Sewage treatment is a type of wastewater treatment which aims to remove contaminants from sewage to produce an effluent that issuitable for discharge to the surrounding environment. It can also be intended for reuse application thereby preventing water pollution from raw sewage discharges. Sewage contains wastewater from households and businesses and possibly pre treated industrialwastewater. There are large number of sewage treatment processes to choose from. These can range from decentralized systems to largecentralized systems involving a network of pipes and pump stations which convey the sewage to a treatment plant. The sewers can alsocarry urban runoff to the sewage treatment plant in cities that have a combined sewar. A larger number of sewage treatment technologieshave been developed. Very broadly they can be grouped into high tech versus low tech options although some technologies might fall intoeither category. To decide which sewage treatment process to choose engineers and decision makers need to take into account technical and economical criteria as well as quantitative and qualitative aspects of each alternative. Often the main criteria for selection aredesired effluent quality or expected construction and operating costs along with availability of land and energy requirements andsustainability aspects. For example, broadly speaking the activated sludge process achieves a high effluent quality but is relativelyexpensive and energy intensive compared to waste stabilization ponds which are a low cost treatment option but require a lot of land. Indeveloping countries countries and in rural areas with low population densities sewage is often treated by various onsite sanitation systems and notconveyed in sewers. These systems include septic tanks connected to drain fields or onsite vermifilter systems and many more. Anadvanced fairly expensive sewage treatment plant in a high income country may include three levels of treatments. Primary treatment toremove solid material. Secondary treatement to digest dissolved and suspended organic material.