Drainage below ground

5.3.9Septic tanks and cesspools

Septic tanks and cesspools shall be correctly installed and be suitable for their intended use. Issues to be taken into account include:

  1. capacity
  2. access and ventilation
  3. permeability of septic tanks and cesspools
  4. connections to septic tanks and cesspools.

A septic tank is a form of treatment plant and requires a suitable outfall for treated effluent discharge, which is agreed with the relevant authority.

A cesspool is a tank which stores effluent and has to be emptied periodically.


The capacity of the septic tank should be based on the number of people it will serve, using the formula: C = 180P + 2000

C = Capacity of tank in litres. Minimum 2700L.

P = Design population/potential occupancy. Minimum four occupants.

Cesspools are required to be at least 18m³ capacity. A 45-day holding capacity calculated at 150 litres/head/day should be provided.

Access and ventilation

Septic tanks and cesspools should:

  • be covered and ventilated
  • be provided with access points for inspection, emptying, de-sludging and cleaning
  • have the access points with lockable covers and no dimension less than 600mm.

The inlet and outlet of a septic tank should be provided with access for inspection.The inlet of a cesspool should be provided with access for inspection. Cesspools should have no openings except the inlet, the vent and the inspection access.

Permeability of septic tanks and cesspools

Septic tanks and cesspools should be impermeable to their contents and to subsoil water. They should be constructed of brickwork, concrete, glass reinforced concrete, glass reinforced plastics or steel.

Brickwork should be of engineering bricks, laid in cement mortar at least 220mm thick. In-situ concrete should be at least 150mm thick.

Connections to septic tanks and cesspools

The entry flow velocity should be restricted to reduce disturbance in the tank. Where the drain into the septic tank is less than 150mm in diameter; it should have a gradient no steeper than 1:50 for at least 12m.

Rodding and cleaning facilities should be provided at the connection with the tank.