All humans rely on safe drinking water. Salt can be removed from sea water to make it safe to drink. Waste water must be treated before being released into the environment.
Waste water from homes, industry and agriculture must be treated before being released into the environment. Here are some examples of pollutantA toxic chemical or object that causes damage to the land, air or water. which might be present in waste water:
human waste contains harmful bacteriaSingle-celled microorganisms, some of which are pathogenic in humans, animals and plants. Singular is bacterium. and high levels of nitrogen compoundA substance formed by the chemical union of two or more elements. which can harm aquaticWithin water. ecosystemA community of animals, plants and microorganisms, together with the habitat where they live.
industrial waste water may contain harmful chemicals such as toxicPoisonous. metal compounds
agricultural waste water may contain fertiliserA nutrient added to the soil to increase the soil fertility. or pesticideA chemical used to kill pests, such as the potato cyst nematode which is a pest that destroys potato crops. which can disrupt sensitive ecosystems
Sewage treatment involves the following steps:
screening and grit removal to remove large particles
sedimentation allows tiny particles to settle out from still water, which produces sewage sludge and effluent (the liquid which remains on top)
the sewage sludge is digested anaerobically by specific bacteria
the effluent is treated with aerobic bacteria to reduce the volume of solid waste
