San Francisco became the first in the country to require new developments (more than 250,000 square feet) to use onsite water reuse systems for anything that requires non-potable water.

This means that when it comes to the water used in buildings for things such as cooling towers, irrigation and toilet flushing, developers can't use pristine drinking water.

Instead they must implement onsite treatment systems to make use of rainwater, greywater (which includes water from bathroom sinks, bathtubs and washing machines), blackwater (sewage), stormwater or foundation drainage (nuisance groundwater) to meet their needs for non-potable water.

The ordinance also requires smaller buildings of 40,000 square feet or more to do a reuse assessment and in five years the city will need to use non-potable water for all irrigation needs and cleaning of public spaces.

More details here.