The cleaning solution is constantly screened and re-circulated in the unit and there is no discharge. Eventually, the solution can become saturated with the material being removed and require changing.
Different options are available to prolong solution life.
An oil skimmer can be added to remove tramp oils. Depending on what material is being washed off, some customers never do more that just add make-up water and chemicals to the unit. This depends entirely on the material being removed. Most municipalities will permit discharge of PH-neutral solutions comprised of biodegradable chemicals and having no suspended solids into their disposal systems. Obviously, wastes containing hazardous materials such as heavy metals or toxic compounds require different handling.