Gray water is any water, except toilet water, that has been used for bathing, washing dishes and clothes. Re-using gray water is a good thing to do in almost any setting.
In East Africa where long periods without rain must be accepted and water is not plentiful, it is especially important, to re-use water. Water from bathing and washing- virtually all uses except for raw sewage - can be sent directly into the garden. An easy and useful way of re-using gray water is through the garden. Plants do not need or even prefer potable water to grow.
They prefer water with nutrients. Gray water is often full of soap and detergent. People worry about that. Many plants grow faster and stronger when irrigated with detergent water. This is because some detergents contain phosphates and others contain nitrogen, important plant nutrients. While detergents can increase plant growth in the short term, they may damage soil quality in the long run.
Most conventional laundry detergents have high levels of salt, dyes, perfumes, petroleum products, and bleaches such as chlorine. This salt is carried into the soil where it builds up over time and the chemicals are not good for the environment. When available it is best to use green detergents, which are designed to biodegrade without hurting the environment, However, in general, gray water is not harmful to plants.
The basic concept of gray water gardening is simple - divert this nutrient rich water away from the sewage system and into the garden. Gray water can be used in a garden for many uses from irrigation to creating damp spots where plants can more easily survive. The methods used vary according to setting and need. In some high-income countries, using residential gray water in a small area may entail storage tanks, pumps, filters and other kinds of high-tech equipment.
However gray water can be used more economically, ecologically, and simply than that in East Africa. Without much effort, bananas, papayas, sugar cane and many kinds of greens such as matembele will comfortably produce around a water tap. This type of system becomes most effective when the people using the tap participate in the process and throw their waste water where it is most needed at any given time.
Also very suitable in East Africa are low-tech gravity run systems bringing used water from inside the house to the outside garden. It is simple to construct them when building - the plumbing directing the gray water is directed through a channel to where you want it in the garden, rather than into the septic tank or sewer. In already constructed buildings, the plumbing leading from showers and sinks can be redirected to pipes leading outside.
Raymer in her book "Down to Earth; a practical guide to gardening in East Africa" describes a very useful "waste-water garden". Choose a spot straight from where the pipes leave the house - at least one meter away from the house so as not to damage walls by the damp - where you would like extra vegetation. Dig a pit 1-3 meters wide depending on the amount of water the pit will receive.
Then, use a string line to mark the shortest distance between the waste pipe and pit, and dig a trench about 8 inches deep along it. Lay a pipe with enough slope to gravity-feed into the pit. Refill the trench. Break up a ring of soil 2-3 meters wide around the pit site, removing all grass roots and digging in a bucketful of compost.
Add a layer of organic material and scatter a little more compost or manure on top, layering excavated soil and compost on the prepared ground. Water at intervals if possible, making a doughnut shaped raised bed, and finish with an application of mulch. Into the pit throw garden and household waste that would otherwise go to the compost pile. Composting takes place in the pit.
Processes in the soil and compost bacteria cleanse the impurities in the gray water. Water-soluble nutrients percolate up, as evaporation draws them towards the root zones. To maintain fertility, keep the soil mulched by adding materials to the pit. The plants you choose will depend on the amount of water draining there, and your personal preferences.
Moisture loving fruit trees such bananas, papayas, bungo, will do well if there is a steady amount of water. If your use of the outlet will be periodic, for example perhaps you will go on vacation and so nobody will be using the tap, then consider plants such as Aloe that can tolerate a wide range of water availability.