A sanitary sewer (also called a foul sewer) is a type of an underground carriage system, (the 'system of sewers' is called sewerage), for transporting sewage from houses or industry to treatment or disposal. In some areas, sanitary sewers are separate sewer systems specifically for the carrying of domestic and industrial wastewater, and are operated separately and independently of storm drains, which carry the runoff of rain and other water which wash into city streets. Sewers carrying both sewage and storm water together are called combined sewers.

In the twenty-first century developed world, sewers are usually pipelines that begin with connecting pipes from buildings to one or more levels of larger underground trunk mains, which transport the sewage to sewage treatment facilities. Vertical pipes, called manholes, connect the mains to the surface. The manholes are used for access to the sewer pipes for inspection and maintenance. They also facilitate vertical and horizontal angles in otherwise straight pipelines. Sewers are generally gravity powered, though pumps may be used if necessary. The most commonly used sanitary pipe is SDR-35 (standard dimension ratio), with smaller sized laterals interconnected within a larger sized main

Sewage is water-carried wastes, in either solution or suspension that is intended to flow away from a community. Also known as wastewater flows, sewage is the used water supply of the community. It is more than 99.9% pure water and is characterized by its volume or rate of flow, its physical condition, its chemical constituents, and the bacteriological organisms that it contains. Depending on their origin, wastewater can be classed as sanitary, commercial, industrial, agricultural or surface runoff.

The spent water from residences and institutions, carrying body wastes, washing water, food preparation wastes, laundry wastes, and other waste products of normal living, are classed as domestic or sanitary sewage. Liquid-carried wastes from stores and service establishments serving the immediate community, termed commercial wastes, are included in the sanitary or domestic sewage category if their characteristics are similar to household flows. Wastes that result from an industrial process or the production or manufacture of goods are classed as industrial wastes. Their flows and strengths are usually more varied, intense, and concentrated than those of sanitary sewage. Surface runoff, also known as storm flow or overland flow, is that portion of precipitation that runs rapidly over the ground surface to a defined channel. Precipitation absorbs gases and particulates from the atmosphere, dissolves and leaches materials from vegetation and soil, suspends matter from the land, washes spills and debris from urban streets and highways, and carries all these pollutants as wastes in its flow to a collection point.

Wastewater from all of these sources may carry pathogenic organisms that can transmit disease to humans and other animals; contain organic matter that can cause odor and nuisance problems; hold nutrients that may cause eutrophication of receiving water bodies; and can lead to ecotoxicology. Proper collection and safe, nuisance-free disposal of the liquid wastes of a community are legally recognized as a necessity in an urbanized, industrialized society^.

In many European countries, citizens are obliged to connect their home sanitation to the national sewerage system (where possible). This has resulted in large percentages of the population being connected to a sewerage system. For example, the Netherlands have 99% of the population connected to the sewerage system, and 1% has an individual excrement disposal system (eg septic tank,). Others have slightly lower (though still substantial) percentages; eg 96% for Germany and 86% for Belgium.

Of these countries connected to the sewerage system however, purification of the sewerage is not always fully implemented. For example, in Belgium, only 64% of the sewerage is actually purified.

Given the great financial costs of a sewerage system however, much of the excrement is not efficiently (and sometimes not at all) recycled, and (in case no purification is implemented) great environmental damage may occur using this system. Many people have thus opted to discontinue the setup of the system in nations/areas where no sewerage has yet been build. In addition, even in nations where the system has been set up, calls have been made to keep the system to drain rainwater, yet implement composting toilets to process the faeces.

 http://en.wikipedia.org/wiki/Sewer

 http://en.wikipedia.org/wiki/Sanitary_sewer