Wastewater that is produced by a community of people
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. Sewage consists of wastewater discharged from residences and from commercial, institutional and public facilities that exist in the locality. 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 is also added to the sewage. Sewage contains macro-pollutants and micro-pollutants, and may also incorporate some municipal solid waste and pollutants from industrial wastewater.
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 from 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 and water pressure.
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. These can be considered to be the main macro-pollutants in sewage. Sewage contains pathogens which stem from fecal matter. The following four types of pathogens are found in sewage: pathogenic bacteria, viruses, protozoa (in the form of cysts or oocysts) and helminths (in the form of eggs). In order to quantify the organic matter, indirect methods are commonly used: mainly the Biochemical Oxygen Demand (BOD) and the Chemical Oxygen Demand (COD).
Management of sewage includes collection and transport for release into the environment, after a treatment level that is compatible with the local requirements for discharge into water bodies, onto soil or for reuse applications. Disposal options include dilution (self-purification of water bodies, making use of their assimilative capacity if possible), marine outfalls, land disposal and sewage farms. All disposal options may run risks of causing water pollution.
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Terminology.
Sewage and wastewater.
Sewage (or domestic wastewater) consists of wastewater discharged from residences and from commercial, institutional and public facilities that exist in the locality. Sewage is a mixture of water (from the community's water supply), human excreta (feces and urine), used water from bathrooms, food preparation wastes, laundry wastewater, and other waste products of normal living.
Sewage from municipalities contains wastewater from commercial activities and institutions, e.g. wastewater discharged from restaurants, laundries, hospitals, schools, prisons, offices, stores and establishments serving the local area of larger communities.
Sewage can be distinguished into "untreated sewage" (also called "raw sewage") and "treated sewage" (also called "effluent" from a sewage treatment plant).
The term "sewage" is nowadays often used interchangeably with "wastewater" – implying "municipal wastewater" – in many textbooks, policy documents and the literature. To be precise, wastewater is a broader term, because it refers to any water after it has been used in a variety of applications. Thus it may also refer to "industrial wastewater", agricultural wastewater and other flows that are not related to household activities.
Overall appearance.
The overall appearance of sewage is as follows: The temperature tends to be slightly higher than in drinking water but is more stable than the ambient temperature. The color of fresh sewage is slightly grey, whereas older sewage (also called "septic sewage") is dark grey or black. The odor of fresh sewage is "oily" and relatively unpleasant, whereas older sewage has an unpleasant foul odor due to hydrogen sulfide gas and other decomposition by-products. Sewage can have high turbidity from suspended solids.
The pH value of sewage is usually near neutral, and can be in the range of 6.7–8.0.
Pollutants.
Sewage consists primarily of water and usually contains less than one part of solid matter per thousand parts of water. In other words, one can say that sewage is composed of around 99.9% pure water, and the remaining 0.1% are solids, which can be in the form of either dissolved solids or suspended solids. The thousand-to-one ratio is an order of magnitude estimate rather than an exact percentage because, aside from variation caused by dilution, solids may be defined differently depending upon the mechanism used to separate those solids from the liquid fraction. Sludges of settleable solids removed by settling or suspended solids removed by filtration may contain significant amounts of entrained water, while dried solid material remaining after evaporation eliminates most of that water but includes dissolved minerals not captured by filtration or gravitational separation. The suspended and dissolved solids include organic and inorganic matter plus microorganisms.
About one-third of this solid matter is suspended by turbulence, while the remainder is dissolved or colloidal. For the situation in the United States in the 1950s it was estimated that the waste contained in domestic sewage is about half organic and half inorganic.
Organic matter.
The organic matter in sewage can be classified in terms of form and size: Suspended (particulate) or dissolved (soluble). Secondly, it can be classified in terms of biodegradability: either inert or biodegradable. The organic matter in sewage consists of protein compounds (about 40%), carbohydrates (about 25–50%), oils and grease (about 10%) and urea, surfactants, phenols, pesticides and others (lower quantity). In order to quantify the organic matter content, it is common to use "indirect methods" which are based on the consumption of oxygen to oxidize the organic matter: mainly the Biochemical Oxygen Demand (BOD) and the Chemical Oxygen Demand (COD). These indirect methods are associated with the major impact of the discharge of organic matter into water bodies: the organic matter will be food for microorganisms, whose population will grow, and lead to the consumption of oxygen, which may then affect aquatic living organisms.
The mass load of organic content is calculated as the sewage flowrate multiplied with the concentration of the organic matter in the sewage.
Typical values for physical–chemical characteristics of raw sewage is provided further down below.
Nutrients.
Apart from organic matter, sewage also contains nutrients. The major nutrients of interest are nitrogen and phosphorus. If sewage is discharged untreated, its nitrogen and phosphorus content can lead to pollution of lakes and reservoirs via a process called eutrophication.
In raw sewage, nitrogen exists in the two forms of organic nitrogen or ammonia. The ammonia stems from the urea in urine. Urea is rapidly hydrolyzed and therefore not usually found in raw sewage.
Total phosphorus is mostly present in sewage in the form of phosphates.They are either inorganic (polyphosphates and orthophosphates) and their main source is from detergents and other household chemical products. The other form is organic phosphorus, where the source is organic compounds to which the organic phosphorus is bound.
Pathogens.
Human feces in sewage may contain pathogens capable of transmitting diseases. The following four types of pathogens are found in sewage: 
In most practical cases, pathogenic organisms are not directly investigated in laboratory analyses. An easier way to assess the presence of fecal contamination is by assessing the most probable numbers of fecal coliforms (called thermotolerant coliforms), especially "Escherichia coli". "Escherichia coli" are intestinal bacteria excreted by all warm blooded animals, including human beings, and thus tracking their presence in sewage is easy, because of their substantially high concentrations (around 10 to 100 million per 100 mL).
Solid waste.
The ability of a flush toilet to make things "disappear" is soon recognized by young children who may experiment with virtually anything they can carry to the toilet. Adults may be tempted to dispose of toilet paper, wet wipes, diapers, sanitary napkins, tampons, tampon applicators, condoms, and expired medications, even at the risk of causing blockages. The privacy of a toilet offers a clandestine means of removing embarrassing evidence by flushing such things as drug paraphernalia, pregnancy test kits, combined oral contraceptive pill dispensers, and the packaging for those devices. There may be reluctance to retrieve items like children's toys or toothbrushes which accidentally fall into toilets, and items of clothing may be found in sewage from prisons or other locations where occupants may be careless. Trash and garbage in streets may be carried to combined sewers by stormwater runoff.
Micro-pollutants.
Sewage contains environmental persistent pharmaceutical pollutants. Trihalomethanes can also be present as a result of past disinfection. Sewage may contain microplastics such as polyethylene and polypropylene beads, or polyester and polyamide fragments from synthetic clothing and bedding fabrics abraded by wear and laundering, or from plastic packaging and plastic-coated paper products disintegrated by lift station pumps. Pharmaceuticals, endocrine disrupting compounds, and hormones may be excreted in urine or feces if not catabolized within the human body.
Some residential users tend to pour unwanted liquids like used cooking oil, lubricants, adhesives, paint, solvents, detergents, and disinfectants into their sewer connections. This behavior can result in problems for the treatment plant operation and is thus discouraged.
Typical sewage composition.
Factors that determine composition.
The composition of sewage varies with climate, social and economic situation and population habits. In regions where water use is low, the strength of the sewage (or pollutant concentrations) is much higher than that in the United States where water use per person is high. Household income and diet also plays a role: For example, for the case of Brazil, it has been found that the higher the household income, the higher is the BOD load per person and the lower is the BOD concentration.
Concentrations and loads.
Typical values for physical–chemical characteristics of raw sewage in developing countries have been published as follows: 180 g/person/d for total solids (or 1100 mg/L when expressed as a 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). The typical ranges for these values are: 120–220 g/person/d for total solids (or 700–1350 mg/L when expressed as a concentration), 40–60 g/person/d for BOD (250–400 mg/L), 80–120 g/person/d for COD (450–800 mg/L), 6–10 g/person/d for total nitrogen (35–60 mg/L), 3.5–6 g/person/d for ammonia-N (20–35 mg/L) and 0.7–2.5 g/person/d for total phosphorus (4–15 mg/L).
For high income countries, the "per person organic matter load" has been found to be approximately 60 gram of BOD per person per day. This is called the population equivalent (PE) and is also used as a comparison parameter to express the strength of industrial wastewater compared to sewage.
Values for households in the United States have been published as follows, whereby the estimates are based on the assumption that 25% of the homes have kitchen waste-food grinders (sewage from such households contain more waste): 95 g/person/d for total suspended solids (503 mg/L concentration), 85 g/person/d for BOD (450 mg/L), 198 g/person/d for COD (1050 mg/L), 13.3 g/person/d for the sum of organic nitrogen and ammonia nitrogen (70.4 mg/L), 7.8 g/person/d for ammonia-N (41.2 mg/L) and 3.28 g/person/d for total phosphorus (17.3 mg/L). The concentration values given here are based on a flowrate of 190 L per person per day.
A United States source published in 1972 estimated that the daily dry weight of solid wastes per capita in sewage is estimated as in feces, of dissolved solids in urine, of toilet paper, of greywater solids, of food solids (if garbage disposal units are used), and varying amounts of dissolved minerals depending upon salinity of local water supplies, volume of water use per capita, and extent of water softener use.
Sewage contains urine and feces. The mass of feces varies with dietary fiber intake. An average person produces 128 grams of wet feces per day, or a median dry mass of 29 g/person/day. The median urine generation rate is about 1.42 L/person/day, as was determined by a global literature review.
Flowrates.
The volume of domestic sewage produced per person (or "per capita", abbreviated as "cap") varies with the water consumption in the respective locality. 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