Our planet has massive water resources, but easy access to high-quality water is limited. As our global water supply becomes dirtier, the amount of energy required to deliver clean, usable water continues to rise.
Traditional plumbing systems deliver high-quality potable water to homes, where it is used once and then discarded. This outdated practice is not sustainable, wasting time, money and energy. The average American home wastes approximately 28 gal per person per day of water (approximately 32% of drain outflow) that is not as dirty as sewer water and can be easily repurposed. Because a reasonable percentage of domestic water does not need to be potable, we should embrace efficiency enhancements like greywater reuse.
By definition, greywater is water that has been used once and would normally go down the drain, but is not contaminated with high concentrations of organic matter and bacteria. Greywater, then, is wastewater from sources like showers, bathtubs, washing machines and hand sinks, but not from toilets, dishwashers or garbage disposals.
Greywater reuse is legal for irrigation and flushing toilets. The most common use of recycled greywater in the U.S. is irrigation, but because commercial green building rating systems now allow for greywater systems as part of overall points earned, toilet flushing applications will likely become more widespread.
The premise of modern greywater reuse is simple: Capture the water, then store it, treat it and use it. The successful implementation of a greywater system, however, requires careful planning and a solid understanding of the technologies, practicalities and legalities.
A greywater reuse system typically consists of two key functions: capture and storage.
Capture. The best greywater designs capture outflows from acceptable points directly, without the use of manual diversion valves and other complicated devices. Do not capture greywater if someone in the building has an infectious disease, or if cloth diapers are being washed without comprehensive disinfection.
Storage. Bacterial overgrowth can turn greywater into blackwater quickly, which is why some locales do not allow storage before treatment. Where it is allowable and feasible, a storage tank can be helpful in accumulating greywater belowgrade before it is pumped up for treatment, making it simpler and more affordable to provide adequate treatment in larger commercial applications.
I recommend allowing for up to 24 hours of discharge as a surge tank for high-volume applications. Naturally, the ventilated storage tank must have an overflow to sanitary sewer in the event of a pump failure or overload. A best practice is to prescreen the water for hair and other fibers before it enters the storage tank. Day tanks should at least be periodically disinfected, or ozonated on a continuous basis to prevent bacterial overgrowth with the resultant biofilm that can clog emitters and cause service issues.
Greywater is dirty, and it needs to be properly treated before use. Treatment will vary based on whether the water is used for irrigation or flushing.
Irrigation Applications. If the water is only being used for sub-surface irrigation, it usually requires no further treatment beyond the removal of solids and hair that could clog emitters. Remember that soaps and detergents raise water alkalinity, and sodium and potassium levels in the soil. This increase can be detrimental to some plants, but the additional nitrates and phosphates can be a significant boon to vegetative growth. Shade-loving plants like hydrangeas, ferns, primroses and violets prefer acidic soils and generally are not suitable for greywater irrigation. Sandy, well-drained soils are less affected by greywater application than poorly drained clay soils.
If the building has a sodium-based water softener, I generally advise against deploying greywater recovery systems in high-hardness areas (20 grains per gal or more) without taking adequate precautions to keep the sodium adsorption ratio as low as possible. Switching to potassium chloride salt can help, and offsetting the sodium/potassium with calcium-based soil amendments is possible, but it is always best to consult with a qualified horticulturist if there are any concerns about damaging plants or soil.
Toilet Flushing Applications. Greywater treatment for toilet flushing must be more aggressive and thorough, because there is greater potential for human contact, and water quality affects the flush actuator mechanisms.
Logically, the most important concern to practitioners is the heavy organic loading with resultant biological oxygen demand that promotes the growth of anaerobic bacteria. Oxidation is the best way to break down the organics and living organisms. Chlorine and ozone are the most common oxidizing technologies used in advanced greywater recovery systems, and I frequently utilize advanced oxidation processes to ensure comprehensive oxidation with removal of odor.
The next thing to address is the inevitable sediment and other particulate matter that may be found in this type of water. Bag filters and self-backwashing multimedia filters are commonly used to clarify the water and remove particulates greater than 30 µ.
Steps to be taken after treatment depend on the intended use.
Irrigation Applications. Irrigation water should be used as quickly as possible to prevent bacterial growth.
Toilet Flushing Applications. Recovered greywater must be dyed to clearly identify it as non-potable. It can be stored for future use if it is properly disinfected. If it is to be stored, a best practice is to incorporate ongoing disinfection in the storage tank to minimize any chance of anaerobic bacterial growth.
Pumps and pipe should always be rated to accommodate residual oxidants and fluctuating pH levels, as well as meet code requirements for identification.
Irrigation Applications. When site elevation allows, many irrigation systems are gravity-flow from the surge tank or collection point. When not gravity fed, the water must be repressurized by a pump with appropriate control logic. Water then is distributed on the surface or via subsurface emitters. Subsurface emission is my preferred method, and is the only legal method in many locales due to the greater potential for harm from surface irrigation methods. Greywater should never be used for sprinklers. Never connect to existing irrigation plumbing without adequate, code-compliant backflow prevention.
Toilet Flushing Applications. Potable water makeup should be included to ensure an uninterrupted water supply in case the greywater system fails. This makeup water piping also must incorporate adequate backflow protection.
Remember that while greywater does not contain as much organic, bacteria or fecal material as blackwater, it is still a potential breeding ground for pathogens and should be properly processed before use, while always avoiding human contact or ingestion. The best practice is to carefully label all greywater piping, storage and distribution points, as well as properly train end users and maintenance staff in safe handling procedures.
Even though the practice of water reuse is as old as humankind, greywater reuse for modern homes and businesses is still an emerging technology. Regulations governing greywater will continue to evolve as more systems are put in place and the body of practical knowledge grows. Always consult with the local code enforcement authority before installing a greywater reuse system. Regardless of where you are, remember the following:
- Code compliance and inspection: All plumbing must be compliant with local code and the installation might be subject to onsite inspection.
- Signage: Signs must be posted where greywater is used to indicate that it is not safe for drinking.
Greywater reuse not only reduces the amount of potable water used on a property, but it also can lower downstream infrastructure demand and environmental impact by reducing sewer outflows. We cannot take our seemingly endless supply of clean water for granted anymore. Clean water scarcity is a growing problem. Reducing water consumption and reusing what we use will help us be part of the solution.