Last reviewed 16 Sept 2026 · 11 min read
Objectives and levels of treatment
Sewage treatment removes suspended solids, biodegradable organic matter (BOD), pathogens and nutrients so that the effluent can be safely discharged or reused.
| Level | Units | Removes |
|---|---|---|
| Preliminary | Screens, comminutors, grit chambers, skimming tanks | Large floating matter, grit, oil and grease — protects pumps and later units |
| Primary | Primary sedimentation tanks | Settleable organic and inorganic solids — roughly 50–70% of SS and 25–40% of BOD |
| Secondary (biological) | Trickling filters, activated sludge, ponds, UASB with secondary clarifiers | Dissolved and colloidal organic matter — overall BOD removal about 85–95% |
| Tertiary / advanced | Nutrient removal, filtration, activated carbon, disinfection, membranes | Nitrogen, phosphorus, residual SS, pathogens, refractory organics — for sensitive waters or reuse |
A typical flow sheet: screens → grit chamber → primary settling tank → aeration tank or trickling filter → secondary settling tank → disinfection → discharge/reuse, with sludge from settling tanks sent to thickening, digestion and dewatering.
Preliminary treatment
Screens
| Screen | Clear openings (typical) | Purpose |
|---|---|---|
| Coarse (rack) screens | About 50 mm or more | Remove large floating objects |
| Medium screens | About 20–50 mm | General screening at pumping stations and plants |
| Fine screens | Less than about 10 mm (down to a few mm) | Remove smaller solids; reduce load on later units |
- Bars are inclined (commonly about 30–60° to the horizontal) and cleaned manually or mechanically.
- Velocity through screens is kept moderate — high enough to avoid deposition in the channel, low enough not to force material through.
- Comminutors (macerators/shredders) cut solids in the flow instead of removing them.
= velocity through the screen openings, = approach velocity (m/s); in m.
Grit chambers
Remove heavy inorganic particles — sand, gravel, cinders (typically particles of about 0.2 mm and larger, specific gravity about 2.65) — which cause abrasion of pumps and accumulate in digesters.
- Horizontal-flow (velocity-controlled) grit chambers: the horizontal velocity is held near 0.3 m/s at all flows — fast enough to keep lighter organic solids in suspension, slow enough to let grit settle. A proportional flow weir (Sutro weir) or Parshall flume at the outlet maintains constant velocity as flow varies.
- Theoretical length , increased for turbulence and inlet/outlet effects.
- Aerated grit chambers — spiral flow created by air; also remove grease and pre-aerate sewage.
- Detritus tanks — square tanks with mechanical scrapers.
Skimming tanks
Remove oil, grease, soap and floating fats that would form scum and interfere with treatment. Air is blown in to help grease rise; the floating scum is skimmed off. Grease traps serve individual buildings.
Primary sedimentation
- Flocculent (Type II) settling of organic suspended solids.
- Typical design (indicative): detention time about 1.5–2.5 h; surface overflow rate of the order of 25–50 m³/m²/day (at average and peak flows respectively); side water depth about 2.5–3.5 m.
- Rectangular (horizontal flow) or circular (radial flow) tanks with sludge scrapers and scum removal.
- Primary sludge is removed regularly to prevent septic conditions.
- Chemically assisted primary treatment — coagulants improve removal.
Principles of biological treatment
Microorganisms (mainly bacteria) use organic matter as food, converting it to CO₂, water and new cells (biomass). The biomass is then separated by settling.
| Attached growth (fixed film) | Suspended growth |
|---|---|
| Microorganisms grow as a film on media (stones, plastic) | Microorganisms kept in suspension as flocs by mixing and aeration |
| Trickling filter, rotating biological contactor, biofilters | Activated sludge process, oxidation ditch, aerated lagoon, SBR |
Under aerobic conditions, bacteria need oxygen; under anaerobic conditions (UASB, digesters), organics are converted to methane and CO₂.
Trickling filters
A bed of coarse media (crushed stone about 25–75 mm, or plastic media) over which settled sewage is sprinkled by a rotary distributor. A biological slime layer (zoogleal film) grows on the media; as sewage trickles over it, organic matter is adsorbed and oxidised. Air enters through the under-drains by natural draft.
- The film thickens and periodically sloughs off; the sloughed solids (humus) are removed in a secondary settling tank.
- Standard (low-rate) filter — low hydraulic loading (about 1–4 m³/m²/day), no recirculation, deep beds; good BOD removal (about 80–90%) and some nitrification; intermittent dosing.
- High-rate filter — much higher hydraulic loading (about 10–40 m³/m²/day) with recirculation of effluent; smaller area; continuous dosing; BOD removal of a single stage somewhat lower (about 65–85%); two-stage filters for higher efficiency.
= BOD removal efficiency (%) of filter and its secondary clarifier; = BOD load applied (kg/day); = volume of filter media (m³); = recirculation factor:
Operational problems: ponding (clogging of voids by excessive slime or fine media), filter flies (Psychoda) in low-rate filters, odours (anaerobic conditions), ice formation in cold climates. Recirculation and flooding the filter help control flies and ponding.
Rotating biological contactors (RBC)
Large closely spaced plastic discs mounted on a horizontal shaft, about 40% submerged and slowly rotated; biofilm on the discs alternately contacts sewage and air. Compact, low energy; sensitive to shaft/mechanical failures.
Activated sludge process (ASP)
Settled sewage is mixed with return activated sludge in an aeration tank where air (or oxygen) is supplied; microorganisms form flocs that oxidise organic matter. The mixed liquor flows to a secondary clarifier; settled sludge is partly returned to the aeration tank and the excess (waste activated sludge) is removed.
- Hydraulic retention time:
- Volumetric (organic) loading: (kg BOD/m³/day)
- Food-to-microorganism ratio: ( = MLSS or MLVSS)
- Mean cell residence time (sludge age, SRT):
- Sludge volume index: (mL/g)
- Return sludge concentration: (mg/L)
- Return ratio:
Typical ranges (conventional ASP, indicative): MLSS about 1500–3000 mg/L; F/M about 0.2–0.4 per day; SRT about 5–15 days; HRT about 4–8 hours; SVI 50–150 mL/g indicates good settling (above about 200 — bulking); BOD removal about 85–95%.
Modifications
| Process | Feature |
|---|---|
| Conventional (plug flow) | Long narrow tanks; high oxygen demand at inlet |
| Tapered aeration | More air at the inlet, less towards the outlet — matches oxygen demand |
| Step aeration | Sewage fed at several points along the tank — spreads load |
| Complete mix | Uniform conditions throughout — resists shock loads and toxic inflows |
| Contact stabilisation | Short contact tank followed by re-aeration (stabilisation) of return sludge |
| Extended aeration | Long HRT (about 18–36 h), low F/M, long SRT (about 20–30 days) — little excess sludge, well-stabilised; for small communities |
| High-rate aeration | High F/M, short HRT — partial treatment |
| Oxidation ditch (Pasveer ditch) | Extended aeration in an oval channel with cage rotors/brush aerators; simple and robust; widely used for small towns |
Oxygen supply: diffused air systems (fine or coarse bubble diffusers) or mechanical surface aerators.
Operational problems
- Sludge bulking — poor settling due to filamentous organisms (low DO, low F/M, nutrient deficiency, septic sewage) or viscous bulking; sludge is lost with the effluent.
- Rising sludge — denitrification in the secondary clarifier releases nitrogen gas that floats sludge; remedy — faster sludge removal, reduced SRT.
- Foaming/scum — from detergents or filamentous Nocardia.
- Pin floc — very long SRT, small flocs that do not settle.