How Gray Water Reclamation Works

By: Robert Lamb

According to the University of Arizona's Water Resources Research Center, between 60 and 65 percent of the water that goes down a home's drain has the potential to be reused.
According to the University of Arizona's Water Resources Research Center, between 60 and 65 percent of the water that goes down a home's drain has the potential to be reused.
© Saulyte-Laurinaviciene

Ever stop to wonder how much water goes down the drain each time you take a shower, throw in a load of laundry or just give your hands a quick wash? For the average U.S. household, the answer is quite staggering: close to 280 gallons (1,060 liters) a day [source: Environmental Protection Agency]. More than half of that can potentially be reused for irrigation in the place of fresh tap water.

After all, why waste all that drinkable water on your yard when dish water will do instead? Why drain your tub directly into the sewer system when there are thirsty plants right outside your door?


That's the idea behind gray water reclamation -- simply getting the most out of your water through reuse.

The water use in most homes has long been thought of in terms of clean white water coming in and sewage, or black water, going out. Gray water, as the name implies, is something in between. By most domestic definitions, gray water is tap water soiled by use in washing machines, tubs, showers and bathroom sinks. It's not sanitary, but it's also not toxic and generally disease free. Gray water reclamation is the process by which households make use of gray water's potential instead of simply piping it into overburdened sewage systems with all the black water.

The advantages of gray water reclamation for your wallet include lower water and sewage bills. Additionally, reusing gray water's otherwise wasted nutrients from soap (nitrogen and phosphorous) and food (potassium) can sustain plant life and recharge topsoil. However, recycling gray water requires more effort, brings the risk of contamination and pollution if mismanaged, and adds installation and upkeep costs for more involved systems. Also, gray water should not be used where children or pets may come into contact with it.

­So how can t­he water that goes down the drain after a bath be of any use to plants? Read on to find out.

Types of Water: Gray Water, Black Water and White Water

Sure, gray water sounds like something worth reusing, but what's in it exactly? First, let's draw the line between gray and black.

The key difference between the two is that black water has come into contact with fecal matter. Fecal matter is a haven for harmful bacteria and disease-causing pathogens. Additionally, this waste doesn't break down and decompose in water fast or effectively enough for use in domestic irrigation without the risk of contamination.


Gray water, on the other hand, has not come into contact with solid human waste. This greatly decreases the risk of disease and increases the speed at which it can be broken down and safely reabsorbed into an active garden or lawn.

The line between white and gray, however, comes down to a number of possible additions made in the acts of washing, bathing, cooking and cleaning. Unlike white water, gray water may contain soap particles, fat and oil from cooking, hair, and even flakes of human skin. The exact contents of gray water depend heavily on the household producing it, so if you want to start reusing your gray water, you have to start regulating exactly what you send down the drain.

­If ­the household chemicals in gray water are kept to a minimum, most plants will be able to handle it. You can keep chemical contamination to a minimum by using environmentally friendly, biodegradable soaps and detergents whenever possible.

When everything you send down the drain winds up in your backyard, "environmentally friendly" certainly hits much closer to home. But still, just how much is your rose garden going to like drinking your old shower water? We'll explore the answer to that question next.

Gray Water and Gardens

For the most part, plants aren't as picky as people when it comes to drinking water.
For the most part, plants aren't as picky as people when it comes to drinking water.
© Tryggvason

To understand how gray water is absorbed by soil and plants, imagine emptying your backpack on the subway. That pile of stuff is much like gray water. It consists of various items that are useful or useless to the environment you introduced them to. There are certain items that will lie untouched -- perhaps your smelly gym clothes or a really boring book. Plants and soil are much like the other train commuters. They're ready to snatch the items they have the most use for and leave the less attractive ones behind.

Plants and soil work hard to break down gray water. Soil filters out many contaminants through a basic process:


  • ­As water passes through layers of sand or granulated rock, larger water contaminants are caught in the grit of the dirt's solids. This process is like straining solids out of soup with a colander, on a smaller scale. (If this sounds far fetched, remember that one key component in commercial water filters is charcoal.) The dirt itself helps filter out nutrients and biodegradable materials, which can then be absorbed by plants and bacteria.
  • Microorganisms and bacteria in the ground feed off of carbon and pathogens, leaving water, carbon dioxide and non-polluting insolubles.
  • The rest of the water, now purged of major pollutants, is absorbed by plants or seeps down to recharge the groundwater.

It's important to remember that plant life varies greatly, and some species are unable to deal with the chemicals, salt or acidity levels in gray water. Other plants just call for careful watering and care to begin with. In many situations, drainage from kitchen sinks and dishwashers is too contaminated by grease and high acidity to be used at all.

Even without coming into contact with human waste or strong chemicals, gray water can contain food particles, grease, bacteria and some pathogens due to contact with our food, soaps and bodies. The yard can handle these elements, but that doesn't mean you can.

Take care to prevent gray water from coming into contact with any fruits and vegetables due to the contamination risk (especially if the produce may be consumed raw). Fruit and nut trees are generally considered safe picks due to the distance between the ground and the food, but all other food gardens are best irrigated with white water or rain water. Additionally, gray water usage should be suspended altogether when a household resident is sick, as this only increases the contamination risk.


To further prevent contamination, don't store gray water for reuse. If allowed to sit, gray water quickly turns into a stagnant, sludge-filled concoction of bacteria and pathogens -- these elements thrive on some of the same nutrients a garden could benefit from. This feeding frenzy needs to take place in the soil, as described above, not in your tank. If gray water is collected and stored without treatment, it effectively becomes black water in as little as 24 hours.

But what kind of system gets all that gray water out to the garden? How much is the installation cost? It can all be as simple as grabbing a bucket.


Irrigating with Gray Water Systems

Gray water reclamation doesn't have to be difficult. It can be as easy as taking a bucket with you into the shower.
Gray water reclamation doesn't have to be difficult. It can be as easy as taking a bucket with you into the shower.
Photo by Ryan Klos/

Most methods of gray water reclamation, from lugging around a full bucket to diverting the water through a costly treatment system, involve a simple cycle:

  1. White water is pumped into the home and is used in showers, bathtubs, sinks, dishwashers, laundry rooms and toilets.
  2. White water that comes into contact with human waste becomes black water; the rest becomes reusable gray water.
  3. Gray water is reused for domestic purposes when appropriate. Black water is sent into a septic tank or sewage system, along with any unused gray water.

When pipes are used to divert gray water, take care to prevent pumps and filters from clogging with bits of hair, skin and food. When clogs do occur, it is worth remembering that chemical clog removers are just the kind of harsh chemicals you don't want to send directly into your garden. Natural solutions, such as boiling water or vinegar and baking soda treatments, might be less damaging to plant life. Additional safety precautions often depend on the specifics of the reclamation system in place, desired vegetation and the home's residents. Gray water laws also vary from place to place. This issue will be discussed later.


Three basic gray water reclamation systems are commonly used. The main differences involve the scope, complexity and cost of the reclamation process.

Manual bucketing

The most low-tech system of gray water reclamation, manual bucketing is exactly what it sounds like. You simply drain your gray water directly into portable containers that you can use to water lawns, gardens and potted plants. The process can be as simple as taking an empty bucket in when you shower, then watering a tree with the gray water you collected.

The irrigation can still be tricky, though. Make sure you evenly distribute gray water among your desired plants and that the water does not pool on the surface or run off the property.

Simply pumping gray water out onto the lawn would be potentially hazardous. But the small quantities of gray water involved in manual bucketing and the hands-on, human discretion make watering on the surface a­ viable option.


Basically a more permanent, hands-free approach to gray water reclamation, the diversion method takes the principals of manual bucketing and lets the plumbing do most of the work. Instead of manually lugging the gray water left over from a bath to the garden, a diversion system drains gray water directly from your bathtub and distributes it outside.

As the diversion system involves the use of more water than manual bucketing and is typically a more independent and hands-free system, subsurface irrigation is used to evenly distribute gray water. Otherwise, gray water pooling on the surface or running off the property could result in building damage, bad odors, mosquitoes and even pollution.


Potentially the most expensive method of gray water reclamation, the treatment method takes the diversion system one step further by routing gray water through a treatment system that cleanses the water -- sometimes enough to be safely stored or used to flush toilets inside the home. Simpler systems employ a physical filtration system, but more advanced systems further remove contaminants through the use of chemical treatment. Chemical treatment basically involves another level of breaking down contaminated water, only on a much smaller scale. More than a dozen different varieties of chemicals are used to treat waste. Some chemicals specialize in killing harmful microorganisms; others break down various unwanted chemicals within the mix. The downside to these systems is that the cost involved can easily place gray water treatment outside the budgets of most households for the foreseeable future.

To further understand the diversion and treatment systems, we'll take a look at just how gray water is piped out from the drain and into the garden in the next section.­

Gray Water Usage: Diversion Systems

While the particulars of a gray water diversion or filtration system can vary greatly, several basic steps are commonly used.

A hand-activated valve serves as an important first step in many systems, especially the more basic diversion systems. This valve allows a person to decide when water from a bathroom sink, tub or washing machine will be diverted to the garden and when it will go into the sewage system or septic tank. This level of regulation comes in handy to avoid overwatering during times of heavy rain or sending harmful chemicals or diaper water out to the garden.


After gray water is diverted down a system of pipes, its first stop on its trip to the garden is a basic filter -- generally a mesh screen -- to eliminate larger particles, before entry into a surge tank. The surge tank is used to help regulate flow by temporarily storing large amounts of gray water. This helps ensure a bathtub's worth of drainage doesn't all rush into the garden at once, while also preventing it from backing up into the home.

Gray water can go two places from a working surge tank: out to the garden or down through a set of sewer drains. The bottom drain and the overflow drain (see above) work just like their counterparts in a bathroom sink, where the central drain provides constant drainage at the bottom while the higher, overflow drain keeps the water from rising above an undesired level.

The drains on a surge tank are always open because it is not a storage tank. To keep it from becoming black water, any excess water left at the bottom has to be allowed to drain off into the sewage system. The remainder of the gray water, at this point, flows or is pumped out to the garden or into a filtration system.

Filtration systems vary greatly in scope and price. The more advanced systems actually allow for treated gray water to be re-routed back into the home for use in toilets, laundry machines and even cooling systems. Some also eliminate the need for a surge tank. Simpler filtration systems are only aimed at further eliminating undesired chemicals and greases before water reaches the plants. Many of these simply involve letting gray water drain through boxes of layered sand, dirt or charcoal.

Sinkpositive, from Environmental Designworks, sits atop your toilet tank, releasing clean water for you to use. Fresh water comes in through the faucet and drains directly into the bowl.
Sinkpositive, from Environmental Designworks, sits atop your toilet tank, releasing clean water for you to use. Fresh water comes in through the faucet and drains directly into the bowl.
Photo courtesy Environmental Designworks

But how do you deliver gray water to an underground root system? To accomplish this, two methods of subsurface irrigation are used frequently:

  • ­Subsurface drip irrigation simply involves sending water out through a system of underground pipes that release water at designated spots, usually next to a plant's root system.
  • Leach field or drain field irrigation is associated with septic tanks, which use irrigative methods to drain off excess water -- the principle is the same with a gray water irrigation system. Water from the surge tank is routed out through perforated pipes, buried in gravel-filled trenches below lawns or garden root systems.

But no matter what the scope or cost, the most important parts of any gray water reclamation system are the people and the plants sharing the water. There is no universal standard when it comes to reusing waste water, so much of it comes down to personal needs and commitment levels. On top of that, the technology, methods and laws governing gray water reclamation are constantly changing.

Gray Water Laws

Would widespread gray water reclamation help or hinder sewage treatment plants such as this one in Santiago, Chili? Some say that without gray water to move waste along, sewage might not make it all the way to the treatment facility.
Would widespread gray water reclamation help or hinder sewage treatment plants such as this one in Santiago, Chili? Some say that without gray water to move waste along, sewage might not make it all the way to the treatment facility.
Photo by Peter Essick/Aurora/Getty Images

Until recently, many cities had no legal distinction between gray and black water, rendering most domestic reclamation efforts technically illegal. But due to increased concerns over droughts and water shortages, some residents and local leaders are pushing for updated laws to allow the regulated use of gray water reclamation.

And where there's demand, industry can't be far behind. Companies are now offering gray water systems, pre-construction consultation and custom installation, in addition to working on new technologies.


Still, a gray water reclamation system that is legal in one city may be deemed a health hazard in another. Some regions still classify gray water as sewage, while others provide residents with basic health and safety guides for reusing gray water.

Across the globe, the scope of gray water reclamation varies greatly. Australia and New Zealand have been ahead of the game for years, providing the local regulations, guidance and education needed for residents to make the most of their waste water. Mexico has begun using treated gray water for irrigation. However, in some less-developed nations, things are less encouraging. While gray water reclamation may not be prohibited by law in poorer nations, often there are far worse substances than soap suds draining into the soil. Ironically, the strict laws gray water advocates are fighting to overcome in developed nations were originally instituted to protect the environment and public health.

While some people are concerned over the potential health hazards involved with gray water reclamation, its effects on our sewage systems concerns others. As most sewer systems were designed before gray water reclamation was an option, a lot of that wasted water is used to carry waste to sewage treatment facilities. If less water is available for treatment, less clean water will then be pumped back out into the water supply [source: Colorado State University Extension].

There's a fragile balance between wasting less fresh water and reusing more gray water, and making sure water and sewage systems can keep pace with the changing ways residents approach water use. Most experts agree that it won't be home systems which have this kind of impact, but large-scale efforts by cities and major corporations.

But on the home front, gray water reclamation is a very user-specific practice. What works for a family of three might not fit well with the needs of a multi-family apartment dwelling. The details of a gray water reclamation system vary significantly, depending on geographic location, size of household, intended reuse and the level of commitment users will devote to making the most out of his or her waste water and protecting the environment.

Lots More Information

Related How Stuff Works Articles­­

  • How Irrigation Works
  • How Sewer and Septic Systems Work
  • What is gray water and can it solve the global water crisis?
  • Plumbing
  • Watering Plants
  • Water Tips for Gardeners

­More Great Links­


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