Settled sewerage (small diameter)

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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.

Settled sewerage (small diameter), in Egypt (for credits, click the picture)
Advantages Disadvantages
Less dependent on active user involvement.

All kind of wastewater can be transported.
Little water needed to transport excreta through the small diameter pipe.
Sewers can be laid at flat gradients.
Excreta `out of sight'.

Institutional operation and maintenance required.

Interceptor tanks need to be desludged periodically.
Potential risk of blockages due to illegal connections that by-pass the interceptor tank.
High water consumption for excreta removal.


  • 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.

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