Waste Water treatment is not one process, but rather a series of biological and chemical treatments that aim to render the water as benign as possibly within the economic limitations of the process. It is possible, through ultra-filtration and reverse osmosis to remove everything that is not water from the water, however it is slow and expensive. The same result can be achieved by evaporation and this is the chosen technology of ours.
Simply simmering the waste water and continuously drawing off the vapour allowing more evaporation effectively separates the water from everything else. This leaves the contaminants concentrated in a small volume and the clean vapour can be condensed back into water, or released into the air as humidity.