<P> Sewage treatment may also be referred to as wastewater treatment, although the latter is a broader term which can also be applied to purely industrial wastewater . For most cities, the sewer system will also carry a proportion of industrial effluent to the sewage treatment plant which has usually received pretreatment at the factories themselves to reduce the pollutant load . If the sewer system is a combined sewer then it will also carry urban runoff (stormwater) to the sewage treatment plant . Sewage water can travel towards treatment plants via piping and in a flow aided by gravity and pumps . The first part of filtration of sewage typically includes a bar screen to filter solids and large objects which are then collected in dumpsters and disposed of in landfills . Fat and grease is also removed before the primary treatment of sewage . </P> <P> The term "sewage treatment plant" (or "sewage treatment works" in some countries) is nowadays often replaced with the term "wastewater treatment plant". </P> <P> Sewage can be treated close to where the sewage is created, which may be called a "decentralized" system or even an "on - site" system (in septic tanks, biofilters or aerobic treatment systems). Alternatively, sewage can be collected and transported by a network of pipes and pump stations to a municipal treatment plant . This is called a "centralized" system (see also sewerage and pipes and infrastructure). </P> <P> Sewage is generated by residential, institutional, commercial and industrial establishments . It includes household waste liquid from toilets, baths, showers, kitchens, and sinks draining into sewers . In many areas, sewage also includes liquid waste from industry and commerce . The separation and draining of household waste into greywater and blackwater is becoming more common in the developed world, with treated greywater being permitted to be used for watering plants or recycled for flushing toilets . </P>

Which is the most common stage of wastewater treatment