Wastewater treatment in WESS plants: compartments, chemistry, and diagnosis

A WESS (Water-Efficient Sanitation System) is a packaged, decentralised treatment plant: it takes the full sewage stream of a building or small settlement, treats it on site through a train of compartments, and returns the water for non-potable reuse — typically toilet flushing. Because the water recirculates rather than discharging to a sewer, a WESS behaves differently from a conventional works in three ways that matter for everything in this guide:

First, it is a closed loop. Anything the biology and physical polishing steps cannot remove — dissolved salts above all — accumulates cycle after cycle. A municipal works never sees this; a WESS lives with it, and "reboot" (partial or full replacement of the loop volume with fresh water) is a legitimate, scheduled operation rather than a failure.

Second, it is small. Hydraulic and organic shock loads that a large works would absorb — a weekend of heavy use, a drum of harsh cleaning chemicals, a power outage — can wipe out the microbial community of a plant with a few cubic metres of biological volume. Recovery then needs reseeding, not patience.

Third, it is operated lightly. There is no resident process engineer. The plant must therefore be diagnosable: a structured set of measurements, taken on a site visit with field instruments plus a small laboratory suite, must be enough to say what is going wrong, where in the train, and what to do about it. That structure — compartment taxonomy, measurement bands, diagnosis catalogue, mitigation library — is the WESS protocol, and it is what the rest of this guide teaches.

Every WESS variant is described against the same ten-compartment archetype. A given plant will not have all ten — a small septic-plus-wetland system may only have C1, C3 and C9 — but every real compartment maps onto one archetype, and every measurement band, flag and diagnosis in the protocol is anchored to that mapping. The train runs from raw influent to reuse product:

First principles: oxidation, reduction, and where the energy comes from

A redox reaction is an electron transfer. The species that loses electrons is oxidised; the species that gains them is reduced (the schoolroom mnemonic OIL RIG — Oxidation Is Loss, Reduction Is Gain — is all you need). There is nothing biological in the definition: iron rusting is iron oxidised by oxygen. What biology adds is harnessing. A microbe is an engine that couples one oxidation to one reduction — it strips electrons from a fuel (the electron donor) and delivers them to an electron acceptor, capturing the energy released in between as ATP. Everything the biological compartments do is a version of this one transaction.

In a WESS the dominant fuel is the sewage itself:

The electrons must land somewhere, and the acceptors are not equivalent: each pays a different energy price. Ranked from richest to poorest, they form the acceptor ladder that organises the whole train:

The ordering is thermodynamic, not managerial: no valve enforces it. While oxygen is present every other acceptor waits, because the organisms using oxygen out-compete the rest for the same electrons; when oxygen runs out the community shifts to nitrate, then sulphate, then CO₂. (In the gap below −100 mV, fermenters also work without any external acceptor at all, splitting organics into the volatile fatty acids that feed methanogenesis — the ABR chain of §3.) This self-sorting is why compartment zoning works: the designer engineers the acceptor supply — blow air into C4, exclude it from C5, keep C3 sealed — and the right community assembles itself in each tank.

The redox ladder: one number that places you in the train

The oxidation–reduction potential (ORP) is the field measurement of all of the above. A platinum electrode dipped in the compartment adopts a voltage set by the dominant redox couples in the liquid, read against an Ag/AgCl reference: strongly positive means the water is electron-poor and oxidising (acceptors abundant), strongly negative means electron-rich and reducing (donors queuing for scarce acceptors). It is a summary voltage, not a concentration — but because each regime on the acceptor ladder holds a characteristic window, the one number places the compartment on that ladder. The protocol's compartment windows are simply the ladder mapped onto the train — which is why a single probe dip can tell you a compartment is in the wrong regime before any laboratory result comes back. An anoxic zone reading +150 mV is not "slightly high"; it has become an aerobic zone, and its denitrifiers have stopped working.

Compartments in full: hardware, chemistry, readings and mitigations

Each WESS compartment is treated here as a complete unit: the hardware as built, the chemistry it runs, the values the field kit and the laboratory should return when it is healthy, and the failure modes with their mitigations. The bands repeat the protocol’s operational triggers, so a reading taken at any of these units can be placed immediately. Thresholds are decision aids under calibration rather than validated regulatory limits.

C1 · Conservancy / septic tank

A sealed reinforced-concrete box receives the raw stream and holds it in near-total oxygen absence (ORP −300 to −100 mV). Two processes begin at once. Coarse and settleable solids drop into a sludge blanket, taking roughly half the suspended load out of the flow within a day of quiet residence. In the liquid above, hydrolysis breaks particulate organics into soluble molecules and ammonification converts organic nitrogen to ammonium, which is why NH₄-N rises through this compartment while COD begins to fall.

Urine carries most of the nitrogen, and urea hydrolyses within hours:

Failure here is almost always arithmetic rather than biological: the blanket grows until the tank stops settling, and raw load passes through to compartments never sized for it. The worked case of §8 shows the signature (septic COD of 5 691 mg/L against the 3 000 action level) and the catalogue names it SEPTIC_OVERDUE. The mitigation is M01, a desludge with a standing sludge-judge cadence so the next one is scheduled rather than discovered. Composite sampling (M11) belongs here whenever a reading will carry weight, since a grab sample near the inlet of a stratified tank can say almost anything. Persistent grit and rag loads point back to what the community puts down the drain, which is M13 territory.

C3 · Anaerobic reactor (ABR / UASB)

This compartment does the heavy COD removal at zero aeration cost. Influent passes up (UASB) or over-and-under (ABR baffles) through a retained anaerobic biomass, and the three-stage chain of §3 runs to completion: hydrolysis, fermentation to volatile fatty acids, then methanogenesis at ORP −400 to −200 mV. Most of the methane leaves by the acetate route, the rest from hydrogen:

Two failure modes dominate. ABR_WASHOUT (TSS above 500 leaving the reactor) follows hydraulic surging or a collapsed blanket; the response is M06, an integrity check with chemically-enhanced backwash where membranes follow, together with M01 to restore the solids inventory. Souring is the chemical failure: an organic shock makes acid faster than the methanogens clear it, pH slides below 6.5, and the methanogens quit, which makes more acid. That loop does not self-correct. Breaking it needs load relief, alkalinity, and in the worst case a reboot with live-culture reseed (site-walk step 10). The causes worth ruling out on the same visit are toxic loads and chlorinated water routed back through the biology, both M13 and plumbing questions rather than dosing ones.

C4 · Aerobic reactor

Air changes everything. At DO of 2–4 mg/L and ORP of +50 to +200 mV, heterotrophs burn the residual soluble COD and the nitrifiers run the first leg of the nitrogen relay. Both reactions consume something besides oxygen, and both costs appear in the readings:

AERATION_FAIL is the acute failure: a blower or diffuser outage stops nitrification and aerobic COD removal within hours and the whole plant slides down the redox ladder. The mitigation is M02, an aeration audit and rebalance of setpoint and blower duty. The chronic failures are quieter. Complete ammonia removal with acidic effluent is unbuffered nitrification; the acid of the first reaction has exhausted the water’s alkalinity, and below pH 6.5 the nitrifiers throttle themselves, trading an acid problem for an ammonia one. Over-aeration wastes power, shears floc, and poisons C5 through the recycle. Filamentous bulking grows out of low DO, low F:M or surfactant loading and is read at the settler but caused here; the settling root-cause tree of site-walk step 4 assigns it, and M10, M13 and M14 carry the responses that are not aeration.

C5 · Anoxic reactor

Between −100 and 0 mV, with oxygen excluded, facultative heterotrophs switch to nitrate respiration and run the relay’s second leg. The unit needs three things in the same water at the same time: nitrate (delivered by the recirculation), soluble carbon (delivered by the influent), and the absence of dissolved oxygen (protected by the mixer doing what a diffuser must not).

DENIT_FAIL announces itself as high effluent nitrate under low effluent ammonia: the first leg worked and the second never ran. Three causes cover the field cases. Under-recirculation starves the zone of nitrate, and M03 raises the recycle to 3–4 times forward flow. Carbon starvation follows over-aerated or over-settled influent, and M04 doses an external carbon source. Oxygen poisoning rides the recycle from an over-aerated C4, which makes the cure M02 upstream rather than anything in this tank. The worked case is the standing reminder that plumbing outranks chemistry here: EP had carbon in abundance (septic COD 5 691) and still put 70.9 mg/L of nitrate in its effluent, because the reactants never met.

C6 · Secondary settling tank

No reaction is intended here; the unit is physics applied to a biological product. Bacteria that grew at the right sludge age flocculate into dense aggregates bridged by extracellular polymer, and an hour of quiet lets them fall while clarified water leaves over the weir. The settled biomass returns to C4 to work again, which makes this tank half of the activated-sludge loop rather than a mere polisher.

One unintended reaction matters. Sludge that sits too long in the hopper goes anoxic, denitrifies whatever nitrate it carried in, and the nitrogen bubbles float whole rafts of solids to the surface. Rising sludge in an otherwise healthy plant is that mechanism, and the fix is faster sludge return rather than anything chemical.

The diagnostic pair SVI/DSVI separates the two failure families. SVI of 210 with DSVI of 95 settles fine once diluted, so the tank simply holds too much sludge: an inventory finding, M01 territory, fixed by wasting cadence. SVI and DSVI both high is bulking proper, and the step-4 root-cause tree assigns it to low DO, low F:M or surfactants, which routes the mitigation to M02, M13 or a reboot-and-reseed after community failure. Solids that escape this tank do double damage, first as effluent TSS, then as the particulate shielding that lets E. coli survive a correct chlorine residual at C9.

C7 · Membrane filtration (UF / MBR)

The membrane is the train’s one absolute barrier. An ultrafiltration wall carries pores of roughly 0.01–0.1 µm, and rejection at that scale is geometric: water and dissolved species pass, solids and bacteria cannot, the smallest viruses are marginal. Nothing biological or chemical is asked of the unit. In a WESS it either polishes the settler’s effluent or, in the MBR form drawn above, replaces the settler entirely, in which case the SVI questions of C6 stop mattering and membrane care takes their place.

The physics that runs the unit fits in one law, a resistance in series: