So far, the discussion on efficient and sustainable use of water has focused on conserving water use. Another approach is to reuse or recycle water. Reusing water is a sustainable practice that has all the same benefits of using less water. Recycling water is the term used to describe the practice of treating wastewater, usually in a centralized location, and then using the water for a variety of purposes, including landscaping. Graywater describes the practice of collecting household water from drains and then reusing it onsite for landscaping irrigation or toilet flushing.

The interest in graywater is growing, particularly in the southwestern and western United States where water supplies are limited. Effective use of graywater reduces the demand for treated drink­ing-quality water and decreases the water going into wastewater treatment facilities.

The model 2012 International Residential Code (IRC) details the design of graywater recycling systems in the section on Sanitary Drainage. Some local codes now require new residential con­struction to include connections for graywater plumbing. According to the 2012 IRC, discharge water may be collected from bathtubs, showers, lavatories, clothes washers, and laundry trays for a graywater system. The water collected is then used for flushing toilets and urinals or landscape irrigation.

Including a graywater system in a bathroom design can challenge a designer. The system needs to be correctly sized. There are requirements for additional plumbing pipes. A collection reservoir or storage tank is needed. While the opportunity for implementing an important sustainable practice is exciting, advanced and careful planning is needed. Familiarity with all applicable codes and/or permits, and perhaps additional expert advice may be needed.

An innovative design for water recycling and graywater use combines a small hand-washing lava­tory directly over the toilet tank (see Figure 3.10). After hand-washing, the water flows into the toilet tank, to be used for the next flush of the toilet.