Sewage treatment is a type of wastewater treatment which aims to remove contaminants from sewage to produce an effluent that is suitable 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 pretreated industrial wastewater. There are large number of sewage treatment processes to choose from. These can range from decentralized systems to large centralized systems involving a network of pipes and pump stations which convey the sewage to a treatment plant. The sewers can also carry urban runoff to the sewage treatment plant in cities that have a combined sewer. A large number of sewage treatment technologies have been developed. Very broadly they can be grouped into high tech versus low tech options although some technologies might fall into either 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 are desired effluent quality or expected construction and operating costs along with availability of land and energy requirements and sustainability aspects. For example, broadly speaking the activated sludge process achieves a high effluent quality but is relatively expensive and energy intensive compared to waste stabilization ponds which are a low cost treatment option but require a lot of land. In developing countries and in rural areas with low population densities sewage is often treated by various onsite sanitation systems and not conveyed in sewers. These systems include septic tanks connected to drain fields or onsite vermifilter systems and many more. An advanced fairly expensive sewage treatment plant in a high income country may include three levels of treatments. Primary treatment to remove solid material. Secondary treatment to digest dissolved and suspended organic material. Tertiary treatment to remove the nutrients such as nitrogen and phosphorus for disinfection and possibly even a fourth treatment state to remove micro pollutants.However, sewage treatment rates are highly unequal for different countries around the world. The treatment of sewage is part of the field of sanitation. Sanitation includes the management of human waste and solid waste as well as storm water management. The term sewage treatment plant is nowadays often replaced with the term wastewater treatment plant or wastewater treatment station. Strictly speaking the latter is a broader term that can also refer to industrial wastewater. The aim of treating sewage is to prevent water pollution. Improving sewage treatment across the globe is crucial for achieving water quality improvements. Sewage treatment removes the contaminants from sewage to produce liquid and solid suitable for discharge to the environment or for reuse. It is a form of waste management. Sewage treatment results in sewage sludge which requires sewage sludge treatment before safe disposal or reuse. Under certain circumstances the treated sewage sludge might be termed bio solids and can be used as a fertilizer. Therefore, sewage treatment can also help agriculture. With regards to biological treatment of sewage the objectives are more specific such as transform dissolved and particular biodegradable components into acceptable end products or incorporate colloidal solids into a biological film. It can transform and remove nutrients and, in some cases, trace organic constituents.