, oils, grease (50–100 mg/L)	Triglycerides, free fatty acids (oleic, stearic), calcium soaps	Coats diffusers and weirs, congeals in sewers, feeds stable scums and filamentous foam	Oil & grease separator (§4)
Organic matter (BOD ≈ 250–400, COD ≈ 500–800 mg/L)	Carbohydrates (starch, sugars — glucose C₆H₁₂O₆), proteins and their amino acids, fats, urea; the texts’ CH₂O is shorthand for the mixture	Oxygen theft: bacteria burning it in the river strip the dissolved oxygen fish and invertebrates live on	Primary settling + the biological reactors (§5–6)
Nitrogen (TKN ≈ 40–80 mg/L)	Urea CO(NH₂)₂ (most of it), ammonium NH₄⁺ / free ammonia NH₃, protein-bound organic N; nitrite and nitrate only in traces on arrival	Three harms: un-ionised ammonia is directly toxic to fish; nitrifying it in the river costs 4.6 g O₂ per g N (more oxygen theft); and as nitrate it fertilises algae and renders water unfit for infants	Nitrification then denitrification (§6)
Phosphorus (TP ≈ 6–12 mg/L)	Orthophosphate PO₄³⁻ / HPO₄²⁻, detergent polyphosphates, organically bound P	The scarcest algal nutrient, so the one that controls blooms: a single gram grows roughly a hundred grams of algae — eutrophication runs on P	Chemical precipitation at settling; some biological uptake
Sulfur (SO₄²⁻ ≈ 30–100 mg/L + protein S)	Sulphate SO₄²⁻ from supply water and detergents; sulfur-bearing amino acids (cysteine, methionine); sulphide HS⁻/H₂S wherever the flow goes septic	Harmless as sulphate — lethal as sulphide: H₂S gas kills sewer workers, its re-oxidation on pipe crowns makes sulphuric acid that eats concrete, and the odour is the works’ public face	Kept oxidised by aeration; managed, not removed (see the sulfur loop below)
Pathogens (E. coli 10⁶–10⁸ CFU/100 mL as indicator)	Organisms, not compounds: bacteria (E. coli, Vibrio, Salmonella), enteric viruses (rota-, noro-), protozoan cysts (Giardia, Cryptosporidium), helminth eggs (Ascaris)	Disease: the reason wastewater treatment exists at all	Settling and membranes remove; disinfection kills (§8)
Suspended solids (TSS ≈ 200–400 mg/L)	Organic detritus and cellulose fibres, clay and silt (aluminosilicates), the particulate share of everything above	Smothers river beds, carries adsorbed organics and metals, and shields embedded pathogens from disinfectant	Settling at both ends of the train, membranes (§5, 7)
Dissolved salts (TDS 500–1 200 mg/L)	Na⁺, Cl⁻, SO₄²⁻, HCO₃⁻, Ca²⁺, Mg²⁺, K⁺	Conservative: nothing in a conventional works touches them — the one group that passes through, and the reuse loop’s eventual limit	Not removed (dilution, or membranes beyond UF)

The chemistry we are buying

Strip the plant to its purpose and it is a small set of reactions, chosen because each one converts a harmful species into a harmless one. The works is a place where those reactions are made to happen fast, in tanks, instead of slowly, in the river.

The harm the works pre-empts. Downstream of an untreated outfall the river runs the same oxidation the aerobic reactor runs on purpose — on its own dissolved oxygen. The sag curve: clean water, degradation, the damage zone where only air-gulpers survive, and the slow recovery kilometres later.

The reaction we are pre-empting — oxygen theft in the river

\ce{CH2O + O2 ->[\text{river bacteria}] CO2 + H2O}

The same oxidation the aerobic reactor runs deliberately (§6) happens for free in any river that receives raw sewage — and the oxygen it consumes is the river’s. A strong discharge drives dissolved oxygen through the sag that kills everything downstream of the outfall. The entire carbonaceous side of the works exists to run this reaction in a tank, on blown air, instead.

Nitrogen: from toxic to inert

\ce{NH3 <=> NH4+}

\ce{NH4+ ->[\text{nitrify}] NO3- ->[\text{denitrify}] N2 ^}

Un-ionised ammonia (the NH₃ side of the equilibrium, favoured at high pH) is acutely toxic to fish at fractions of a mg/L. The works walks nitrogen down the whole chain: ammonium to nitrate (removing both toxicity and the 4.6 g/g oxygen debt), then nitrate to nitrogen gas — the same inert N₂ that is four-fifths of air. Section 6 gives both reactions in full.

The sulfur loop — septicity, odour and the acid that eats sewers

\ce{SO4^2- + 2CH2O ->[\text{no oxygen, no nitrate}] HS- + 2CO2 + H2O + OH-}

\ce{HS- + H+ <=> H2S ^}

\ce{H2S + 2O2 ->[\textit{Thiobacillus}] H2SO4}

\ce{HS- + 2O2 -> SO4^2- + H+}

Sulfur cycles rather than leaves. When sewage goes septic — long rising mains, deep septic tanks, any corner the redox tower’s bottom reaches — sulphate-reducing bacteria respire SO₄²⁻ and make sulphide. The equilibrium releases H₂S gas on the acid side of neutral: the rotten-egg odour, and at sewer-atmosphere concentrations a worker-killing toxin. On the damp crown of the pipe, Thiobacillus oxidises that gas to sulphuric acid, which dissolves the concrete — crown corrosion is how sewers are eaten from the inside. The works’ defence is the last reaction: keep the flow aerated and sulphide is oxidised harmlessly back to sulphate, which is why septicity control starts at the headworks, not at the complaint line.

Phosphorus: the reaction biology cannot do

\ce{Al^3+ + PO4^3- -> AlPO4 v}

\ce{Fe^3+ + PO4^3- -> FePO4 v}

Ordinary biomass is only ~1–2% phosphorus, so a biological train barely touches TP. Removal is bought chemically: alum or ferric dosed at settling precipitates phosphate as an insoluble salt into the sludge. Jar-test first — real sewage consumes coagulant on side reactions before this one.

Pathogens: oxidative kill

\ce{HOCl + $\text{cell}$ -> $\text{inactivated}$}

Hypochlorous acid oxidises through cell walls and proteins; UV instead wrecks DNA directly. Either way the barrier is §8 — and it only works on organisms the solids removal has left exposed.

Eutrophication in four panels. Nutrients arrive, algae bloom and block the light, the bloom dies and its digestion strips the oxygen, and the pond ends green, anoxic and dead — with the multiplier that makes phosphorus the control knob: one gram grows about a hundred grams of algae.

The four pathogen classes to scale — virus, bacterium, protozoan cyst, helminth egg across three orders of magnitude — and the barrier that catches each: settling takes the heavy egg, the 0.05 µm membrane wall stops bacteria and cysts and most viruses, chlorine and UV finish what passes.

What "removed" means Almost nothing is destroyed. The screens fill skips; the settlers and the biology convert dissolved pollution into sludge; the precipitated phosphorus leaves in the same sludge; only nitrogen (as N₂) and carbon (as CO₂ and CH₄) actually leave as gas. Everything else is concentrated and carted away — which is why the sludge line of §9 is half the works, and why "treatment" is mostly the art of moving pollution from a large volume of water into a small volume of solids.

Who removes what. The section’s table drawn as a matrix: each contaminant group against the unit that captures or destroys it — and the dissolved-salts row empty end to end, because nothing in a conventional works touches them.

The works at a glance

A treatment works is two trains sharing one site. The liquid train runs left to right: physical protection (screens, grit, fats), balancing, primary settling, the biological reactors, final clarification, disinfection, and discharge or reuse. The sludge line runs underneath it: everything the liquid train removes — primary sludge and wasted biomass — is thickened, stabilised, dewatered and carted away. Two internal recycles stitch the trains together: the nitrified-liquor recycle (MLR) and return activated sludge (RAS) keep the biology supplied, while filtrate from dewatering returns its load to the head of works.

The whole works in one line. Liquid train left to right with its two return streams — the MLR feeding nitrate back to the anoxic zone and the dashed RAS returning biomass — and the sludge line beneath: primary sludge and WAS to thickener, digester (yielding biogas), dewatering and cake off-site, with filtrate returning to the head of works. Every box below is drawn in full in the sections that follow.

Arrivals — by sewer and by tanker

Most of the load arrives by gravity sewer, but a works also serves everything the sewer network does not reach: conservancy tanks and septic systems whose contents arrive by vacuum tanker and are discharged at the headworks. The conservancy tank is the works’ off-site outpost — a sealed holding tank whose only treatment is quiet settling between collections. Its condition (and its emptying cadence) decides how septic, how strong, and how gritty the tankered load is when it hits the screens.

Conservancy tank, x-ray view. A sealed RCC holding tank in service: inlet, overflow and valved outlet, waterproofing strip between tank and base. What settles here between tanker visits arrives at the works as concentrated septage — the strongest, most variable feed the headworks sees.

How a septic tank actually works

The septic tank is the conservancy's discharging cousin, and it is a working treatment unit rather than a store. It is a flow-through vessel: every flush that enters pushes an equal volume out the far end, so the tank holds each day's flow for one to three days and no longer. What it does with those days is physics first, biology second.

The physics is self-organisation into three layers. Fats and greases float to a scum crust; settleable solids fall to a sludge blanket; between them sits a comparatively clear liquor zone. The inlet tee kills the incoming jet and delivers flow below the scum so nothing short-circuits across the surface; the outlet tee draws only from the clear middle, leaving both blankets behind. On that geometry alone the tank removes half to two-thirds of the suspended solids and a third of the BOD.

The biology happens in the blanket the physics builds. The settled sludge digests itself anaerobically — the same hydrolysis, fermentation and methanogenesis chain as the digester of §9, run cold and slow — destroying perhaps half of its own volume as gas. The bubbling lifts flecks of sludge that resettle, the urea arriving with the flow hydrolyses to ammonium, and sulphate reduction supplies the signature odour:

The septic chain — cold, slow, unmanaged digestion

\ce{CO(NH2)2 + 3H2O -> 2NH4+ + HCO3- + OH-}

\ce{$\text{settled organics}$ -> $\text{VFAs}$ -> CH4 + CO2}

\ce{SO4^2- + $\text{organics}$ -> HS- + H2S ^}

Urea to ammonium (why septic effluent is ammonium-rich), the digestion chain shrinking the blanket, and the sulphide that announces the process. Nothing here is aerobic: the tank ships its effluent OUT still anaerobic, still strong, still pathogen-laden.

The septic tank, working and failing. Inlet tee below the scum, three self-organised layers, outlet tee drawing from the clear middle, effluent to the soakaway where soil biofilm runs the aerobic half — and the failure inset: blanket too high, solids over the outlet, soakaway blinded. The depth gauge at 30–40% is the desludge trigger.

That last point is the part most owners never hear: a septic tank does not make clean water. Its effluent is anaerobic, ammonium-rich and carries roughly half the original load; the actual treatment is the soakaway or drain field downstream, where the soil percolation bed runs the aerobic half of the job — an unpowered trickling filter of soil biofilm that oxidises the organics and ammonia and adsorbs what it cannot burn. The tank's real product is effluent clear enough not to clog that soil.

Which is also its fatal failure mode. The blanket and crust grow until they crowd the clear zone; solids then carry over the outlet and blind the soakaway, and a blinded soakaway is a rebuilt soakaway — the one component that does not recover with a pump-out. Hence the sludge-judge cadence: desludge at 30–40% blanket depth, before the geometry fails, not after the garden smells. Where there is no soakaway at all, the same vessel built sealed is the conservancy tank above, and the tanker to this works replaces the drain field.

Preliminary treatment — protecting everything downstream

Nothing biological happens here; preliminary treatment exists so that everything downstream can. Screens take the rags and plastics that would wrap a pump shaft; the grit chamber slows the flow until sand drops out before it can scour impellers and fill digesters; the oil-and-grease separator floats off what would otherwise coat diffusers and feed filamentous foam; and the equalisation tank flattens the morning peak into a feed the biology can ride. Each unit is cheap insurance against a specific, expensive downstream failure.

Screening / headworks. Coarse (20–50 mm) then fine (6–10 mm) bar screens with a raked monorail; screenings are washed, compacted and skipped. Everything the screens miss becomes somebody else’s blockage downstream.

Grit chamber. Velocity drops to 0.2–0.3 m/s so sand and grit settle into the hopper while organics stay in suspension — protecting pumps and reactors from abrasion and keeping inert solids out of the digesters.

Oil / grease / scum separator. Three zones — gravity separation, coalescing media, clarified water — with skimmer arms taking the float and a sump taking the sinkers. Kitchen and industrial fat loads that defeat it reappear later as aeration-basin foam and settling trouble.

Equalisation tank. A mixed buffer (typically 2–8 h) that flattens hydraulic and organic shocks into a steady feed — the difference between a plant sized for the average day and one sized for its worst hour.

Primary settling — the cheapest treatment on site

An hour or two of quiet buys the removal of roughly half the suspended solids and a third of the BOD, at almost no operating cost: gravity does the work, a slow scraper collects the result. Primary sludge is the energy-rich half of the digester’s diet, so this tank is where the sludge line truly begins.