Greywater-harvesting stub-outs are built-in plumbing connections allowing for easy future access to a home’s drain-water stream…
Incorporating greywater-harvesting drains/stub-outs at the time of construction (or major renovation) of a home is generally the easiest and least expensive time to do so. That way you don’t need to later cut and jack-hammer through walls and floors to get to the plumbing. See (figs. 1A, B, C).
The following is adapted from, and meant as a supplement to, chapter 12 of Rainwater Harvesting for Drylands and Beyond, Volume 2, 2nd Edition
Ideally the upper end of the stub-out is located for easy and convenient access beside the greywater source, such as inside the bathroom beneath the sink, rather than an inconvenient placement outside or under a solid floor. The lower/downstream end of the stub-out pipe is routed to sections of yard that have ample space to install water-harvesting basins. Stub-outs remain capped until a greywater distribution system is set up in the landscape, then they can be connected to the drain plumbing and the greywater can be put to use.
Often it is ideal if the stub-outs are connected with a three-way valve, or a multi-drain system for washing machines (no valve needed), (see chapter 12 of Rainwater Harvesting for Drylands and Beyond, Volume 2, 2nd Edition), enabling the user to direct the greywater to the landscape or the sewer/septic depending on what goes down the drain, the weather, or the saturation of the soil in the landscape.
Greywater lines originating inside a home should be diverted downstream from P-traps and vents to inhibit the entry of insects, vermin, or odors into the house via greywater lines. Three-way diverter valves can often be installed as high in the system’s flow as the vent/sewer stack, downstream of the P-trap.
Greywater lines must be diverted upstream of toilet sewage flow. Flushed toilet water is blackwater or sewage, and must never mix with the greywater you use in your landscape.
Existing homes lacking stub-outs can typically be retrofitted fairly easy where interior sinks are against exterior walls, with cooler-bleed-off harvesting, with a multi-drain or laundry-to-landscape system for washing machines, and with outdoor showers (all described in Rainwater Harvesting for Drylands and Beyond, Volume 2, 2nd Edition). See what works for you, your site, and your lifestyle. You don’t have to do it all, just start somewhere. Anything is an improvement, and the more you do, the better!
See here for some of the City of Tucson’s resources/illustrations on how stub-outs might be designed and installed in new-home construction. See here for the City of Tucson’s greywater-harvesting stub out ordinance
See chapter 12 of Rainwater Harvesting for Drylands and Beyond, Volume 2, 2nd Edition available at deep discount direct from the author