What is wastewater and where does it come from? Wastewater (or sewage) is dirty, used water that goes down the drains and toilets of homes, schools, businesses and factories.
What is screening? When wastewater first arrives at a wastewater treatment works, it passes through metal bar screens. These screens trap large objects (like rags, papers and plastics) but allow the rest of the wastewater to pass through.
Grit Removal (1.6MB)
How is the grit removed? After passing through the bar screens, the grit (stones, sand and soil) is removed in the grit chamber.
Primary Settling (1.5MB)
What happens in the primary settling stage? The wastewater flows very slowly in large tanks. This allows the fine solids (from human wastes) and some of the germs to settle to the bottom of the tank forming a thick grey layer of sludge (primary sludge).
Biological Treatment (1.5MB)
What is biological treatment? This is a process that uses a special mixture of tiny organisms to feed on the dissolved nutrients in the wastewater.
Secondary Settling (1.5MB)
What happens in the secondary settling stage? The tiny organisms that feed on the nutrients in the bioreactors, and some of the germs from human waste naturally cluster together and form heavy clumps.
What is disinfection? This involves the use of chemicals like chlorine or ozone gas, or exposure to ultraviolet (UV) light, to kill tiny living organisms in water.
Where does the water finally go? After disinfection, the water is put into a river. In this way it is returned to the natural water cycle.