Sewage (or domestic sewage, domestic wastewater, municipal wastewater) is a type of wastewater that is produced by a community of people. It is typically transported through a sewer system.[1]:175 Sewage consists of wastewater discharged from residences and from commercial, institutional and public facilities that exist in the locality.[2]:10 Sub-types of sewage are greywater (from sinks, bathtubs, showers, dishwashers, and clothes washers) and blackwater (the water used to flush toilets, combined with the human waste that it flushes away). Sewage also contains soaps and detergents. Food waste may be present from dishwashing, and food quantities may be increased where garbage disposal units are used. In regions where toilet paper is used rather than bidets, that paper may be added to sewage rather than placed with municipal solid waste. Sewage may contain micro-pollutants and pollutants from industrial wastewater.

Raw sewage arriving at a sewage treatment plant in Syria

Sewage usually travels from a building's plumbing either into a sewer, which will carry it elsewhere, or into an onsite sewage facility. Collection of sewage of several households together usually takes places in either sanitary sewers or combined sewers. The former is designed to exclude stormwater flows whereas the latter is designed to also take stormwater. The production of sewage generally corresponds to the water consumption. A range of factors influence water consumption and hence the sewage flowrates per person. These include: Water availability (the opposite of water scarcity), water supply options, climate (warmer climates may lead to greater water consumption), community size, economic level of the community, level of industrialization, metering of household consumption, water cost, water pressure and system losses in the water supply network.[2]:20

The main parameters in sewage that are measured to assess the sewage strength or quality as well as treatment options include: solids, indicators of organic matter, nitrogen, phosphorus, and indicators of fecal contamination.[2]:33 The following four types of pathogens from fecal matter are found in sewage: bacteria, viruses, protozoa, helminths and their eggs.[3][4] In order to quantify the organic matter, indirect methods are commonly used: mainly the Biochemical Oxygen Demand (BOD) and the Chemical Oxygen Demand (COD).[2]:36 Typical values for physical–chemical characteristics of raw sewage in developing countries have been published as follows: 180 g/person/d for total solids (1100 mg/L concentration), 50 g/person/d for BOD (300 mg/L), 100 g/person/d for COD (600 mg/L), 8 g/person/d for total nitrogen (45 mg/L), 4.5 g/person/d for ammonia-N (25 mg/L) and 1.0 g/person/d for total phosphorus (7 mg/L).[2]:57

Sewage treatment is part of the broad term sanitation which includes not only the management of wastewater but also the management of human excreta, solid waste and stormwater. Disposal options include dilution (self-purification of water bodies), marine outfalls, land disposal and sewage farms. All disposal options may run risks of causing water pollution.