In most plants the cleaning agents are circulated until their cleaning power has eventually decreased and reached the maximum tolerable contaminant level. In order to delay the necessary bath exchange as much as possible there are sophisticated treatment attachments in use, removing contaminants and the used up agents from the system. At the same time fresh cleaning agents or parts thereof have to be supplemented, which requires a bath control. The latter is more and more facilitated online and thus allows a computer aided adjustment of the bath. With the help of oil separators, demulsifying agents and evaporators aqueous processes can be conducted 'waste water free'. Complete exchange of baths becomes only necessary every 3 to 12 months.