Greywater & Rainwater Systems
"Greywater" is the waste water produced from baths, showers, and clothes washers (as distinguished from "blackwater" which is the output from toilets, dishwashers and kitchen sinks). Today, many municipalities are realizing that harnessing greywater could bring real results in both water and cost savings.
Greywater systems can be tricky to retrofit into existing structures since they require a distinct set of outflow pipes to be separate from blackwater, but it's relatively easy to accomplish in new construction.
Legal greywater systems are usually limited to households of a certain size (those that generate less than 400 gallons per day, for instance) and are usually restricted to underground systems where the water is used to irrigate trees and gardens.
Photo courtesy of GreywaterAction.org
A word of caution: Most commercial bath and cleaning products contain chemicals that can be harmful to plants (and, of course, to humans as well). Even so-called "natural" products can contain harmful ingredients. Households with greywater systems should use only pure, bio-based bath and cleaning products, with no bleach, no dyes, no salts and no toxic chemicals.
Barnstable, MA has an entire webpage dedicated to providing residents with information regarding greywater and greywater disposal systems.
Tempe, AZ offers residents a rebate for the installation of a greywater system.
Oasis Design is the country's leading advocate for greywater systems, and they have lots of educational materials on their site. They also have a Greywater Policy Center with links to model state policies.
Greywater Action has an active website where you can learn how to install simple greywater systems, watch free webinars from leading greywater experts, download presentations and manuals on system design, check out studies on greywater, sign up for a workshop, or post your questions on a forum to get expert advice. (They also have info and presentations in Spanish and Chinese.)
 Duttle, M. (1994) Safe Use of Household Greywater. http://aces.nmsu.edu/pubs/_m/M106/
 Brain, R., et al. (2015). Defining Terms: Greywater, Blackwater and Clearwater (Rep.). Logan, UT: Utah State University, 1-4.
 Pereira, T. (2009). Sustainability: An integral engineering design approach. Renewable and Sustainable Energy Reviews, 13(5), 1133-1137.
 Friedler, E., & Hadari, M. (2006). Economic feasibility of on-site greywater reuse in multi-storey buildings. Desalination, 190(1-3), 221-234.
 Allen, L., et al. (2010). Overview of Greywater Reuse: The Potential of Greywater systems to Aid Sustainable Water Management (Rep.). Oakland, CA: Pacific Institute, 1-41.
 National Academies of Sciences, Engineering, and Medicine, et al. (2016). Using graywater and stormwater to enhance local water supplies: An assessment of risks, costs, and benefits. Washington, DC: National Academies Press, 62-67.