Using pure fresh water in the garden while sending only “slightly used” grey water into the septic or sewer system is considered wasteful in some situations. Where water is extremely scarce, some people consider it insane. Here’s how to make use of that grey water in the garden.
The term “grey water” is often used to describe all the waste water from non-toilet uses such as dishwashing and laundry. (Water from toilets is called “black water.”) Grey water doesn’t require extensive chemical or other treatments to be used, and is an excellent way to start recycling water at home. Grey water may contain objectionable items such as grease, hair, detergent residue, food particles and even human pathogens. These substances must be considered when planning a grey water system for home use. Scale your plan for recycling water at home to the size of your garden. Collect and use only as much grey water as it can reasonably handle. Any excess waste water should be released into the sewer or septic tank in the conventional manner. A prudent rule is that a square foot of loamy garden soil rich in organic matter is capable of handling a half gallon of grey water per week. Sandy, lighter soils will require more water, while heavy clay soil can handle less. If the garden area suitable for grey water application is 500 square feet, it will take up to 250 gallons per week. This rate will be higher in the summer when surface evaporation and plant transpiration are considerable. Adjust downward in the winter.
Soaps & Detergents
As a general rule, soaps are less harmful than detergents. But either substance may present problems over time. A common problem with cleaning agents is that they contain sodium salts that are harmful to soils if present in excessive amounts in grey water. It can damage soil structure, create an alkaline condition and burn plant leaves. Soaps tend to contain less sodium and other substances than detergents. Gentle soaps (such as soap flakes) are preferred to those heavy in lanolin, perfumes and other chemicals. When detergents must be used, avoid those which are heavy in sodium-based compounds such as “softening powders.” If you plan to re-use washing machine water, minimize or eliminate bleach. Boron products should be eliminated. Phosphates in detergent are actually a plus for plant growth. Unfortunately, high-phosphate products usually contain the highest amount of sodium. Ammonia is preferred to scouring products that contain chlorine.
Applying Grey Water
Whether carried in containers or transported through plumbing systems, several principles govern the best usage of waste water for irrigation. Apply waste water directly to the soil surface. Don’t use an overhead sprinkler or allow the water to splash off the soil and contact the above-ground portion of the plants. Grey water isn’t suitable for drip irrigation, as the solid matter it conveys would clog the pipe. Distribute grey water to flat areas of the garden. Avoid steep slopes where runoff might be a problem. Disperse waste water over a broad area. Avoid concentrating it on one particular site. Use fresh water for garden irrigation on a rotating basis when available to aid the soil in leaching of contaminants. Apply thick compost mulches to areas receiving grey water. This will facilitate natural decomposition of waste residues. When irrigating food plants, use grey water only for application to the soil around plants such as corn, tomatoes or broccoli, where the above-ground portion of the plant is eaten. Don’t apply it to the soil around leafy vegetables or root crops. Ideally, grey water should be used for ornamental or non-food plants. Use waste water on healthy, well-established plants. Seedlings and house plants can’t tolerate the impurities in household waste water. Waste water from the shower, bath and washing machine may contain disease-carrying organisms. However, when grey water is applied to soil, potentially harmful bacteria and viruses are immobilized by the abundant soil organisms better suited to the soil environment. If the pathogens were to survive, it is unlikely that they would be assimilated by the plant roots and transported to the edible portion of food plants. For safety, restrict the use of bathing and washing machine water to ornamental plants and lawns.
Over extended periods, sodium levels may build up in the soil. This could lead to poor drainage and potential plant damage. A pH test can be used to check for high sodium levels in the soil.
Getting it to the Garden
There are several easy and inexpensive ways to get grey water to the garden. Carrying buckets from the sinks and bathtub to the garden is an obvious choice. A more graceful method would involve siphoning water from the bathtub or basin to the yard through a garden hose. Equipment for this purpose may be obtained from hardware stores. The trap from the bathroom sink may be dislodged and rotated away from the drain pipe to permit collection of waste water in a five-gallon bucket. This water can be hauled to the garden or used to flush the toilet by pouring the full bucket of water into the bowl. Don’t fill the toilet tank with grey water, as it may clog flushing devices, create objectionable odors, or even contaminate the water supply. If your washing machine discharges into a utility sink which is at a sufficient height above the yard to permit gravity feed, the water may be collected in the basin and siphoned to the yard with a garden hose. Don’t attach the garden hose directly to the discharge line of the washing machine, as the water flow will be too heavy for even distribution. Water from the wash cycle is heavily polluted and should be used only when diluted with rinse water. Washing machine waste water from loads containing baby diapers must not be used. In some situations, it’s possible and practical to modify the plumbing to divert grey water out of the house, perhaps to a sand or gravel filter and then to a holding tank or the garden.
Originally published in 1999 and regularly vetted for accuracy.