Water Efficiency Series: Part 2- Utilizing Graywater
Graywater, or “light graywater” is generally defined as water that has not been in contact with human waste and organics, water that has been used once in sinks, showers or washing machines. This differs from “dark” graywater, which includes water from kitchen sinks or dishwashers, and blackwater, which is wastewater from toilets.
It is estimated by the US EPA that the average household of four uses about 400 gallons of water per day. Approximately 70% of this is for indoor use and over 10% is usable graywater. As many states and countries are facing droughts more often, reusing graywater could prove to be a very beneficial strategy.
Graywater is most easily and commonly reused for irrigation purposes. The safest method is subsurface irrigation, where the water does not come in contact with humans and where amounts are generally not significant enough to infiltrate the drinking water supply. Whitewater (potable water that meets the EPA’s drinking water standards) should still be used for irrigation of food crops other than fruit and nut trees where the crop is far away from the ground. Subsurface irrigation ensures that the graywater will not pool on the surface or runoff, which could result in odors, mosquitoes, pollution, building damage and unsanitary conditions.
According to the U.S Geological Survey, about 1/3 of residential water use is used outside (mostly for landscape irrigation), which is about 7.8 billion gallons a day! (Source: U.S. EPA’s Water Efficiency Landscaping publication). This means there is a huge opportunity to recycle water for irrigation purposes.
Graywater systems for reuse generally consist of collection plumbing, treatment methods, disinfection and distribution components.
Dual plumbing is required in order to separate wastewater from graywater and blackwater sources. If graywater is used for subsurface irrigation, the system can be very simple, with just a three way manual valve installed that can cut off graywater to irrigation if an anticipated load is too heavily contaminated. Soil must also be tested prior to design to determine if a surge tank is needed to handle peak water loads. The surge tank is not a long term storage tank. Graywater should be treated or distributed immediately as the organic material in the water will cause it to become unusable and possibly harmful in as little as 24 hours.
For complex systems the next step is treatment. These scenarios include but are not limited to: graywater used for surface irrigation or indoor use, large collection sources, systems that store water and areas with stricter permitting regulations.
After a filtration pretreatment, treatment methods can include mulch basins, media filtration, filtration membranes, biological treatment and constructed wetlands. I highly recommend reading Sustainable Infrastructure: The Guide to Green Engineering and Design to learn more about each method as I could write an entire blog about each one!
After treatment, water should also be disinfected if it will be used indoors or aboveground. Water can be disinfected using ultraviolet irradiation or by adding chemicals such as chlorine, chlorine dioxide or iodine. Non-chemical methods are preferable, but also more expensive.
There are many ways to distribute graywater. In the simplest of systems, gravity carries water from the building to the lower-elevation landscape. Advanced systems generally require pumping in order to force water through treatment equipment. Water can be distributed through single or branch pipe systems directly to the landscape, to mulch basins, to small leachfields, through a perforated pipe to gravel filled trenches, to subsurface drip systems, or to subsoil infiltration galleys.
Graywater can be an excellent water resource; however, many factors contribute to ensuring a high-quality system. First of all, thorough planning and sound engineering are essential for a successful system. The soil conditions, climate, anticipated loads and end use are just a few of the many issues that must be analyzed during the design process.
After construction, the user must carefully maintain the system and be conscious of what is entering the system. Before doing research on graywater systems, I was very concerned about how soaps and detergents can be okay for plant life. I learned that although it is important to use environmentally-friendly products and be aware of what is going down the drain, generally the chemicals and salts introduced to plants and soils in small-scale graywater irrigation projects are usually too minimal to have any negative effects.
In fact, some of the articles I read stated that the nitrogen and phosphorous from soap and the potassium found in food actually nourishes plant life and recharges top soil. It is most important, however, to avoid soaps with sodium, chlorine, boron, borax, bleach and high levels of phosphates.
As policies allowing and promoting graywater use are beginning to increase, there are more and more government resources to read and learn all about graywater use. The EPA water recycling and reuse website, the Graywater Guidebook released by the State of California and Sustainable Infrastructure: The Guide to Green Engineering and Design are a few resources to start with. I hope graywater use will continue to expand and be more widely accepted as a water resource with the general public, as water recycling will help alleviate some of the stress on our current infrastructure. Water scarcity is an growing problem that will only increase in the years to come, we need to start implementing solutions now.
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