Greywater

Last updated: July 24, 2009.

Next time you're flushing your toilet, washing your car, firing the sprinkler over your garden or cleaning your windows, spare a thought for the one billion or so people (15 percent of the world's population) who still have no safe supply of clean water and the 2.5 billion people (37 percent of the population) who don't have proper sanitation. Imagine how people in some remote village in Africa would feel if they could see you squandering gallons of sparkling, clean, highly treated water that you're not even going to drink!

On one level, this is an utterly ludicrous comparison: even if you save water, it doesn't help people in Africa one iota. But on another level, conserving water is incredibly important: as global warming and climate change kick in, virtually all of us will find our water resources under much greater pressure. Saving water obviously saves water, but it also saves energy (because cleaning water is very energy intensive), protects rivers (because water ultimately comes from there), and helps the environment on which we all depend. If you're billed for every unit of water you use, saving water also helps your pocket. That's why many people are interested in greywater: a way of collecting and recycling some of your household water and using it for less important things like flushing the toilet. Let's take a closer look at greywater and find out how it works!

Photo: We waste terrific amounts of water every day. If you had to walk for an hour to fetch your water, would you let so much run down the drain?

What is greywater?

You've probably seen those medical articles suggesting you need to drink 1-2 liters of water a day (that's roughly 2.5–5 pints or 6–8 glasses) to stay healthy, so a typical family of four would need a maximum drinking water supply of maybe 8 liters (2 gallons). But according to the US Environmental Protection Agency (EPA), the same family can use an astonishing 1500 liters (400 gallons) of water a day in total. Even allowing for cooking and hand-washing, where we need to use clean water, you can see there's a huge mismatch between how much water we use in total and the amount we need that has to be scrupulously clean. Greywater systems try to address this.

Photo: A simplified diagram showing the basic idea of greywater.

Greywater (sometimes spelled graywater in the United States) is the idea of having two separate household water systems. First, you have a normal household water supply of clean, fresh water (sometimes called whitewater or mains water), which you use for drinking, cooking, and so on. But you also have an extra tank that collects the used water from your bath, shower, clothes washing machine, (and sometimes your outside roof). This is your greywater. It's used for flushing the toilet (automatically), but you can also use it for washing the car, watering the garden, and anything else that doesn't need absolutely clean water. Sometimes water from the kitchen sink (dark greywater) is reused too, but it's more contaminated and unhygienic than water from your bath or shower. Water from the toilet (known as blackwater) is never reused: it's discharged to the sewer in the usual way. Trials by the UK's Environment Agency (a similar organization to the US EPA) have found that systems like this can save 5-36 percent of total household water consumption, though much less (a maximum of about 20 percent) in new homes.

Photo: Greywater typically means installing an extra tank. This large tank is being put into a school; you wouldn't need a tank quite so big for a home. Photo by Felix Garza Jr. courtesy of US Navy.

Advantages and disadvantages of greywater systems

Photo: Imagine what people in developing countries would make of using drinking water to irrigate a golf course! Fortunately, this course at Fort Meade, Maryland is being kept lush and healthy with reclaimed water from a wastewater plant. Photo by Christine Frankovitch courtesy of US DOE/NREL (Department of Energy/National Renewable Energy Laboratory).

Greywater sounds like a brilliant idea—for all sorts of reasons. First, it reduces the fresh water you need to consume, so it could help to cut your water bill. If you're consuming less water, the sewage and wastewater plants have to recycle less (using less energy) and less water has to be removed ("abstracted") from rivers—so greywater is also good for the environment. If you have a septic tank, switching to greywater reduces the amount of water you're passing through the tank for treatment, extending its life.

But there are disadvantages too. First, the cost of installing a greywater system can be significant compared to the savings in water bills you actually make. More seriously, storing used water as greywater allows microorganisms to breed—especially if it's warm water to begin with—and that can present a health hazard. So graywater has to be stored carefully with hygiene in mind, typically either filtered before being stored, chemically disinfected, or stored for only relatively short periods of time (greywater systems automatically flush their tanks and refill with clean white water if they're unused for too long) to reduce the chance of bacterial contamination.

Alternatives to greywater

Purification, disinfection, and periodic draining clearly reduces the benefit of having a second water system—so much so that, for small households, there may be no benefit at all. You can probably achieve greater savings more quickly and economically simply by using fresh water more carefully: by flushing your toilet less often (or converting to a water-saving dual-flush), turning off the faucet while you brush your teeth, installing a low-flow shower nozzle (one that mixes a lot of air with the water), using a water butt to collect rainfall for your garden, and so on. Water savings like this are really easy to make; many are instant and free. One really good way to save water is to ask your utility company to install a water meter on your property (if you don't have one already). Seeing how much water you use each month or quarter (and how much it costs, on your bill) really focuses the mind on making savings—and you can see just how effective you're being.

Environmentalists tend to see things a little bit differently. The very concept of wasting a resource as precious as water is galling to people who truly value the planet, so some green-minded people insist on installing greywater systems in eco-homes as a matter of principle. Environmentalists or not, the message is clear: in a world of growing water scarcity, all of us have a responsibility to use this important resource more carefully. It's worth bearing mind that in the future, saving water may not be a matter of choice.

Photo: With a bit of ingenuity, you can save water in all kinds of ways with little or no effort or expense. This low-maintenance garden at DOE/NREL in Golden, Colorado is a good example of how office gardens can be redesigned to save the need for wasteful irrigation. Photo by Warren Gretz courtesy of US DOE/NREL (Department of Energy/National Renewable Energy Laboratory).

Further reading

• WaterSense, a program sponsored by the US Environmental Protection Agency that encourages people to save water and protect the environment.
• Reusing greywater and harvesting rainwater: Useful guidance from the UK Environment Agency
• WaterAid: Find out how you can help people in developing countries who lack clean water and sanitation.