 water reuse system design is the cycling of grey LOW-IMPACT SYSTEMS water from the shower/bath and laundry into a reservoir for use on demand by the toilet. Excess grey water is routed to a disposal system (which could be an irrigation system). Depending on the flush volume of the toilet and the frequency of use, homes without laundry facilities may not generate enough shower/bath water to service the toilet. In this case, fresh water may have to be provided as make-up water. At this point, it is instructive to talk briefly about cross connec- tions. Where a fresh water source comes into direct contact with a reservoir of not-so-fresh water, a cross connection can occur. The problem is this: suppose you put a valve in the toilet reservoir to admit grey water and another valve to admit fresh water when the grey water is not available. Now suppose that the toilet reservoir is three-quarters full of grey water when the fresh water valve opens to make up the rest of the water needed in the reservoir. Then a fire hydrant is opened up down the street or the automatic washing machine lets water into itself. There could be a brief negative pres- sure at the fresh water valve in the toilet reservoir, sucking the grey water from the reservoir into the fresh water pipes. Such cross connections have caused disease outbreaks in large apartment houses as well as single residences. Whether you do the plumbing yourself, or whether you hire a plumber, be sure that there is no way that a fresh water supply pipe, hose, or valve can come into contact with contaminated water. As an aside: perhaps the most common cross connection is when people add water to a bucket of pesticide or other chemical by putting a garden hose into the mix- ture while adding water. Now back to the grey water system design. Table 7.4 shows some average volumes of liquids associated with grey water systems. A fresh water supply is generally needed where there is no laundry. And where there are a number of frequently used grey water pro- ducing appliances, no