Water that is polluted by humans, through residential, industrial or commercial activity, is called wastewater. These pollutants are often toxic substances and a hazard to the health of the people who consume or use it. It’s a hazard to the environment if left to sink into the ground. This gives rise to the need for treatment of wastewater, in order to remove the pollutants from water, and make it safe for consumption, and use. It must be treated even if merely let out in the environment, say to sink into the ground or to mix with sea water.