Industrial companies either lead their wastewater into municipal wastewater treatment plants (indirect dischargers) or they treat it in on-site treatment plants and directly discharge into water bodies (so called direct dischargers).
Industrial and trade companies often use highly specified wastewater treatment procedures. In 2004, more than 950 industrial companies discharged wastewater cleaned on site directly into the Rhine. This wastewater may contain easily degradable organic compounds, but sometimes also persistent substances such as halogenated organic compounds or heavy metals.
The thermal stress of waters caused by cooling water inputs and discharges from wastewater treatment plants is a particular case.