Wastewater is water that has been changed in its properties through estate, trade, agricultural and other usage and drains off during dry weather (sewage) as well as that water that arises from precipitation and drained off and collected from built and sealed areas (storm water). Sewage is also defined as liquids that are discharged and collected from plants that treat/store waste and liquids from waste.
You can find the definition in the Water Act § 54 WHG.