Recycling Gray Water for Home Gardens

Sometimes plants in a vegetable garden or flowerbed will need more water than is provided through normal rainfall. Usually at those times a gardener just turns on an outside faucet and waters the garden, drawing on community water supplies, or from a private well.

But-below normal rainfall and predictions that the Northeast may be going through a drought cycle are causing people to find other ways to provide gardens with the moisture they need without using what may become scarce supplies of fresh water.

One method is to use the wastewater, usually referred to as gray water, produced in the home. The following are answers to some basic questions about how to safely use gray water in the home garden.

First, what is gray water?

Gray water is all the non-toilet wastewater produced in the average household including the water from bathtubs, showers, sinks, washing machines, and dishwashers. Although gray water does not need extensive chemical or biological treatment before it can be used in the garden as irrigation water, it still must be used carefully because it usually contains grease, hair, detergent, cosmetics, dead skin, food particles and small amounts of fecal matter.

How much gray water can be used in the home garden?

First, collect only as much wastewater as you will need to meet the water requirements of your garden. The rest should go into your sewer or septic system.

A good rule-of-thumb for deciding how much gray water to use on your garden is that a square foot of well-drained, loamy soil can handle about a half gallon of gray water per week. In other words, if your garden area is 500 square feet, then you can put up to 250 gallons of gray water on your garden each week.

If you can be choosy about the gray water you recycle on your garden, then use shower and bathtub water first, followed in decreasing order of desirability by water from the bathroom sink, utility sink, washing machine, kitchen sink and dishwasher. Water from the kitchen sink and dishwasher is not desirable because of the larger proportion of grease, food particles and other materials it will contain. If there is no way you can avoid using water from the kitchen sink and dishwasher, try to limit the amount of grease and solid food particles that go down the drain. Do not recycle water from a washing machine that has been used to wash baby diapers because it may contain fecal matter.

What about soaps and detergents? Are they harmful to the soil and plants?

Soaps and detergents are biodegradable, but they can present problems when gray water is used over an extended period. The main problem with most cleaning agents is that they contain sodium salts which, if present in excessive amounts, can damage the soil structure, can create an alkaline condition, and can also damage plants.

Avoid detergents that advertise "softening power," because they will have a large proportion of sodium-based compounds. The phosphates in detergents can be good for plant growth, but unfortunately, the detergents highest in phosphates usually contain the greatest amount of sodium. If you re-use washing machine water, cut down or eliminate the amount of bleach you use and do not use detergents or additives that contain boron, which is especially toxic to plants.

When doing your household cleaning, use ammonia, or products that contain ammonia, instead of chlorine as the cleaning agent.

What precautions can I take to protect the soil from damage when I use gray water over a long period of time?

As mentioned earlier, a great danger in using gray water is the build-up of sodium in the soil. You can discover if the sodium levels are high by having the pH of your soil tested. A pH of 7.5 or above indicates that your soil has become loaded with sodium. You can correct or avoid this problem by spreading gypsum (calcium sulfate) over the soil at a rate of two pounds per 100 square feet about once a month. Rainfall, or rotating gray water applications with fresh water, will help leach the soil of sodium and excess salts.

Is there any danger of spreading disease by using gray water in the garden?

Recycled water from the bath, shower, or washing machine could contain organisms causing diseases in humans. However, when gray water is poured onto soil that has an abundance of organic matter, the potentially harmful bacteria and viruses die quickly. If any should survive, it is unlikely that they would be taken up by the roots of garden plants and transferred to the edible portion of food plants. Nevertheless, for safety, you could use gray water to irrigate lawns and ornamental plants only.

How should I apply gray water to the soil?

Whether you carry your gray water to the garden by hand in buckets or modify your household plumbing for direct delivery of water to the area where it is needed, a number of guidelines should be followed in applying the water. They include:

• If possible, use gray water for your ornamental plants and shrubs and use what fresh water is available for your vegetable garden. If you need to use gray water for irrigating food plants, restrict its application to the soil around plants such as corn, tomatoes, broccoli, or other vegetables of which only the above ground part is eaten. Do not apply gray water to leafy vegetables or root crops.
  
  
• Apply the gray water directly to the soil surface. Do not use an overhead sprinkler, or allow the recycled water to splash off the soil and contact the above-ground portion of the plants. If you have a drip irrigation system, do not use gray water in it since any solid matter it might contain could clog the emitters in the pipe.
  
  
• Pour the gray water on flat garden areas; avoid steep slopes where runoff could be a problem.
  
  
• Apply the wastewater over a broad area; avoid concentrating it on one particular site.
  
  
• When possible, rotate applications of gray water with fresh water. The fresh water will help leach out any soil contaminants that might be building up.
  
  
• Apply thick compost mulches to areas where you use gray water. They will speed the natural decomposition of waste residues.
  
  
• Use gray water on well-established plants only. Seedlings can not withstand the impurities of the waste water.
  
  
• Do not use gray water, which is alkaline, on acid-loving plants such as rhododendrons and azaleas. Be sure to rotate your use of gray water with fresh water on lawns and fruit trees.

How can I get gray water from the house to the garden?

Gray water can be transported to the garden in a number of ways, the most basic being to bucket the water from the sinks and bathtub into pails and hand carry it to the garden. More sophisticated systems involve siphoning or pumping water from the bathtub or other deep basins (sumps) to the yard through a garden hose, or removing the trap from the bathroom sink drain pipe and putting a five-gallon, or larger, bucket beneath the sink. If you decide to adapt your plumbing system to allow you to get the gray water to the garden, be sure to have your local board of health inspect your work to insure that no sanitary codes are violated.

Prepared by: Allen V. Barker, Professor; Jean E. English, Graduate Student, Department of Plant and Soil Sciences, University of Massachusetts, Amherst.

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