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Water use, sustainability, and efficiency by choosing quality systems and materials, and providing environmentally friendly solutions.
Greywater is wastewater from baths, showers and hand basins (and in some cases, from laundries) that is reused for irrigation or, with appropriate treatment, toilet flushing.
On this page:
- Code requirements
- Safety considerations
- Water quality
AS/NZS 1547:2012 On-site domestic-wastewater management defines greywater as the domestic wastes from a bath, shower, basin, laundry and kitchen, but excluding toilet and urinal wastes. However, some people prefer not to reuse wastewater from clothes washing machines because of the enzymes, detergents and other chemicals in it. Some systems do not reuse kitchen wastewater because it contains fats and other organic matter (especially if there is a food disposal unit in the sink). Kitchen wastewater also has the potential to carry contamination from organisms such as Campylobacter that come from preparing raw meat for cooking.
It can be recycled for use in garden irrigation and, if treated, for toilet flushing. Recycling greywater:
- reduces the load on sewerage systems including on-site treatment systems
- provides a garden water supply, reducing demand on other sources of water.
The Kapiti Coast District Council has reported that with residential water use, 68% (160 litres per person per day) is greywater that could be reused. There is clearly huge potential here to take pressure off the reticulated water supply network.
While recycling greywater has benefits, maintenance in particular, cleaning or replacing filters is required to keep these recycling systems operating properly. Specific maintenance tasks should be pointed out to clients considering this option, without discouraging them, before they make a final decision.
There are no regulations or national guidelines that specifically and comprehensively cover greywater reuse from end to end. AS/NZS 3500.2.2021 Plumbing and drainage Part 2: Sanitary plumbing and drainage (section 7) specifies requirements for the installation of greywater plumbing and drainage. AS/NZS 1547:2012 On-site domestic wastewater management also applies.
Some local authorities in New Zealand encourage greywater recycling, but other authorities do not. The local authority should be contacted before you begin to design a system.
The sanitary plumbing connected to an on-site wastewater treatment system within the house must comply with the requirements of NZBC clause G13 Foul water.
Greywater recycling must be designed and installed with care as it is potentially unsafe in some situations. These systems must be installed by a certifying drainlayer. Proprietary systems are available in the New Zealand market. To reduce risk, greywater used for garden irrigation should deliver water below the soil surface.
Public health authorities in New Zealand typically do not support the use of greywater.
BRANZ does not recommend greywater use for:
- washing clothes
- garden irrigation by sprinkler
- use on vegetables or salad plants.
Collected rainwater is a preferred option for toilet flushing than greywater. Where greywater is used for toilet flushing it should be filtered, treated to remove odours and disinfected. The water is likely to be cloudy.
Most greywater systems use the water immediately or within 24 hours. Untreated greywater should not be stored for any longer than this.
A greywater system should have a bypass switch so that the greywater can be sent directly into the sewer or on-site blackwater treatment system if there is something in it that house occupants do not want to go into the garden. Water with faecal matter from bathing very small children should be diverted, for example. There should also be provision for overflow to be discharged to a sewer or on-site blackwater treatment system.
Some proprietary systems have a sensor that diverts greywater if there is already enough water in the garden.
One research project (BRANZ Study Report 420) investigated the water quality of greywater from residential and commercial properties. Taken weekly over 6 months, greywater samples were tested for the bacteria Escherichia coli (E. coli) and Pseudomonas aeruginosa (P. aeruginosa). There are no New Zealand guidelines in this area, but the test results were assessed against South Australian guidelines. The hand basin and laundry sample E. coli numbers were consistent with category A reclaimed water in the guidelines. In South Australia, this can be used for residential non-potable uses such as toilet flushing, garden watering and exterior washing, amongst other end uses.
Updated: 18 August 2021