Zero discharge water: The principle of ‘zero discharge’ is recycling of all industrial wastewater. This means that wastewater will be treated and used again in the process. Because of the water reuse wastewater will not be released on the sewer system or surface water.

Zeta potential: An electrokinetic measurement which can be used for the control of coagulation processes.

Zone of aeration: A region in the Earth above the water table. Water in the zone of aeration is under atmospheric pressure and will not flow into a well.

Zone of saturation: The space below the water table in which all the interstices (pore spaces) are filled with water. Water in the zone of saturation is called groundwater.

Zooplankton: Tiny aquatic animals eaten by fish.

Zwitter ions: Act as cations or as anions according to the environment in which they find themselves. In water technology they are usually organic macromolecules.