Wastewater from households and different industries represent a significant pressure on the environment and treatment is normally required before discharge. The indicator assesses the proportion of wastewater that undergoes different (primary, secondary and tertiary) levels of treatment. It includes the volume of wastewater treated at public wastewater treatment plants, industrial wastewater treatment plants and by independent wastewater treatment systems.

For treated wastewater from households (sometimes mixed with industrial wastewater in a public collecting system) to be considered acceptable it should undergo at least secondary treatment either at a public wastewater treatment plant, an independent wastewater treatment plant or in an industrial wastewater treatment plant (where usually the industrially generated wastewater is dominating).

Industrial wastewater needs to undergo a treatment process which is to remove the specific pollutants generated by the production process to a limit which does not negatively affect the aquatic environment or human uses (in the case of direct discharges), or allows a proper treatment together with wastewater originating from household activities in a public wastewater treatment plant (indirect discharges).