Sewage (or domestic wastewater, municipal wastewater) is a type of wastewater that is produced by a community of people. It is characterized by volume or rate of flow, physical condition, chemical and toxic constituents, and its bacteriologic status (which organisms it contains and in what quantities). It consists mostly of greywater (from sinks, bathtubs, showers, dishwashers, and clothes washers), blackwater (the water used to flush toilets, combined with the human waste that it flushes away); soaps and detergents; and toilet paper (less so in regions where bidets are widely used instead of paper). 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.
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Sewage usually travels from a building's plumbing either into a sewer, which will carry it elsewhere, or into an onsite sewage facility. Whether it is combined with surface runoff in the sewer depends on the sewer design (sanitary sewer or combined sewer). The reality is that most wastewater produced globally remains untreated, causing widespread water pollution, especially in low-income countries: a global estimate by UNDP and UN-Habitat is that 90% of all wastewater generated is released into the environment untreated. In many developing countries the bulk of domestic and industrial wastewater is discharged without any treatment or after primary treatment only.
The term sewage is nowadays regarded as an older term and is being more and more replaced by "wastewater". In general American English usage, the terms "sewage" and "sewerage" mean the same thing. In common English usage, and in American technical and professional English usage, "sewerage" refers to the infrastructure that conveys sewage.
Sewage is made up of the wastewater from residences and institutions, carrying bodily wastes (primarily feces and urine), washing water, food preparation wastes, laundry wastes, and other waste products of normal living. This is classified as sewage or domestic wastewater.
Sewage in cities also includes the liquid-carried wastes from stores and service establishments serving the immediate community, termed commercial wastes. However, wastewater that result from industrial processes such as the production or manufacture of goods are classed as industrial wastewater, not as sewage and is usually collected and treated or pre-treated separately.
The term sewage is nowadays regarded as an older term and is being more and more replaced by "wastewater", domestic wastewater or municipal wastewater. In the context of sanitation, wastewater is any water that has been contaminated by human use. Wastewater is "used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff or stormwater, and any sewer inflow or sewer infiltration".
Sources and types
Sources of municipal wastewater include households, municipalities, industries and urban runoff. At the household level, the following liquids and solids might be discarded to the sewer pipes:
- Human excreta (feces, urine, blood and other bodily fluids) often mixed with used toilet paper or wet wipes; this is known as blackwater if it is collected from flush toilets
- Washing water (personal hygiene, clothes, floors, dishes, cars, etc.), also known as greywater or sullage
- Surplus manufactured liquids from domestic sources (cooking oil, pesticides, lubricating oil, paint, cleaning detergents, etc.).
At the municipality level, the following liquids and solids might end up in the sewage: Urban runoff from highways, roads, railway tracks, car parks, roofs, pavements (contains oils, animal feces/manure, food waste, litter, petrol, diesel or rubber residues from tyres, soapscum, metals from vehicle exhausts, de-icing agents, herbicides and pesticides from gardens, etc.)
Dilution and mixing
Sewage can be diluted or mixed with other types of water through the following mechanisms:
- Seawater ingress (high volumes of salt and microbes)
- Direct ingress of river water
- Rainfall collected on roofs, yards, hard-standings, etc. (generally clean with traces of oils and fuel)
- Groundwater infiltrated into sewage
- Mixing with other types of wastewater or fecal sludge
- Mixing with industrial wastewater or with pre-treated industrial wastewater (activities at city-level producing industrial wastewater include: Industrial processing waters, organic or biodegradable waste including waste from hospitals, abattoirs, creameries, and food factories, organic or non bio-degradable waste that is difficult-to-treat from pharmaceutical or pesticide manufacturing etc.)
Sewage can get mixed with surface runoff in combined sewers unless stormwater is collected separately. Surface runoff is also known as storm flow or overland flow. It 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 and may mix it with sewage.
Sewage may include stormwater runoff or urban runoff. Sewerage systems capable of handling storm water are known as combined sewer systems. This design was common when urban sewerage systems were first developed, in the late 19th and early 20th centuries.:119 Combined sewers require much larger and more expensive treatment facilities than sanitary sewers. Heavy volumes of storm runoff may overwhelm the sewage treatment system, causing a spill or overflow. Sanitary sewers are typically much smaller than combined sewers, and they are not designed to transport stormwater. Backups of raw sewage can occur if excessive infiltration/inflow (dilution by stormwater and/or groundwater) is allowed into a sanitary sewer system. Communities that have urbanized in the mid-20th century or later generally have built separate systems for sewage (sanitary sewers) and stormwater, because precipitation causes widely varying flows, reducing sewage treatment plant efficiency.
As rainfall travels over roofs and the ground, it may pick up various contaminants including soil particles and other sediment, heavy metals, organic compounds, animal waste, and oil and grease. Some jurisdictions require stormwater to receive some level of treatment before being discharged directly into waterways. Examples of treatment processes used for stormwater include retention basins, wetlands, buried vaults with various kinds of media filters, and vortex separators (to remove coarse solids).
The composition of sewage varies widely. This is a partial list of pollutants that may be contained in sewage:
Organic pollutants and nutrients
Sewage is a complex mixture of chemicals, with many distinctive chemical characteristics. These include high concentrations of ammonium, nitrate, nitrogen, phosphorus, high conductivity (due to high dissolved solids), high alkalinity, with pH typically ranging between 7 and 8. The organic matter of sewage is measured by determining its biological oxygen demand (BOD) or the chemical oxygen demand (COD).
Sewage also contains organic matter that can cause odor and attract flies.
Biological pollutants / pathogens
All categories of sewage are likely to carry pathogenic organisms that can transmit disease to humans and animals. Sewage contains human feces, and therefore often contains pathogens of one of the four types:
- Bacteria (for example Salmonella, Shigella, Campylobacter, Vibrio cholerae),
- Viruses (for example hepatitis A, rotavirus, coronavirus, enteroviruses),
- Protozoa (for example Entamoeba histolytica, Giardia lamblia, Cryptosporidium parvum) and
- Parasites such as helminths and their eggs (e.g. ascarisAscaris (roundworm), ancylostomaAncylostoma (hookworm), trichurisTrichuris (whipworm))
Sewage can be monitored for both disease-causing and benign organisms with a variety of techniques. Traditional techniques involve filtering, staining, and examining samples under a microscope. Much more sensitive and specific testing can be accomplished with DNA sequencing, such as when looking for rare organisms, attempting eradication, testing specifically for drug-resistant strains, or discovering new species. Sequencing DNA from an environmental sample is known as metagenomics.
Sewage also contains environmental persistent pharmaceutical pollutants. Trihalomethanes can also be present as a result of past disinfection.
Chemical or physical pollutants
The following chemical or physical pollutants can occur in sewage due to household activities or due to industrial wastewater mixing:
- Heavy metals, including mercury, lead, and chromium
- Organic particles such as feces, hairs, food waste, vomit, paper fibers, plant material, humus, etc.;
- Inorganic particles such as sand, grit, metal particles, rubber residues from tires, ceramics, etc.;
- Soluble inorganic material such as ammonia, road-salt, sea-salt, cyanide, hydrogen sulfide, thiocyanates, thiosulfates, etc.;
- Macro-solids such as sanitary napkins, nappies/diapers, condoms, needles, children's toys, dead animals or plants, etc.;
- Emulsions such as paints, adhesives, etc.;
- Toxins such as pesticides, poisons, herbicides, etc.
- Pharmaceuticals, endocrine disrupting compounds, hormones, perfluorinated compounds, siloxanes, drugs of abuse and other hazardous substances
- Microplastics such as polyethylene and polypropylene beads, polyester and polyamide
- Thermal pollution from power stations and industrial manufacturers
- Radionuclides from uranium mining, processing nuclear fuel, operating nuclear reactors, or disposal of radioactive waste.
Management of sewage includes its collection, treatment, reuse or disposal. It 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.
Management of sewage may include collection for release to surface water, infiltration to groundwater, or reuse, with or without treatment. Wastewater produced by activities within a municipality may be collected and transported in a sanitary sewer or in a combined sewer that conveys stormwater runoff, sewage and pre-treated industrial wastewater. After treatment at a sewage treatment plant, treated wastewater (also called effluent) is discharged to a receiving water body. The terms "wastewater reuse" and "water reclamation" apply if the wastewater is used for another purpose. Wastewater discharged to the environment without suitable treatment can cause water pollution.
Wastewater quality indicators are laboratory test methodologies to assess suitability of wastewater for disposal, treatment or reuse. Tests selected vary with the intended use or discharge location. Tests measure physical, chemical, and biological characteristics of the wastewater. Physical characteristics include temperature and solids. Chemical characteristics include pH value, dissolved oxygen concentrations, biochemical oxygen demand (BOD) and chemical oxygen demand (COD), nitrogen, phosphorus, chlorine. Biological characteristics are determined with bioassays and aquatic toxicology tests.Both the biochemical oxygen demand (BOD) and chemical oxygen demand (COD) tests are a measure of the relative oxygen-depletion effect of a waste contaminant. Both have been widely adopted as a measure of pollution effect. Any oxidizable material present in an aerobic natural waterway or in an industrial wastewater will be oxidized both by biochemical (bacterial) or chemical processes. The result is that the oxygen content of the water will be decreased.
Both words are descended from Old French assewer, derived from the Latin exaquare, "to drain out (water)".
- Fecal sludge management
- History of water supply and sanitation
- List of waste-water treatment technologies
- Reuse of excreta
- Sanitary sewer overflow
- Sewage pumping
- Water management
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