Last reviewed 16 Sept 2026 · 9 min read
Methods of disposal
- Disposal by dilution — discharge of (treated) sewage into rivers, lakes or the sea, relying on dilution and natural self-purification.
- Disposal on land — application of sewage or effluent on land for irrigation (sewage farming) or land treatment.
- 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):
| Dilution factor | Treatment needed |
|---|---|
| Above 500 | No treatment necessary (raw sewage may be discharged) |
| 300 – 500 | Primary treatment (plain sedimentation) |
| 150 – 300 | Screening, sedimentation and chemical precipitation |
| Below 150 | Complete treatment |
In modern practice, effluent discharge standards must be met regardless of dilution.
Applies to BOD, DO, temperature and other conservative (fully mixed) parameters; = sewage, = river.
Self-purification of natural streams
A polluted stream gradually purifies itself downstream by natural processes:
| Type | Processes |
|---|---|
| Physical | Dilution and dispersion; sedimentation of suspended solids; reaeration from the atmosphere; sunlight (UV kills bacteria; supports algae photosynthesis); temperature effects |
| Chemical | Oxidation of organic matter (aerobic); reduction under anaerobic conditions |
| Biological | Bacteria decompose organics; protozoa feed on bacteria; algae supply oxygen; predation and die-off of pathogens |
Factors: dilution, current velocity, temperature (higher temperature — faster decomposition but lower DO saturation), sunlight, depth, turbulence.
Zones of pollution
| Zone | Characteristics |
|---|---|
| Zone of degradation | Just below the outfall; water turbid, DO begins to fall; fungi appear; algae die |
| Zone of active decomposition | DO falls to its minimum (may become zero — anaerobic); H₂S and CH₄ odours; black colour; no fish; anaerobic organisms |
| Zone of recovery | Reaeration exceeds deoxygenation; DO rises; nitrates and algae reappear; fish return |
| Zone of clear water | Natural conditions restored; DO near saturation; normal aquatic life (pathogens may still survive) |
Oxygen sag curve
Downstream of an outfall, two processes act simultaneously:
- Deoxygenation — oxygen consumed by BOD exertion (rate ).
- Reaeration — oxygen absorbed from the atmosphere, proportional to the oxygen deficit (rate ).
The DO profile dips to a minimum (critical point) and then recovers — the oxygen sag curve.
Deficit at time :
Critical time:
Critical (maximum) deficit:
Self-purification constant:
= ultimate BOD of the mixture; = initial deficit = saturation DO − DO of mixture. Distance to critical point stream velocity.
- Reaeration coefficient increases with velocity and turbulence and decreases with depth; e.g. O'Connor–Dobbins: (base , per day, in m/s, in m, at 20 °C).
- Temperature correction: (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.