Sewage Wastewater is a complex mixture of natural inorganic and organic material mixed with man-made substances. It contains everything discharged to the sewer, including material washed from roads and roofs, and of course where the sewer is damaged groundwater will also gain entry. It is this complex mixture that ends up at the wastewater treatment plant for purification. The strength and composition of sewage changes on an hourly , daily and seasonal basis.