Waste water reduction comes from the fact that you've reused half the water from inside your residence, or perhaps more from some commercial types of buildings, thereby keeping out the same volume of water from the sewage treatment plant, reducing that plant's cost of operations, mainly by reducing energy demands, and decreasing chemical discharges into the environment.  If your home is on a septic system, you reduce impacts to your precious leach fields by up to 50%!  This is accomplished by not only removing the sheer volume of water and placing it at the root zones for plant usage, but also by introducing greywater's inherent solids, which are almost exclusively organic, into the upper layer of the soil, which has a high degree of microbial activity.  There, indigenous microbes in the soil ingest those solids, producing mainly fulvic and humic acids, which are literally plant food. 

Some sewer districts have formal rate reduction schedules for our systems, especially in their commercial rate schedules.  All that is usually required for receiving the reduction is a greywater meter and documenting the use for them.  Most residential sewer rate schedules are based on your fresh water usage in the supposedly non-irrigated winter months; with that type of rate schedule, the lower your winter water use is, the lower your year-round sewer bill automatically becomes.  In California, state law requires that the rate they charge must consider the actual amount and type of wastewater they have to treat.