A sewage treatment plant (STP) is a facility that cleans sewage — the wastewater from homes, offices and industries — before it is released back into rivers, lakes or the ground. Sewage contains suspended solids, organic matter, harmful microorganisms and dissolved impurities. If untreated sewage is released into water bodies, it pollutes the water and spreads disease. In a sewage treatment plant, the wastewater passes through several stages — preliminary, primary, secondary and (sometimes) tertiary treatment — that gradually remove these impurities so that the treated water is safe to discharge or reuse.
A sewage treatment plant (STP) cleans wastewater before it is released into the environment.
Sewage contains suspended solids, organic matter, microbes and dissolved impurities.
Treatment has stages: preliminary, primary, secondary and tertiary.
Primary treatment is physical (settling); secondary treatment is biological (microbes).
Aerobic bacteria break down organic matter in aeration tanks during secondary treatment.
Sludge is digested to produce biogas and can be used as manure.
Treated water is disinfected and then discharged or reused for irrigation.
Sewage is the used (waste) water from houses, public buildings and industries. It is a complex mixture containing: • Suspended solids and grit • Organic matter (food waste, human waste) • Harmful microorganisms (bacteria, viruses) • Dissolved salts and nutrients (nitrogen, phosphorus) • Oils and detergents
The process of removing these impurities from sewage is called sewage treatment or wastewater treatment, and it is carried out in a sewage treatment plant (STP).
Preliminary treatment: Large floating objects and grit are removed by passing sewage through bar screens and grit chambers.
Primary treatment (physical): The sewage is sent to settling tanks (sedimentation tanks) where solids settle down as primary sludge and oils/grease float on top and are skimmed off. This removes a large part of the suspended solids.
Secondary treatment (biological): The water is treated with the help of microorganisms (bacteria) that feed on the remaining organic matter. Air is supplied (in aeration tanks) so that aerobic bacteria break down the organic waste. The microbes form 'activated sludge', which is later settled out.
Tertiary treatment (advanced): The water is further cleaned by removing remaining nutrients, microbes and dissolved impurities, often by chemical treatment, filtration and disinfection (such as chlorination or UV).
Sludge: The solid waste (sludge) collected from the settling tanks is treated separately — often in digesters where anaerobic bacteria break it down and produce biogas (which can be used as fuel). The dried sludge can be used as manure.
Treated water: The cleaned (treated) water is disinfected to kill remaining germs and is then: • Released into rivers, lakes or the sea, or • Reused for irrigation, gardening, industrial cooling or flushing.
This protects water bodies from pollution and helps in water conservation.
Sewage treatment has four main stages: (1) Preliminary treatment — removing large objects and grit with screens; (2) Primary treatment — settling out suspended solids in sedimentation tanks; (3) Secondary treatment — using microorganisms to break down organic matter biologically; and (4) Tertiary treatment — advanced cleaning to remove remaining nutrients and microbes, followed by disinfection.
Primary treatment is a physical process in which suspended solids settle down in sedimentation tanks and oils are skimmed off. Secondary treatment is a biological process in which aerobic bacteria, supplied with air, break down the dissolved and remaining organic matter to form activated sludge that is then settled out.
The sludge (settled solids) is treated separately, usually in digesters where anaerobic bacteria break it down. This process produces biogas, which can be used as a fuel, and the dried sludge can be used as manure.
Sewage treatment is important because untreated sewage pollutes rivers and lakes, kills aquatic life and spreads water-borne diseases. Treating sewage removes solids, organic matter and harmful microbes, so that the water is safe to release into the environment or to reuse for irrigation, which also helps conserve water.
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