50 shades of greywater
You collect water from your shower or when washing vegies, then tip the water onto your garden. Is that okay, and what are the long-term e ects on the soil and plants? ANGUS STEWART explains the di erent types of greywater, and which plants can handle it
Prolonged drought has seen many of us take steps to use water more wisely. Although summer rain has provided relief in some areas, a lot of the habits we’ve developed to cope with limited access to water for our gardens are good common sense, and should be continued. Water collected while washing vegies or waiting for the shower to warm up, for example, is too good to waste down the drain.
But, as with many things, when using greywater, the devil is in the detail.
Greywater is a very variable substance. Depending on which products you use around the home, it may add unwanted chemicals to your soil and plants. In some instances, it contains disease-causing microbes. Here are some guidelines to follow when using greywater.
FRESH IS BEST
There are two main types of greywater, namely fresh and soapy.
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, depending on the products used, and must be applied to the garden with care because it can affect soil life.
The golden rule is to 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.
Another type of greywater, including water from the dishwasher and washing up in the kitchen sink, is dark greywater.
This can contain significant amounts of harmful bacteria, and needs to go through a professional treatment system before being used in the garden.
Treatment systems are a whole other subject beyond the scope of this article. If you want to go down that path, check first that your state government and local council permit these systems where you live, and what regulations exist.
HOW TO USE GREYWATER
The easiest and cheapest way to use greywater is to collect it in buckets.
It’s a great way to save fresh greywater from washing and cooking vegetables, and waiting for the shower to warm up. If you’re using buckets to collect soapy water, too, make sure you use separate buckets for each.
Alternatively, you could consider installing a diversion device in your plumbing system that enables you to choose whether the water goes into pipes, or into a small tank connected to a garden irrigation system with pipes under the soil. Dirtier water, such as the first dump from your washing machine, can be directed into the sewerage system. That way, you can use the cleanest water, and any that is more heavily contaminated can be safely treated in the public sewerage system.
There are a few guidelines to observe when using soapy greywater in the garden. Importantly, don’t put soapy water on top of the soil. Instead, 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. Use only fresh greywater on vegie and herb gardens. Soapy greywater can, however, 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.
Soapy greywater can have a detrimental effect on soil salinity, pH and phosphorus levels. Most washing products contain salts, including sodium phosphate, which create alkaline conditions that dissolve stains and suspend dirt, but these substances interact with the soil’s biology, causing symptoms in plants such as discoloured growth, including the yellowing of new foliage.
A build-up of salinity in the soil from the various components of cleaning products, such as sodium, results in the scorching of leaf margins and growing tips, and the collapse of the structure of the soil, especially clay. This can be treated by leaching the soil with copious fresh water, although it may be difficult to do this if you’re short on water. Sandy soils are much more forgiving of soapy greywater because of their chemically inert nature.
To avoid potential problems with your soil and plants, spread soapy greywater across the largest possible area of your garden via your chosen method of underground channelling. Don’t allow it to pool on the 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. Only water your soil with greywater when it really does need it, to avoid it pooling or running off.
To further safeguard good soil health, regularly add plenty of organic matter, such as compost or worm castings, to areas that are showing symptoms. This will improve the soil’s structure, as well as its pH and nutrient balance.
Biochar is an important soil additive for absorbing nutrients and neutralising any deleterious effects of soapy greywater. You can also treat alkaline soil by adding iron sulfate or wettable sulfur – test your soil pH to determine how much you need. And to minimise problems from the outset, look for the NP logo on packaging when choosing laundry detergents, as this indicates very low or no phosphorus.