How to use greywater

Types of greywater

• Fresh greywater is what you collect while waiting for the hot-water tap to warm up in the bathroom or kitchen. It also includes water drained after cooking pasta or vegetables. This water is clean enough to be used anywhere in your garden, including on fruit and vegetable crops, and doesn’t have any deleterious impact on soil health.
• Soapy greywater is water saved from washing in the bathroom and laundry. It contains soap, detergent and other substances, and must be applied to the garden with care because it can affect soil life. Don’t put on top of the soil. Channel it underground through pipes or other means, to reduce any risk of children and pets being exposed to harmful microbes. Restrict the use of soapy greywater to lawns and ornamental plants, trees and shrubs. While you should use only fresh greywater on vegie and herb gardens, soapy greywater can be used on fruit and nut trees if it is fed directly under the soil’s surface and doesn’t come into contact with the edible parts of the plant.
• Dark greywater is water from the dishwasher and washing up in the kitchen sink. It can contain significant amounts of harmful bacteria, and needs to go through a professional treatment system before being used in the garden. Check that your state government and local council permit these systems where you live, and what regulations exist.

Dos and don’ts

• Never store untreated water for more than 24 hours, as disease-causing microbes, which may be in negligible amounts in newly generated greywater, can multiply to much greater and riskier amounts over time.
• Don’t allow greywater to pool on the soil’s surface, particularly where children or pets might play, and don’t use it within 1m of an in-ground pool, in-ground potable water tank or property boundary.
• Soapy greywater can have a detrimental effect on soil salinity, pH and phosphorus levels. Symptoms include discoloured growth in plants, the yellowing of new foliage, the scorching of leaf margins and growing tips, and the collapse of the structure of the soil, especially clay. To avoid potential problems, spread across the largest possible area of your garden via your chosen method of underground channelling. And look for the NP logo when choosing laundry detergents, as this indicates very low or no phosphorus.
• To safeguard good soil health, regularly add organic matter such as compost or worm castings to areas where greywater is used repeatedly; use the soil additive Biochar to absorb nutrients and neutralise any deleterious effects of soapy greywater; or add iron sulfate or wettable sulfur to treat alkaline soil (test the soil pH to determine how much you need).
• Know that some plants cope with the use of soapy greywater better than others. Plants that prefer alkaline soils or tolerate salty soils adapt more readily than acid-loving and phosphorus-sensitive plants.