Sewage Treatment & Disposal

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.

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

• 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.

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 $L=\frac{H{v}_h}{v_s}$, 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.

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.

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.

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

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.

Methods of disposal

1. Disposal by dilution — discharge of (treated) sewage into rivers, lakes or the sea, relying on dilution and natural self-purification.
2. Disposal on land — application of sewage or effluent on land for irrigation (sewage farming) or land treatment.
3. Reuse of treated wastewater.

The degree of treatment required depends on the discharge standards and the assimilative capacity of the receiving environment.

Disposal by dilution

Favourable conditions

• Sewage is fresh or treated, free of floating and settleable solids.
• The receiving water body has large flow and high dissolved oxygen, with swift currents that mix the effluent and prevent deposits.
• Water downstream is not used for drinking without treatment or for bathing close to the outfall.
• The outfall is submerged and located so that mixing is rapid.

Dilution factor

A classical guideline (Royal Commission on Sewage Disposal, UK):

In modern practice, effluent discharge standards must be met regardless of dilution.

Self-purification of natural streams

A polluted stream gradually purifies itself downstream by natural processes:

Factors: dilution, current velocity, temperature (higher temperature — faster decomposition but lower DO saturation), sunlight, depth, turbulence.

Zones of pollution

Oxygen sag curve

Downstream of an outfall, two processes act simultaneously:

• Deoxygenation — oxygen consumed by BOD exertion (rate $K_D$).
• Reaeration — oxygen absorbed from the atmosphere, proportional to the oxygen deficit (rate $K_R$).

The DO profile dips to a minimum (critical point) and then recovers — the oxygen sag curve.

• Reaeration coefficient increases with velocity and turbulence and decreases with depth; e.g. O'Connor–Dobbins: $k_R=\frac{3.9v^{0.5}}{H^{1.5}}$ (base $e$, per day, $v$ in m/s, $H$ in m, at 20 °C).
• Temperature correction: $K_T=K_{20}{\theta }^{T-20}$ (θ ≈ 1.047 for deoxygenation, about 1.024 for reaeration).
• If the minimum DO falls below the required level (commonly 4–5 mg/L for fish), more treatment or a better outfall location is needed.