Wastewater is water that has been used so that it is no longer potable. Wastewater can be classified as graywater or blackwater. Rainwater and stormwater are other types of non-potable water. See definitions.

Waterless urinals use no water, but instead replace the water flush with a specially designed trap that contains a layer of buoyant liquid that floats above the urine layer, blocking sewer gas and urine odors from the room.

Wave and Tidal Power Systems capture energy from waves and the diurnal flux of tidal power. The captured energy is commonly used for desalination, water pumping, and electricity generation.

Wind power is a form of renewable energy generated by wind spinning turbines.