Settled sewerage (small diameter)
Settled sewerage, also called small diameter or small-bore sewerage is designed to prevent solids in wastewater from entering a communal small bore sewer network. An important condition for the functioning of these sewer networks is that a minimum average of 25 litres per person per day enters the system. First wastewater settles in a small interceptor tank. Later, wastewater is conveyed via small (50 200 mm) diameter sewers of PVC or other durable material. Pipes are laid at various gradients from 0% to 10%. Inspection manholes are limited to minimise unauthorised opening and disposal into the system. Costs can be reduced if a group of households shares one interceptor tank. Although settled sewerage is mainly used to transport wastewater, small diameter sewers are also appropriate to transport urine.
| Advantages | Disadvantages |
|---|---|
| Less dependent on active user involvement. All kind of wastewater can be transported. |
Institutional operation and maintenance required. Interceptor tanks need to be desludged periodically. |
Costs
- Investment per household US$ 150 500 (Honduras, 1990).
- Investment per person US$ 35 85 (North East Brazil).
- Investment 20%-50% less than conventional sewerage in rural areas.
- Where septic tanks already exist, the cost reduction can be 40%70% (USA).
Applying conditions
- The system can be appropriate in high- and low-density areas.
- In areas where elevation differences do not permit gravity flow, pump stations are required.
- The system is appropriate for areas where septic tanks already exist, but effluent is causing public health or environmental risks.
- Understanding of the system hydraulics is required.
- The system needs to be flushed periodically to avoid blockages.
