Water use, sustainability, and efficiency by choosing quality systems and materials, and providing environmentally friendly solutions.

Greywater systems

A greywater system diverts waste water either to an irrigation or a treatment and recycling system.

On this page:

  • how a greywater system works
  • treatment of greywater
  • designing a greywater system
  • proprietary greywater systems

The key consideration is the health and safety of property users. Greywater systems used for irrigation typically comprise a surge tank and a method of discharge to an irrigation system. Greywater systems used for toilet flushing should have a treatment system.

Greywater systems must comply with Building Code requirements.

How a greywater system works

Greywater is water from basins, baths and showers that is piped to a surge tank. The greywater is held briefly in the tank before being discharged to an irrigation or treatment system.

The greywater can be diverted either by gravity or by using a pump.

The surge tank can be any type of container that is suitable for holding (but not storing) the initial surge of water. The surge tank must be emptied completely each time greywater is dispersed to the irrigation or treatment system – greywater must not sit for extended periods of time in the tank.

A gravity system can only be used when there is sufficient fall from the laundry/bathroom drain to the surge tank.

The surge tank should:

  • be vented
  • have a trapped overflow
  • discharge directly into the sewer or to an on-site discharge
  • be sealed
  • be vermin proof.
Diversion of water from washing machine or tub for gravity fed irrigation

A three-way valve manually diverts water from the normal drainage system to the surge tank. The machine discharge pipe must not be more than 300 mm above the top of the machine to avoid overloading the pump, and it must discharge into a 40 mm open pipe to avoid the possibility of water being siphoned from the machine.

Diversion of laundry and bathroom greywater for pumped irrigation

This can only be done if there is sufficient distance between the floor level and the outside ground level to allow a gravity feed to the surge tank. It entails a valve on the appropriate waste pipes to divert the wastewater to the surge tank. The pipes may be individual or combined wastes from the laundry and bathroom (but not from the kitchen). Water is then pumped to the irrigated area.

A pumped system, using a simple submersible pump and float switch, must be used where there is insufficient fall. If necessary, the surge tank may be partially or wholly below ground level.

Treatment of greywater

Treatment of greywater may include:

  • filtering
  • settlement of solids
  • flotation and separation of lighter solids
  • anaerobic or aerobic digestion
  • chemical or UV disinfection.

Greywater used for irrigation should be filtered as it still contains high levels of solids and is otherwise likely to clog the irrigation system.

Simple type of greywater filter

Greywater must be filtered to avoid clogging the system. In a simple filtration device such as this, greywater is discharged into a tank containing the filter material that consists of a layer of bark over a filter-cloth and a sand layer. The water flows continuously through the filter and directly to the irrigation system.

Greywater filters will need to be replaced from time to time, and the solids that settle on top of the greywater must be removed regularly.

Greywater should only be used for toilet flushing if it has been treated to reduce harmful bacteria to an acceptable level.

Designing a greywater system

Consider the:

  • personal habits of the users i.e. what they put in the system
  • quantity of wastewater output
  • size of the site
  • soil conditions of the site
  • type of recycling usage required i.e. whether it is for irrigation only, or for re-use within the home.

Greywater is often reused to reduce the pressure on an on-site treatment system. It does not allow a reduction in on-site treatment capacity, however, because good system design will still allow greywater to be diverted to the treatment system when a tank is full or the garden is waterlogged.

Installations that are designed in accordance with NZBC G13/AS1 Sanitary plumbing are suited to a greywater installation because:

  • greywater and blackwater systems are separate until they are outside the building
  • greywater intended for recycling can be directed to a single gully trap where it can easily be diverted for re-use
  • other wastewater, such as kitchen wastewater, can be directed to a separate gully.

Installations designed in accordance with AS/NZS 3500.2 Plumbing and drainage may not be suitable for a greywater installation because:

  • greywater and blackwater are not necessarily separated, i.e. they may be both connected to the same drain
  • all wastewater may be connected directly to the outfall drainage, i.e. there may be no gully trap.

Under AS/NZS 3500, a purpose-designed plumbing installation must be specified for a greywater recycling system.

Proprietary greywater systems

Commercially manufactured systems that treat greywater to a standard for toilet flushing and/or irrigation are available in New Zealand.

Commercially manufactured units suitable for irrigation typically comprise:

  • a plastic gully with a grating
  • a submersible pump that automatically pumps the greywater to the irrigation system
  • either a manual or remote electrically operated over-ride switch that diverts all the greywater to the sewer if necessary
  • a partially self-cleaning filter.
Treatment of greywater in proprietary system for use in toilet cisterns and irrigation

Greywater used for toilet flushing should be treated to reduce harmful bacteria. Commercially manufactured systems which do this are available.


Updated: 05 November 2018