Re-using greywater

With the right technology, water used to wash yourself and your clothes can be re-used on the garden or for flushing toilets.

By re-using greywater instead of sending it down the drain, you can save water, reduce wastewater charges and cut down on demand for water supplies in your area.

However, it’s important that any system you use for collecting and re-using greywater is properly installed and maintained. Greywater also needs to be kept away from human contact as there are possible detrimental effects on health (see ‘Is greywater safe to re-use?’ below).

While you can have a greywater reuse system that can comply with the building code, you’ll need to check with your local council before installing a system to collect and re-use greywater.

What is greywater?

All household wastewater from kitchen sinks, dishwashers, laundry tubs, washing machines, showers, baths and basins is called greywater. A building consent is required to collect and re-use greywater from baths, showers and washing machines on your garden or for toilet flushing.

Greywater cannot be used for cooking, bathing, brushing teeth, swimming or drinking.

The wastewater from your toilet or bidet is known as blackwater. You can’t re-use it. It has to go into the mains sewage system or, if you have no connection to a mains sewage system, into an on-site sewage system. Kitchen wastewater is also unsuitable for re-use.

Is greywater safe to re-use?

Yes, if a system is properly installed and maintained.  Expert advice is recommended. It is possible that greywater can contain faecal matter and microbes which are harmful to human health. Possible detrimental health effects can come about if drinking water becomes contaminated with greywater or there is direct contact with collected greywater that has become septic.

If you do want to have a greywater recycling system at home:

• it needs to be properly installed and maintained
• the greywater needs to be kept away from direct human contact if you’re using it in the garden
• It should be discharged under the soil (not on top) and it should not be discharged in areas where food plants grow
• untreated greywater should be used within 24 hours. Your system should be set up so that any greywater not used in that time goes to the sewer.

It’s vital that no-one can unwittingly drink from a greywater storage tank. Put locks on taps and put up signage.

How do you collect and re-use greywater?

There is a range of options for collecting and using greywater, depending on the source of the greywater and where you intend to use it.

Using greywater on your garden

In general, a garden greywater system will divert water from your washing machine, shower, bath or basin so that solids such as lint and fats are filtered out. Care needs to be taken when cleaning and maintaining systems (e.g. avoiding contact with solids that can be composted or disposed of).  After filtering, the water then flows to a storage tank or directly through an irrigation system to your garden.

With some simple systems, the greywater flows directly to a storage tank with a filter inside. These systems require a high level of maintenance requirement (e.g. emptying the filter every time the washing machine runs).

Whatever type of system you use:

• the greywater should be discharged below ground, not directly onto the surface of the soil to avoid the risk of people being exposed to bacteria in the greywater. There’s also a risk of the greywater pooling on the ground
• it’s worth having a switch to allow you to bypass the greywater system and have your greywater go straight into the sewer. This is handy if you’re putting something down the drain that you wouldn’t want in the garden.

Flushing the toilet

If you want to re-use your greywater, you’ll need a plumber to install a greywater recycling system that connects to your toilet. It may be difficult to fit one of these systems to an existing home with a concrete floor. Your system will need to comply with local building consent requirements. Refer to the references for details.

Looking after a greywater system

If you install a greywater system:

• make sure faecal matter - for example, from children’s baths or from washing nappies - is diverted to the sewer or on-site sewage system
• use appropriate soaps and detergents - avoid washing powders that whiten or have enzymes, and avoid detergents or cleaners containing boron
• don’t use too much greywater on your garden - if water ponds, harmful microbes can multiply, creating a potential health hazard
• you may want to divert the first flush of water from your washing machine into the sewer to reduce the amount of chemicals you are putting on your garden
• take advise about planting - some plants do not thrive in alkaline conditions and greywater tends to be alkaline. You may need to change plants or avoid watering such plants with greywater.

Testing by Environmental Science and Research (ESR) on greywater systems in New Zealand found the presence of bacteria which can harm your health, even in treated greywater. Water from kitchen sinks was found to be unsafe for re-use because of the risk of contamination from organic matter such as bacteria from meat. Therefore it is recommended that kitchen greywater be discharged to a sewer or on-site sewage system.  ESR findings also suggest that bathroom and laundry wastewater can contain levels of bacteria similar to that of kitchen wastewater. So it’s important for your health to maintain the system to the manufacturers’ recommended guidelines.

What are the legal issues?

Before you install a greywater system, contact your local and/or regional council for advice. You will require a building consent for any plumbing work and possibly a resource consent for any discharges of greywater.

Some local authorities restrict the re-use of greywater, particularly for garden irrigation in some urban locations. Under the Resource Management, Health and Building Acts, you’ll be responsible for any greywater that runs on to neighbouring properties or into waterways.

More information

From Smarter Homes

• Reducing water flow
• Collecting and using rainwater
• Managing stormwater
• Outdoor water use
• Onsite sewage systems

From other sites

The Kapiti Coast District Council has a guide about the safety of re-using greywater. It includes information on a do-it-yourself system to re-use washing machine water. You can download it here Greywater and your health (PDF, 123KB).

Technical References

Building Regulations and Building Code clauses:

G13 Foul Water, G13/VM4, On-site disposal AS/NZS 1547:2000

G12 Water supplies, G12/VM1, Greywater plumbing to toilet, AS/NZ 3500.1: 2003, Section 9