Sewage treatment

Sewage treatment is a type of <a href="/facts/Wastewater_treatment/b5bJ6aJm">wastewater treatment</a> which aims to remove <a href="/facts/Contaminants/RC0L7KRy">contaminants</a> from <a href="/facts/Sewage/aC5N1kls">sewage</a> to produce an <a href="/facts/Effluent/HPS1Akbo">effluent</a> that is suitable to discharge to the surrounding environment or an intended reuse application, thereby preventing <a href="/facts/Water_pollution/SCUhUYqB">water pollution</a> from raw sewage discharges. Sewage contains <a href="/facts/Wastewater/g6Bn8xvT">wastewater</a> from households and businesses and possibly pre-treated <a href="/facts/Industrial_wastewater_treatment/401ph19H">industrial wastewater</a>. There are a large number of sewage treatment processes to choose from. These can range from <a href="/facts/Decentralized_wastewater_system/tlyDwRgF">decentralized systems</a> (including on-site treatment systems) to large centralized systems involving a network of pipes and pump stations (called <a href="/facts/Sewerage/bmbN015l">sewerage</a>) which convey the sewage to a treatment plant. For cities that have a <a href="/facts/Combined_sewer/UUxl1lwB">combined sewer</a>, the sewers will also carry <a href="/facts/Urban_runoff/GEvrwc1f">urban runoff</a> (stormwater) to the sewage treatment plant. Sewage treatment often involves two main stages, called primary and <a href="/facts/Secondary_treatment/t4yq9AL5">secondary treatment</a>, while advanced treatment also incorporates a tertiary treatment stage with polishing processes and nutrient removal. Secondary treatment can reduce organic matter (measured as <a href="/facts/Biochemical_oxygen_demand/8vDcugIo">biological oxygen demand</a>) from sewage,  using aerobic or anaerobic biological processes. A so-called quaternary treatment step (sometimes referred to as advanced treatment) can also be added for the removal of organic <a href="/facts/Micropollutant/CMqzAhcH">micropollutants</a>, such as pharmaceuticals. This has been implemented in full-scale for example in Sweden.
A large number of sewage treatment technologies have been developed, mostly using biological treatment processes. Design engineers and decision makers need to take into account technical and economical criteria of each alternative when choosing a suitable technology.: 215  Often, the main criteria for selection are desired effluent quality, expected construction and operating costs, availability of land, energy requirements and <a href="/facts/Sustainability/9oYH9mAs">sustainability</a> aspects. In <a href="/facts/Developing_country/eTfS4kxl">developing countries</a> and in rural areas with low population densities, sewage is often treated by various <a href="/facts/Sanitation/NLGqBs2T">on-site sanitation</a> systems and not conveyed in sewers. These systems include <a href="/facts/Septic_tank/hHgN3sM3">septic tanks</a> connected to <a href="/facts/Septic_drain_field/f2GtQ1cV">drain fields</a>, <a href="/facts/Onsite_sewage_facility/55CQ0Kb7">on-site sewage systems</a> (OSS), <a href="/facts/Vermifilter/lEJvMn6J">vermifilter</a> systems and many more. On the other hand, advanced and relatively expensive sewage treatment plants may include tertiary treatment with disinfection and possibly even a <a href="/facts/Fourth_treatment_stage/bqLgnLnJ">fourth treatment stage</a> to remove micropollutants.
At the global level, an estimated 52% of sewage is treated. However, sewage treatment rates are highly unequal for different countries around the world. For example, while <a href="/facts/World_Bank_high-income_economy/KTrB8swr">high-income countries</a> treat approximately 74% of their sewage, developing countries treat an average of just 4.2%.
The treatment of sewage is part of the field of <a href="/facts/Sanitation/NLGqBs2T">sanitation</a>. Sanitation also includes the management of <a href="/facts/Human_waste/FkBVmLgc">human waste</a> and <a href="/facts/Solid_waste/x1T17ne7">solid waste</a> as well as <a href="/facts/Stormwater/CX4EcIWu">stormwater</a> (drainage) management. The term sewage treatment plant is often used interchangeably with the term wastewater treatment plant.

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Sewage treatment