Using grey water for many of the water needs in a home or garden that don't require potable water can help reduce stresses on water supplies, lower home water usage and costs, and support a thriving landscape.
If you just crawled out from under a rock, you can be forgiven for not knowing that fresh water is a precious resource, and that using it, and reusing it, wisely, is a key component of trying to live more sustainably. And with the extended drought conditions in many parts of the US, as well as in many other parts of the world, water issues are one of our most pressing challenges.
One effective strategy lies in reducing the overall demand for fresh water, through water-wise landscaping, rainwater harvesting, water-efficient appliances and fixtures, and implementing manufacturing processes with lower water needs than conventional methods. But even after reducing water usage initially, a lot of the water that goes down the drain is essentially wasted, and could be put to good use as grey water in the home and yard.
Grey water, which is the water that comes out of the drains of showers, baths, sinks, and washing machines, is distinctly different from black water, which is what gets flushed down the toilet. Grey water can be used for watering houseplants, landscaping, or even flushing the toilet, so it's a resource we can use twice. The problem is that our modern plumbing doesn't distinguish between the two, but instead combines them and sends onward as sewage, so unless we manually divert or capture it, grey water essentially becomes black water, rendering it useless until it goes through the municipal water treatment process.
Not all grey water is the same, as the water coming from the kitchen sink or dishwasher can contain a lot of organic matter and has the potential for harboring pathogens (and kitchen sink water, under some codes, is actually considered black water and is not to be used), but a bathroom sink or tub often has minimal amounts of organic matter and soap residue. However, with the proper system, such as a biofilter or mulched basin, using grey water from the kitchen sink is an accepted practice.
Catch warm-up water
Before we talk about how to reuse grey water, there's one great way of putting potentially wasted water to work, without worrying about health or safety from contaminants, and that's by capturing and using 'warm-up' water. Warm-up water is all of that water that goes down the drain as you're waiting for the hot water to reach the faucet or shower, which can be quite a bit if your water heater is located far away from the point of use. To capture and use warm-up water, simply put a bucket or large bowl under the faucet when you turn on the hot water, move it out of the way when the water is hot. That water can be used to directly water house or garden plants, as it's just as clean as the rest of the tap water that gets used for those tasks.
How to reuse grey water
[It's important to remember that the rules and regulations about grey water reuse vary quite a bit, so depending on your location, some or all of these ways of reusing grey water may be illegal. In addition, improper management of grey water could lead to odor, pest, or pathogen issues, so it's important to do your homework before putting in any sort of grey water system.]
Grey water from bathroom sinks: Unless you're willing to (and are allowed to, according to local building and health codes) replumb your sink drains into a grey water system, you'll need to either manually bail out full sinks into buckets for use in the landscape or for house plants, or remove the J-trap from beneath the sink and replace it with a bucket to catch the grey water. If you choose to remove the trap and put a bucket under the sink drain instead, you'll need to be really diligent about checking the level of water so it won't overflow onto the floor. This grey water can be used to flush the toilet (pour it into the bowl, not the toilet tank), or to water house plants or outdoor plants or trees, as long as no harsh soaps or detergents are used.
Grey water from the bathtub: If you are serious about integrating grey water reuse into your home, the shower/tub drain is a great place to start, but will require some plumbing work to install a 3-way valve, so drain water can either be sent to a grey water system or directly to the sewer line (which can be desirable during very cold weather, or if you have periods of high water use that will overload a small grey water landscape system). However, even if you don't want to put a lot of work into a grey water system for the bath, you can still reuse the water from showers (assuming you are OK with plugging the drain and letting the bath fill up) or baths, but you'll need to use a bucket to manually bail out the bathtub. If you'd like a system that's still less work than installing a diverter valve and the plumbing to support it, a sump pump and some tubing can be set up to pump the bath water out a window into a barrel after each bath, which can then be used for watering landscaping.
Grey water from laundry: Assuming you don't use harsh laundry detergent or chlorine bleach in your washing machine, a laundry-to-landscape system can be an effective method of reusing grey water to water trees or other landscape plants. At its simplest, removing the discharge hose from the washing machine from the house drain, and connecting it to a longer run of hose that can reach your yard, will enable you to reuse grey water every time you do the laundry. This method has to be manually managed (or you run the risk of creating a bog in your yard from always discharging the water in the same spot) by moving the hose to a different spot with each load of laundry. Care also has to be taken to not let the water that exits the hose from gouging holes in the lawn or plant beds, which can be mitigated by building mulched grey water basins that can deal with that volume of water. The direct method does have issues, as it could lead to burning out the washer pump, or could accidentally back-siphon dirty water into the washing machine. A far better method, which is too extensive to cover here, is laid out at Oasis Design, which has a lot of other great info on grey water implementation.
The ideal method of grey water reuse requires some thought, as a level of filtration or settlement, as well as a method of discharging it either below the surface or in a location where it can slowly filter through the soil without human contact, may be required. If you'd like to implement a larger-scale grey water system at your home, be sure to thoroughly evaluate your options, consult with a grey water professional, and consider investing in a system that complies with local regulations and is appropriate for your needs and your landscape.
Helpful grey water resources: