Recycling Grey Water

Grey water is water that has already been used for some purpose, such as washing, cooking, and cleaning, but not for sewage disposal.

Sewage water, or black water, can be treated and made safe, but it is not usually practicable on a routine household basis. It's much better to have a composting toilet.

In general, grey water is fine for gardens unless it has been previously treated with water softeners, in which case the soil tends to suffer from salinization.

Although they are at the experimental stage, deep soil/leaf mould beds are used in greenhouses; the beds appear promising.

The beds work by diverting water from baths, showers, washing machines, and sinks through a coarse filter (this removes debris such as hairs, buttons, and vegetable peelings) into the beds. Although the water is delivered in large, irregular doses, the soil absorbs it well; and the soil fauna and flora flourish, releasing nutrients.

Running the warm water into the greenhouse has two advantages: (a) you can make better use of the incidental heat, and (b) the system has the potential to work all year round.

Similar systems outside in the garden work well if the ground is not frozen—water is channelled through pipes with intermittent holes or slots. The pipes need occasional maintenance, but are generally kept clear by soil organisms. Alternatively, you can modify your plumbing system to drain water from the sink, bath, shower, or washing machine into a storage tank in the garden, ready for its second use.

If you do decide to divert grey water to your garden, ensure you use environment-friendly washing products, and that you alternate grey water with either mains or another clean water supply

The chief challenge in re-using household grey water comes from the fats, salts, and other residues it might contain; these produce a combination of minor physical and biological problems. You can either ignore the problem and use grey water only for irrigating permanent plantings, or obtain water fit for the vegetable plot.

The simplest approach is to run grey water straight on to permanent plantings when needed. When planning to do this, though, allow for surges and to install several "switch" points. Surges can be dealt with by first running the water into a tank. The first and most important switch point comes before the water enters this tank. In case of hothouse contamination of water, find a way to run the water straight into the normal sewage system.

There should (often, by law) be a valve to prevent grey water flowing back into the household supply.

If you exclude kitchen grey water from the system, the worst problem, fat, virtually disappears. It is, however, possible to install a straw filter in the tank to remove food wastes (including fat). This must be regularly replaced, and the old one dumped on the compost heap.

The outflow pipe from the surge tank needs a gate valve for control. It can be necessary to introduce another switch point here, to run excess water into the main sewer.

Grey water flush toilets

A neat way of reducing the amount of clean water used in toilet flushing is to collect the grey water released after hand washing in the toilet cistern. A simple plumbing modification is all that is needed.

Reed bed system

If you wish to conserve large amounts of grey water, it is worth installing a simple reed bed system. In this case, instead of using a surge tank, the outflow pipe feeds into a much larger tank, containing gravel and planted with a local variety of phragmites (a type of reed).

Water from this processing tank will be "treated" by bacteria; the reed root systems and can pass into a soak away or irrigation system, be used for watering, or be designed to serve either purpose at will.

This system doesn't work well at low temperatures, so it is sometimes necessary to divert grey water from the house to the main sewer in winter.

Using grey water

In times of drought, the vegetable patch generally receives its share of water via the watering can, but trees and other permanent plantings might be neglected.

This is where grey water can serve well.

A simple permanent hand-irrigation system can be set up using lengths of perforated plastic pipe, bent upward, and capped at both ends. It's simple to run in water from your holding tank or phragmites bed to fill up the pipe, then to disconnect after a day or so and leave the water to soak into the ground.

If the water comes direct from the surge tank, rotating from pipe to pipe prevents the build-up of clogging residues and allows a drying-out time, giving soil organisms a chance to break down raw organic matter. This system is especially useful for watering soft fruit bushes

pH problems

Most grey water is alkaline. On acid soils, this is beneficial. Test pH regularly. If it rises above 7, top-dress the soil with gypsum.

WHAT ABOUT HEAVILY ALKALI SOILS?

Hints on using grey water
• Use environment-friendly washing products
• Allow a rest period between waterings
• Apply to permanently planted areas, such as trees and soft fruit bushes.