Sewage treatment refers to the process of removing microorganisms and other types of contaminants from wastewater. In more developed countries, most domestic waste is collected in a sewer system and sent through pipelines to a central sewage treatment plant. At these plants, sewage treatment in done in a multi-step process that removes or changes different types of material in stages, so that the end product, or effluent, is safe to return into the environment.
All residences, businesses, hospitals, and other establishments which use water, produce wastewater in the form of sewage. Sewage is the collective term for water that drains from toilets, sinks, showers, and liquid industrial waste. A typical sewage treatment process involves pretreatment, as well as primary, secondary, and tertiary treatment stages.
Pretreatment includes the process of removing large objects from sewage to avoid the clogging or damaging of equipment further in the treatment process. Items such as sticks, rags, feminine hygiene products, and even fruit, are removed by a mechanical screening process, and are then usually incinerated or sent to a landfill. Sand and rocks, collectively referred to as grit, are allowed to settle out in a holding tank, where they are swept into a device that collects them, after which they are also sent to a landfill.
Primary treatment operates on much the same principle as the pretreatment and screening processes. Its main purpose is to allow particles to settle out in holding tanks, into a "sludge" which is collected and processed elsewhere. Grease and oils also are separated in this stage, since they are lighter than water and will float to the top to be skimmed off. This allows the sewage after this stage to be dealt with as a whole, since it is more homogeneous.
The purpose of secondary sewage treatment is to break down biological matter that is present in the sewage from sources like human waste and detergents. This is accomplished thorough various related techniques, all of which use bacteria and other helpful microorganisms to break down dissolved biological contaminants. These organisms also help other compounds and materials to precipitate out of the sewage.
To further raise the quality of the effluent before it is discharged, many treatment plants employ tertiary treatment. Further filtration and the removal of nitrogen and phosphorous constitute this step. Tertiary treatment often includes disinfection, especially in developed countries. This is usually accomplished either through chlorination or by treating the water with ozone or ultraviolet light, all of which have the effect of eliminating harmful bacteria and other organisms before the water is returned to the environment through a river, ocean, or other avenue.