Grey water is domestic wastewater generated from non-toilet sources, representing a large portion of a household’s total water output. This includes water from bathroom sinks, showers, bathtubs, and washing machines. Unlike black water, which contains human waste, grey water contains fewer pathogens, making it a valuable resource for non-potable uses. Filtering this water for garden reuse is a practical way to conserve water and reduce strain on municipal systems. The process requires careful selection of source water, physical filtration to remove solids, and advanced treatment methods for safe application in the landscape.
Identifying Suitable Sources and Contaminants
The first consideration for a garden grey water system is ensuring the source water is appropriate and minimally contaminated. Acceptable sources include wastewater from showers, bathtubs, bathroom sinks, and laundry machines, as these streams generally contain lower levels of pathogens and organic solids. Water from toilets, soiled diapers, kitchen sinks, and dishwashers must be avoided. Kitchen water often contains high concentrations of grease, food particles, and complex cleaning chemicals that quickly clog filtration systems.
The primary contaminants of concern are sodium, boron, and chlorine bleach, which are toxic to plants and detrimental to soil health. Sodium salts, often found in powdered laundry detergents, degrade soil structure by causing clay particles to disperse, which reduces water infiltration and aeration. Boron is an element toxic to many plants, even at low concentrations, and can accumulate in the soil over time. Chlorine bleach, a common household disinfectant, harms beneficial soil microbes and plant roots.
Proactively managing contaminants before the water enters the system improves quality and reduces the burden on filtration. Homeowners should switch to biodegradable, low-sodium, and phosphorus-free soaps and detergents specifically labeled as “grey water safe.” This minimizes harmful chemicals that are difficult or impossible to remove through simple physical filtration methods. Selecting appropriate household products increases the longevity of the filtration system and mitigates the risk of long-term soil damage.
Primary Filtration Systems
Initial physical filtration is necessary to remove suspended solids, hair, lint, and other large debris that can rapidly clog advanced treatment components or irrigation emitters. The first stage involves coarse screening placed immediately where the grey water exits the house plumbing, such as before a diverter valve. A simple nylon stocking or fine mesh screen in a lint trap effectively captures larger particulates from laundry and shower drains. This initial screen requires frequent cleaning, often weekly, to prevent blockages and maintain flow.
Following coarse screening, a simple media filtration system is employed to remove smaller, suspended particles. A common DIY setup uses a large barrel or container layered with different grades of aggregate materials. The water flows downward through these layers, starting with the coarsest material and progressing to the finest. A typical arrangement involves a base layer of coarse gravel, followed by a thick layer of sand.
This vertical-flow filter physically traps progressively smaller particles. The gravel supports the finer materials and aids drainage, while the sand provides the main filtration surface. The effectiveness of this system depends on maintaining a steady, slow flow rate to prevent channeling and maximize contact time. While this physical process removes a significant amount of solids, media filtration alone does not remove dissolved chemical residues or pathogenic microorganisms. The filter media accumulates trapped solids, necessitating periodic cleaning, backflushing, or replacement to ensure consistent performance.
Advanced Treatment and Application Methods
To further enhance the safety and quality of grey water, particularly for long-term irrigation, advanced treatment for dissolved substances and pathogens is beneficial. Biological filtration, such as a simple constructed wetland or biofilter, offers a low-maintenance, natural polishing step. These systems typically consist of a lined bed filled with gravel or sand planted with water-tolerant vegetation, such as reeds or grasses. The roots of these plants and the surrounding media create a habitat for beneficial microorganisms.
As the grey water flows slowly through the wetland media, the microbial community breaks down residual organic matter and neutralizes some pathogens. This biological process significantly reduces the biochemical oxygen demand (BOD) and total suspended solids (TSS) that remain after primary filtration. For systems seeking to remove specific chemical residues, such as volatile organic compounds or some pharmaceutical traces, a final stage of activated carbon filtration can be incorporated, though this adds complexity and cost due to the need for eventual carbon replacement.
Regardless of the level of treatment, the application of grey water must strictly follow safe irrigation practices to protect human and pet health. The only recommended method for applying grey water is subsurface irrigation, which involves delivering the water directly to the plant root zone through buried drip lines or leaching trenches. This technique prevents contact with the water, eliminates the risk of airborne pathogens through spraying, and minimizes vector breeding by avoiding surface pooling. The drip lines should be buried at a minimum depth of two to four inches and covered with a layer of mulch.
Grey water must never be used on edible root crops, such as carrots or potatoes, or on vegetables where the edible portion touches the ground, like lettuce or strawberries. The safest application is for ornamental plants, turf grass, and fruit trees, ensuring the water does not come into contact with the harvested parts of any food crop.