Using Grey Water in the Garden

Re-using at least some of your wastewater on site fits in with permaculture principles, reducing your impact on the environment, as no resources are being used to process it off site. Many areas of Australia have been in drought for years, with garden water availability either limited or curtailed. In these situations, grey water may be the only way to have a garden.

Course Notes

© Karen Sutherland, Edible Eden Design

Why use grey water?

In many households it's a large and untapped resource, even though its use in gardens is still somewhat new in Australia.

Re-using at least some of your wastewater on site fits in with permaculture principles, reducing your impact on the environment, as no resources are being used to process it off site. Many areas of Australia have been in drought for years, with garden water availability either limited or curtailed. In these situations, grey water may be the only way to have a garden.

Remember that grey water should not be used if there is no need for irrigation normally.

(Note that before considering grey water use in your garden, check with your local council to make sure its use is allowed in your area.)

So can we use this resource to grow produce?

To answer that, we need to understand what grey water is, the basics of safe grey water garden usage and the particular issues that apply to its use on edible plants.

What is grey water?

Grey water is waste water from the laundry and bathroom. The toilet is classified as black water and in Australia kitchen waste water is usually classified as black water. It is advisable not to re-use the water from hand basins and laundry troughs, as it contains many substances not suitable for gardens.

An often over-looked resource is the water saved while warming up the showerwashing fruit and vegetables before eating and vegetable and pasta cooking water. It is possible to save at least 20 litres of this water a day for re-use on pots, herbs and vegetables, and this amount can keep a small vegetable patch alive. The water from rinsing soap from clean dishes is also suitable for use in this way.

Make sure the buckets collecting this are not also used to collect grey water – keep a separate bucket in the house for collecting this water – let's call it second hand water. Keep a couple of buckets for storage at your back door, and a watering can for giving it to your vegetable patch or pots. Using it at the end of each day avoids it becoming stagnant.

If your water needs are large, you may need to consider using your kitchen washing up water sometimes – if you do so, make sure to avoid any oils or fats in the wash – (wipe them off beforehand with a paper towel) and use the least amount of the simplest, gentlest detergent possible. Do not use washing water from a meal containing animal products and let the water cool before putting it on the garden.

Sit a flexible plastic trough in your sink. After you've done your dishes, scoop water out with a plastic jug into 2 buckets, then evenly balanced you can take your waste water out onto the garden. The last bit can be poured out from the flexible trough.

What do we need to know about safe grey water use?

The dangers contained in grey water (for instance it may contain small amounts of faeces, many bacteria and other potentially disease causing organisms), are avoided by following some basic principles:

  1. don't store grey water for more than 24 hours
  2. don't allow pets or children to come into contact with it
  3. make sure to water with it underground or under mulch (never above ground)
  4. wash your hands thoroughly after coming into contact with it (try to use gloves)
  5. don't use grey water if someone in the house has an infectious disease, or you have a new baby in the house
  6. never allow your grey water to run off onto a neighbour's property or into the stormwater system or into a creek or waterway

What are the issues in using grey water on the produce garden?

For the produce garden, it is not safe to use grey water on the edible parts of plants that are going to be cooked. Grey water is therefore best used on trees, shrubs, large herbs or perennials such as Artichokes, where the edible part of the plant is well away from the soil. However you can safely use second-hand water on plants that are to be eaten raw.

What do we need to know about the quality of grey water and its effect on plant and soil health?

It is advisable to test the pH of your grey water (with a simple aquarium tester kit) and ensure it stays between 6.5 and 7.5. A higher pH will lead to too much Phosporous being available in the soil, and may lead to Iron deficiencies, shown by the interveinal yellowing of leaves, with the veins themselves remaining green. (This can be remedied by applying Sulphate of Iron, available from nurseries, at the recommended rate).

In the laundry, use a low salt, low Phosphorous, low pH liquid detergent. Powders have a pH of more than 8.5 and are usually high in salt. The Lanfax Laboratory website has lists of powder and liquid detergents and their ratings in these regards. At the time of printing, they are still re-working their lists for liquid detergents, but a small list from December 2007 will be at the end of the article in the references, along with the web address for their discussions of grey water and gardens. Make sure you refer to the correct chart for either top-loaders or front loaders, depending upon what you have.

If you must use a powder detergent, only use the grey water on lawns.

In the bathroom, use only liquid soaps, as hard soaps will clog up the soil. For other products, according to a recent study by the ATA (Alternative Technology Association), there is no disadvantage for the garden in using 'conventional' bathroom products rather than organic products, or products seen as healthier for the skin. However, using the mildest products, with as little chemicals as possible, is usually best for people's skin, and is better for the environment generally.

How do we ensure good soil health if using grey water on the garden?

Sandy soils will cope far better than clay soils with grey water, so if you have heavy clay soils, use as little soapy water as possible on it.

Add gypsum each year for the calcium it contributes, which helps the soil deal with salinity. Apply at the recommended rate (usually 1 kg per square metre on garden beds, and 50gm per m2 on lawns). Don't add lime or dolomite for calcium as they will both raise the pH of the soil and grey water is already probably doing this.

With all soil types it is important to keep up the levels of micro-organisms, nutrients and organic matter - this will help the soil deal with any potential problems from the grey water. So each year add plenty of compost and good quality mulch, such as composted recycled garden waste sold by councils and nurseries.

Never use grey water on potting mix, as it causes the organic matter in it to breakdown and will clog up its drainage capacity.

What are the other issues involved in grey water use in the garden?

The other main issue is to match the amount of grey water generated to the amount of water required by the garden. In some households, the grey water generated will be more than the garden can use, and it may need to be diverted to the sewer when the garden has had adequate water. Conversely, you may have a large and productive garden, with many fruit trees, where you will have to prioritize the water to various trees as they fruit. We will look at the requirements of individual produce plants later.

How do we get the grey water onto the garden?

There are various devices to access the grey water before it reaches the sewer, and divert it to the garden – visit your local hardware, plumbing or irrigation shop and ask for advice. There must always be a way to re-divert the grey water back to the sewer, for instance during winter, when the water is not needed.

Another issue to consider with grey water use is that you may not have the amount of water you need when you want it. Also, soils have become so dry in drought stricken areas, so that water administered to plants can simply run off. Smaller plants can still survive under these circumstances (although not thrive) but established fruit trees with their greater needs (20 or more litres of water a day when fruiting) may drop fruit or not set it at all.

By fitting slotted flexible plastic agricultural pipes (aggie pipes) around your fruit trees, (as many councils do in street tree planting), you can give them a good deep water when they need it, such as when they are setting fruit (at flowering time) and when approaching harvest time. It is amazing how much you can increase the size of your fruit by watering when the fruit is growing. By watering with the pipes, you can get water down deeply into the root zone – difficult to do otherwise.

The pipes should be fitted 30-50cm deep into the ground around the trees, at a distance of about 30 cm from the trunk. A new tree will need 2 pipes, whereas an older established tree may need 4-6 pipes to receive adequate water. Make sure to leave the pipes sticking out of the ground 50mm or so, so that they don't fill up with mulch too easily. When you are topping up mulch, cover the tops with upturned plastic pots.

Does your home generate enough grey water to use?

In the laundry, top loading washing machines use a lot of water, so their waste water is diluted and quite suitable for garden use. Fill your machine with a 10L bucket, and see how many litres it holds, remembering to add the wash and rinse amounts. Now you can calculate the total grey water generated per day and per week.

The wash water of front loaders is too soapy for most garden use, (except perhaps lawns), but if you can mix it with the rinse water before it goes on the garden, then it would be more suitable, or use the rinse water only. You will need to look at your machine's specifications to find out its water usage.

In the bathroom, do you have a water-saving shower-head? Regular shower heads usually use 15 litres per minute. Water-saving shower-heads use around 7 litres per minute. So if each person has a 3 minute shower, you are generating either 45 or 21 litres per day per person. Now you can calculate the total grey water output per day and per week.

You have enough grey water, now how do you match its production to your climate, garden size and its particular plants?

Hotter drier climates call for more watering and climates with frequent heavy rain may have no need for additional watering.

Your garden may be small and your grey water production large, so you may be looking for plants that tolerate large amounts of water, or you may have a large garden and a small but steady stream of grey water, that you have to share around your garden, so you need to know which plants simply must have the water and which others will survive on less.

Research thoroughly, plan well and keep a watchful eye on your garden and grey water re-use should be very rewarding.

For further reading:

Ludwig, Art; 'Create an Oasis with Greywater'

van Dok, Wendy; 'The water-efficient garden: a guide to sustainable landscaping in Australia' (The home page for Lanfax Laboratories)

From a list from December 2007 on the Lanfaxlab website - some low-salt, low phosphorous, low alkalinity liquid laundry detergents (as the Lanfax Lab website currently has no detergent information)

Enviroclean Liquid Laundry

Earth Choice Laundry Liquid

Earth Choice Wool and Delicates

Ecologic Lavender Laundry Liquid