The legal and most common sources of greywater are the showers, tubs, bathroom sinks, and clothes washer. These sources account for over half of all the water used inside a residence. Other legal sources of greywater are the tail water from non-salt generating water conditioners, Jacuzzis, and utility basins, basically anything but kitchen and toilet water. Never include salt water brine from an ion-exchange water softener.
The most cost effective sources of greywater are the regularly used showers, tubs, and clothes washer, though reverse osmosis systems waste a lot of very good water that can be used in our systems. While you may have many bathrooms in your home, usually only those producing greywater on a regular bases are cost effective to include. If one of your extra bathrooms receives guests more often than other guest bathrooms, it might pay to include its shower/tub. If your home is in the middle of the desert relying on a well that may go dry, plumbing every legal source is probably cost effective for you.
INSTALLATION NOTE: It is always more cost effective to state on your original plumbing plan that you want certain drains separated into a greywater main line, with that mainline taken to where you want the ReWater filter system to sit, and that filter system to be attached there with an overflow to the sewer, and a vent. You should also state you need at least a 1” (ideally 2”) fresh water line brought to that filter location. If you do not state all this on your original plumbing plan, you’ll later be charged for a “change order” by your plumber, and the exact same work will cost you many times more than it would had you included it originally.
Whatever wastewater code you use locally determines the mechanical composition of the plumbing to and from the greywater system itself. In California, you will use the California Plumbing Code (CPC), which is based on the Uniform Plumbing Code. There may a local amendment. Note that the CPC differs from the UPC. In some other states, the UPC is the governing code for the plumbing, but there may be a local amendment as well.
You must install a reduced pressure principle device (RP device) on the fresh water source that supplies water to the filter system, or if in the City of Los Angeles or unincorporated Los Angeles County, an air gap on that supply pipe. This water is used to backwash the sand filter and to provide supplemental water for irrigation, should there not be enough greywater to fulfill the entire irrigation schedule on any given day, like when you go on vacation. Without the RP device or air gap, there is a chance of greywater contacting your fresh water supply in the event of a major upstream water supply disruption that causes a reverse flow in your water lines.
The picture to the right is of a surge tank (foreground) and filter vessel (behind) in a basement. The black ABS pipes are unpressurized and the white PVC pipes will be pressurized while is use. Displacing a filter from the surge tank requires extending all those pipes (and some wiring too).