Water which is "slightly used," which takes its name from the convention of naming water that is unfit for use "black water." Gray water is water from showers, sinks, dishwashers, and laundry. This specifically excludes toilets and any water used to rinse diapers--any water that contains human waste. Because this water is used to rinse biodegradable bits off of our precious stuff--skin cells, leftovers, body odor, etc.--it's actually not too bad for watering plants with, which would save gallons of "white" potable water. Another novel idea is for each home to have a gray water tank, and to use that for the water in your toilet. Other than the cat having to find a new watering trough, this idea could save 25%-40% of water used in America.

Unfortunately, there are some down sides to a plan where water is used twice for irrigation: fibers (from clothing), sodium (water softener), boron, and many other household chemicals shouldn't be allowed to enter the gray water collection system. It's also unsafe to drink, despite its appearance. Some studies suggest using a vegetable dye to artificially identify it, and to run it through differently colored pipes. Proper disinfection and filtering is required, as well--if you let a spoonful of the wrong chemicals get into your landscape every time you water, they'll work their way up the food chain quickly.

AKA Sullage
Gray water

By now most people have heard of grey water recycling. It means reusing water that you've used once, and there are all sorts of new technologies coming out, and it saves water... And that's about all most people know. It's easier to buy a composting toilet than a toilet that recycles grey water. It easy to save rainwater running off your roof to water your garden, but much harder to find information on how to use water running down the drain for the same purpose. Grey water recycling is a great idea that has completely failed to take off. Maybe a writeup on Everything2 will help...

First off, grey water is dirty water. Not really dirty water; grey water comes from shower drains, kitchen drains, and washing machines. Water from toilets and water containing toxic chemicals are black water, and are not as useful as grey water. While grey water is too dirty to use for cleaning or drinking, it is still uncontaminated enough to be useful for some household or industrial purposes. Grey water can be used for gardening, irrigation, and flushing toilets, often with little or no treatment.

One of the biggest challenges in reusing grey water is avoiding bacterial growth. Most grey water has some bacterial contamination, some sort of nutritional content, and it is often warm; perfect for growing bacteria. This makes grey water a potential health hazard, and also stinky. If you are releasing the grey water directly (for example, draining it directly into your garden) you are probably fine, as long as the water is absorbed completely into the soil (it should not be left to pool). It should not be used to water plants that will be eaten raw, especially root vegetables and herbs. You might also have to worry about local regulations dictating the disposal of wastewater, which may restrict both acceptable sources and uses of grey water.

If you are storing the grey water for more than 24 hours you will need to filter and/or treat it in some way. This can involve some fancy equipment, but it doesn't need to. You can disinfect grey water with chlorine, iodine, or bleach. None of these are really good for the environment or your garden, but you don't want to be spreading giardia and other nasties. Chemically treated water is often used to flush toilets. You can also build your own filter; you may wish to do this even if you are using the water immediately, as grey water can contain bits of food, hair, and grease that may do your irrigation system / toilet / plants harm. These filters can be anything from an old sock to a homemade sand filter, but even the best homemade filter will leave some bacteria in the water.

Toilets: Your toilet uses a lot of clean, drinkable water just to empty itself. Unfortunately, there is no neat and cheap way to use grey water for flushing. You don't want to pump grey water into the toilet tank if it is also connected to the house water supply, as it may contaminate your water supply. You also don't want to leave grey water in your tank for very long, lest the bacteria start to stink. (On the plus side, you can chemically treat the water in the tank all you want).

If you are rich and environmentally conscious (great combination!), you may want to simply buy a high-end composting toilet; these are not connected the the house's drainage system, so if you are careful in the soaps you use for dish- and clothes-washing, you can then convert your house's entire wastewater output to grey water. But good composting toilets are both big and expensive. You can also filter and treat all your grey water, and then use it to flush your toilet, but this requires extra plumbing and more expensive filters.

If you are earthy but not rich, you can keep using your standard toilet, but use grey water to flush by simply dumping a bucketful of grey water into your toilet bowl whenever you wish to flush. Not high tech, but effective. You may have to clean your toilet more frequently to keep the grey water residue from growing something unpleasant.

Gardens: Using grey water for gardening is a complex subject; every gardener will have to judge for themselves how clean they want their grey water to be. You can simply drain your sink/shower/washer into your garden, and let the water soak into the ground. This is rather unsanitary, and should not be done if you are eating any of the plants raw or selling them to be eaten. It's fine for flower gardens and landscaping, but still should not be applied by spraying, as airborne droplets may carry bacteria. You can introduce any level of filtering from an old sock over the drain that filters out food particles to a thousand dollar water purification system that will render your water cleaner than the municipal water.

All of the chemicals used to treat grey water are potentially harmful to your plants, although not necessarily deadly. If you are on municipal water you probably have some chlorine in it; this hasn't killed off your plants, but they might do a little better if you filtered out the chlorine. You can get a good chlorine-removing filter for about US$70, but you can't feed raw grey water into this filter. You'll need yet another filter to catch food and grease particles.

Your best bet for watering your garden with grey water is to filter it though a sand filter (you can build it yourself), and then run it under your garden through a modified French drain; bury a perforated pipe in a shallow trench, and cover it with mulch. This water won't be drinkably fresh, but it will be reasonably sanitary, and because it stays underground it won't allow spray to contaminate leaves and fruit. It should not be used to water root vegetables. If you are willing to go to a little (or lot) more expense, you can build a grey water septic tank. A septic tank will clean grey water the same way it cleans the black (toilet) water, and this water can then be used on your garden. (But do not use the same septic tank for grey water and black water!)

Also be warned -- grey water is alkaline, so do not use it on plants that need acidic soils.

Choosing your poisons: Grey water has all kinds of nasty stuff in it. Triclosan is found in many soaps, and breaks down into dioxins and chlorine gas. Some cleaners use boron (borax), which kills plants, or sodium, which is bad both for plants and for soil. You can limit yourself to biodegradable, biocompatible, and natural soaps and detergents, which will not harm any plants (or toilet bowls, for that matter). These are slowly becoming more available, but are still often hard to find, more expensive, or less effective then traditional cleaners. If you don't want to go the rainforest-herb route, you can still adjust your soap usage to be kinder to your soil.
  • Laundry Detergents: Avoid detergents that include sodium and borax. Liquid detergents are usually better than powdered, as sodium is used as a filler in many powdered detergents.
  • Bleach: chlorine bleach or bleach using sodium perborate aren't good for your plants. Use liquid hydrogen peroxide when possible.
  • Hand and Dish Soaps: Most of these are okay. (Shampoo too.) Don't use more than you need to, and avoid those that include triclosan (it will say somewhere on the back label). Remember, you don't need antibacterial soap to wash the dishes or take a shower.
  • Chemicals: Life is full of chemicals, from nail polish remover to used motor oil. Use common sense, and when possible avoid putting anything odd into your grey water.
Heat recovery: Water isn't the only thing going down the drain. When you take a hot shower a lot of heat is lost down the drain, and even if you aren't recycling the water you can still recycle the heat. For a few hundred dollars you can buy a spiffy little section of copper pipe that will transfer heat from the wastewater to the water heater, saving energy and increasing the amount of hot water you can get from the hot water heater at one go. This is essentially a smaller copper pipe wrapped in a spiral around a central, straight shaft. This shaft feeds into the hot water heater, and the shower drains through the spiraling pipe; heat from the wastewater passes through the walls of the pipe and reduces the amount of heat that the hot water heater will need to add to the water to get it up to full temperature.

Gee, Tem, that didn't help me at all! Well, yes. Unless you live in a rather arid climate, you probably aren't going to be recycling your grey water, especially if you don't have a yard. It just takes too much time, planning, and money, and water is still insanely cheap. While your city planners may wish that there was some way to reduce the amount of sewage coming down the pipe, and farmers in your area may dearly wish that all the water wasn't being gobbled up by the cities, the fact is that no one has yet figured out a good and inexpensive way to use grey water recycling in the average household. In the meantime, we should all work on our water conservation.

I would love to see some examples of good grey water usage posted here. You can also /msg me with tips and ideas, and I'll add them to my node.

rootbeer277 reports a toilet top sink from Japan that drains directly into the toilet. It is an excellent invention, and not too expensive either, at US$89 (before shipping). It is a little unergonomic, as you have to reach over the toilet bowl to wash your hands, and it may be of more use in public restrooms than in your home (it would be awkward to brush your teeth or dye your hair in it). It would be perfect for that little half-bath that never has enough room.


Here are some sites to give you ideas for DIY water filters. Do it, I dare you!

Thanks to Oolong for the bathtub-filter ideas!

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