There are two main types of domestic waste water, black water and grey water. The main difference between the two are bacteriological levels and presence of human pathogens in black water. Meaning, black water comes from your toilet and grey water comes from the other drains. Grey water still contains the same pathogens, it just typically has much fewer of them. Sometimes grey water is used to refer to water of almost drinking quality, like collected rain or snow melt water, I prefer to call that raw water.
There are many regulations on uses of grey water. A lot of places make no distinction between them because improperly handled grey water can become black water in less than 24 hours. If you live somewhere that allows grey water recycling, find out what the approved uses are and obtain any permits you need. If you live somewhere that has an abundance of water, you may spend more on the new system than you will save. Grey water recycling gets it’s most benefit where there is a strong need for irrigation, and a short supply of fresh water.
The goal of grey water recycling is to take advantage of the nutrients present before it is mixed with the pathogens in the black water. The other usefu part of grey water is the heat. Plumbing you grey water system to send grey water around (but not connected to) the inlet to your hot water tank will save you some money.
The basic components of a grey water system are the collection drains and plumbing and a distribution system to a biological treatment receptor.
The biological system can be as simple as a field of grass or a pile of mulch and as complicated as constructed wetland.
The distribution system can be one distribution point or a complicated branch design feeding specific plants and areas. Your regional and site characteristics will shape a lot of your options. As will climate, hot arid areas will not be able to have a constructed wetland. In cold climates preventing the system from freezing becomes critical and a solar greenhouse may be the only option.
In general using less water is a good idea. If you want to reduce your ecological footprint grey water recycling can help. I have seen places in arid areas that recycle all water black and grey because the supply is so low. And if treated properly black water can be cleaner then many natural sources.
Some things that should never be done with grey water. Never spray it into the air, like from a sprinkler. Pathogens can become aerosolized and infectious. Also never spray grey water on plants that are going to be eaten directly like berries or apples. Also do not spray it directly on the surface of your lawn as you could also get sick from it if you use your lawn for recreation. When installing any pressurized plumbing be careful not to accidentally connect it to your potable water plumbing, and if the pressure comes from a pump (it most likely will) make sure you haven’t just switched water waste for electrical waste.
To recap, recycling grey water can be beneficial for a self-reliant and environmentally conscious person. Assuming it is legal in your jurisdiction, there are numerous design options to choose from. Choose the one that best suits your needs. Then enjoy the fruits of your labor.