Sewage (ordomestic sewage,domestic wastewater,municipal wastewater) is a type ofwastewater that is produced by acommunity of people. It is typically transported through asewer 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 aregreywater (from sinks, bathtubs, showers, dishwashers, and clothes washers) andblackwater (the water used to flushtoilets, combined with thehuman waste that it flushes away). Sewage also contains soaps and detergents. Food waste may be present fromdishwashing, and food quantities may be increased wheregarbage disposal units are used. In regions wheretoilet paper is used rather thanbidets, that paper is also added to the sewage. Sewage contains macro-pollutants and micro-pollutants, and may also incorporate somemunicipal solid waste and pollutants fromindustrial wastewater.
Sewage usually travels from a building'splumbing either into asewer, which will carry it elsewhere, or into anonsite sewage facility. Collection of sewage from several households together usually takes places in eithersanitary sewers orcombined sewers. The former is designed to excludestormwater 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 ofwater scarcity),water supply options,climate (warmer climates may lead to greater water consumption), community size, economic level of the community, level ofindustrialization,metering of household consumption, water cost and water pressure.[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 These can be considered to be the main macro-pollutants in sewage. Sewage containspathogens which stem fromfecal matter. The following four types of pathogens are found in sewage: pathogenicbacteria,viruses,protozoa (in the form of cysts or oocysts) andhelminths (in the form of eggs).[3][4] In order to quantify the organic matter, indirect methods are commonly used: mainly theBiochemical Oxygen Demand (BOD) and theChemical Oxygen Demand (COD).[2]: 36
Management of sewage includes collection and transport for release into the environment, after atreatment level that is compatible with the local requirements for discharge into water bodies, onto soil or for reuse applications.[2]: 156 Disposal options include dilution (self-purification of water bodies, making use of theirassimilative capacity if possible),marine outfalls, land disposal andsewage farms. All disposal options may run risks of causingwater pollution.
Sewage (or domestic wastewater) consists ofwastewater discharged from residences and from commercial, institutional and public facilities that exist in the locality.[2]: 10 Sewage is a mixture of water (from the community'swater supply),human excreta (feces andurine), used water frombathrooms, 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 fromrestaurants,laundries,hospitals,schools,prisons,offices, stores and establishments serving the local area of larger communities.[2]: 21
Sewage can be distinguished into "untreated sewage" (also called "raw sewage") and "treated sewage" (also called "effluent" from asewage treatment plant).
The term "sewage" is nowadays often used interchangeably with "wastewater" – implying "municipal wastewater" – in many textbooks, policy documents and the literature.[2][5][6] To be precise, wastewater is a broader term, because it refers to anywater after it has been used in a variety of applications.[5]: 1 Thus it may also refer to "industrial wastewater",agricultural wastewater and other flows that are not related to household activities.
Blackwater and greywater are kept separate in "ecological buildings", such asautonomous buildings.Recreational vehicles often have separate holding tanks for greywater from showers and sinks, and blackwater from the toilet.
Greywater (or grey water, sullage,also spelled gray water in the United States) refers todomestic wastewater generated in households or office buildings from streams without fecal contamination, i.e., all streams except for the wastewater from toilets. Sources of greywater includesinks,showers,baths,washing machines ordishwashers. As greywater contains fewerpathogens thanblackwater, it is generally safer to handle and easier to treat andreuse onsite fortoilet flushing, landscape or cropirrigation, and other non-potable uses. Greywater may still have some pathogen content from laundering soiled clothing or cleaning theanal area in the shower or bath.
The application of greywater reuse inurban water systems provides substantial benefits for both thewater supply subsystem, by reducing the demand for freshclean water, and the wastewater subsystems by reducing the amount of conveyed and treated wastewater.[8] Treated greywater has many uses, such as toilet flushing or irrigation.[9]
Pumping station lifting sewage to the treatment plant inBujumbura, BurundiGreywater (a component of sewage) in a settling tank
The overall appearance of sewage is as follows:[2]: 30 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. Theodor of fresh sewage is "oily" and relatively unpleasant, whereas older sewage has an unpleasant foul odor due tohydrogen sulfide gas and other decomposition by-products.[10]: 9–38 Sewage can have highturbidity from suspended solids.
ThepH value of sewage is usually near neutral, and can be in the range of 6.7–8.0.[2]: 57
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 orsuspended solids.[2]: 28 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 bysettling orsuspended solids removed byfiltration 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.[11] The suspended and dissolved solids include organic and inorganic matter plus microorganisms.[2]: 28
About one-third of this solid matter is suspended byturbulence, while the remainder isdissolved orcolloidal. For the situation in the United States in the 1950s it was estimated that the waste contained in domestic sewage is about halforganic and halfinorganic.[10]: 9–38
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 ofbiodegradability: either inert or biodegradable.[2]: 35 The organic matter in sewage consists ofprotein compounds (about 40%),carbohydrates (about 25–50%), oils and grease (about 10%) andurea,surfactants,phenols,pesticides and others (lower quantity).[2]: 35 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 theBiochemical Oxygen Demand (BOD) and theChemical Oxygen Demand (COD).[2]: 36 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.[2]: 55
Typical values for physical–chemical characteristics of raw sewage is provided further down below.
Apart from organic matter, sewage also contains nutrients. The major nutrients of interest arenitrogen andphosphorus. If sewage is discharged untreated, its nitrogen and phosphorus content can lead to pollution of lakes and reservoirs via a process calledeutrophication.[2]: 77
In raw sewage, nitrogen exists in the two forms of organic nitrogen orammonia. The ammonia stems from theurea inurine. Urea is rapidly hydrolyzed and therefore not usually found in raw sewage.[2]: 43
Total phosphorus is mostly present in sewage in the form ofphosphates.They are either inorganic (polyphosphates and orthophosphates) and their main source is fromdetergents and other household chemical products. The other form is organic phosphorus, where the source is organic compounds to which the organic phosphorus is bound.[2]: 45
Human feces in sewage may containpathogens capable of transmitting diseases.[10]: 9–38 The following four types of pathogens are found in sewage:[3][4]
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 themost probable numbers offecal coliforms (called thermotolerant coliforms), especiallyEscherichia 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).[2]: 52
Screening of the sewage with bar screens at a sewage treatment plant to remove larger objects in Norton,ZimbabweScreening of sewage at a sewage treatment plant inBujumbura, Burundi
The ability of aflush toilet to make things "disappear" is soon recognized by young children who may experiment with virtually anything they can carry to the toilet.[13] Adults may be tempted to dispose oftoilet paper,wet wipes,diapers,sanitary napkins,tampons, tampon applicators,condoms, and expiredmedications, even at the risk of causing blockages. The privacy of a toilet offers a clandestine means of removing embarrassing evidence by flushing such things asdrug 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.[14] Trash and garbage in streets may be carried to combined sewers by stormwater runoff.
The composition of sewage varies with climate, social and economic situation and population habits.[2]: 28 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.[5]: 183 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.[2]: 57
Typical values for physical–chemical characteristics of raw sewage indeveloping 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).[2]: 57 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).[2]: 57
For high income countries, the "per person organic matter load" has been found to be approximately 60 gram of BOD per person per day.[6] This is called thepopulation equivalent (PE) and is also used as a comparison parameter to express the strength ofindustrial 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 havekitchen waste-food grinders (sewage from such households contain more waste): 95 g/person/d for totalsuspended 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.[5]: 183
A United States source published in 1972 estimated that the daily dry weight of solid wastesper capita in sewage is estimated as 20.5 g (0.72 oz) in feces, 43.3 g (1.53 oz) of dissolved solids in urine, 20 g (0.71 oz) of toilet paper, 86.5 g (3.05 oz) of greywater solids, 30 g (1.1 oz) of food solids (ifgarbage 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 ofwater softener use.[19]: 234
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.[20] The median urine generation rate is about 1.42 L/person/day, as was determined by a global literature review.[20]
The volume of domestic sewage produced per person (or "per capita", abbreviated as "cap") varies with the water consumption in the respective locality.[2]: 11 A range of factors influence water consumption and hence the sewage flowrates per person. These include: Water availability (the opposite ofwater scarcity),water supply options,climate (warmer climates may lead to greater water consumption), community size, economic level of the community, level ofindustrialization,metering of household consumption, water cost and water pressure.[2]: 20
The production of sewage generally corresponds to the water consumption. However water used for landscape irrigation will not enter the sewer system, whilegroundwater andstormwater may enter the sewer system in addition to sewage.[2]: 22 There are usually two peak flowrates of sewage arriving at a treatment plant per day: One peak is at the beginning of the morning and another peak is at the beginning of the evening.[2]: 24
With regards to water consumption, a design figure that can be regarded as "world average" is 35–90 L per person per day (data from 1992).[5]: 163 The same publication listed the water consumption inChina as 80 L per person per day,Africa as 15–35 L per person per day,Eastern Mediterranean in Europe as 40–85 L per person per day andLatin America andCaribbean as 70–190 L per person per day.[5]: 163 Even inside a country, there may be large variations from one region to another due to the various factors that determine the water consumption as listed above.
A flowrate value of 200 liters of sewage per person per day is often used as an estimate inhigh income countries, and is used for example in the design of sewage treatment plants.[6]
For comparison, typical sewage flowrates from urban residential sources in theUnited States are estimated as follows: 365 L/person/day (for one person households), 288 L/person/day (two person households), 200 L/person/day (four person households), 189 L/person/day (six person households).[5]: 156 This means the overall range for this example would be 189–365 L (42–80 imp gal; 50–96 US gal).
Wastewater quality indicators arelaboratory testmethodologies to assess suitability ofwastewater for disposal,treatment or reuse. 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, indicators of fecal contamination.[21]: 33 Tests selected vary with the intended use or discharge location. Tests can 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) andchemical oxygen demand (COD), nitrogen, phosphorus, chlorine. Biological characteristics are determined withbioassays andaquatic toxicology tests.
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 withDNA sequencing, such as when looking for rare organisms, attemptingeradication, testing specifically for drug-resistant strains, or discovering new species.[22][23][24] Sequencing DNA from an environmental sample is known asmetagenomics.
Sewage has also been analyzed to determine relative rates of use of prescription and illegal drugs among municipal populations.[25] General socioeconomic demographics may be inferred as well.[26]
Acombined sewer is a type ofgravity sewer with a system of pipes, tunnels, pump stations etc. to transport sewage andurban runoff together to asewage treatment plant or disposal site. This means that during rain events, the sewage gets diluted, resulting in higher flowrates at the treatment site. Uncontaminatedstormwater simply dilutes sewage, but runoff may dissolve or suspend virtually anything it contacts on roofs, streets, and storage yards.[27]: 296 As rainfall travels over roofs and the ground, it may pick up various contaminants includingsoil particles and othersediment, heavy metals,organic compounds, animal waste, andoil andgrease. Combined sewers may also receive dry weather drainage from landscapeirrigation, constructiondewatering, and washingbuildings andsidewalks.
Infiltration isgroundwater entering sewer pipes through defective pipes, connections, joints ormanholes.[2]: 26 [5]: 164 Contaminated orsaline groundwater may introduce additional pollutants to the sewage. The amount of such infiltrated water depends on several parameters, such as the length of the collection network, pipeline diameters, drainage area, soil type, water table depth, topography and number of connections per unit area.[2]: 26 Infiltration is increased by poor construction procedures, and tends to increase with the age of the sewer. The amount of infiltration varies with the depth of the sewer in comparison to the localgroundwater table.[10]: 9–1, 9–9 Older sewer systems that are in need of rehabilitation may also exfiltrate sewage into groundwater from the leaking sewer joints and service connections.[5]: 167 This can lead togroundwater pollution.[28]
Combined sewers are designed to transport sewage and stormwater together. This means that sewage becomes diluted during rain events. There are other types of inflow that also dilute sewage, e.g. "water discharged from cellar and foundation drains, cooling-water discharges, and any direct stormwater runoff connections to the sanitary collection system".[5]: 163 The "direct inflows" can result in peak sewage flowrates similar to combined sewers during wet weather events.[5]: 165
Sewage from communities with industrial facilities may include someindustrial wastewater, generated by industrial processes such as the production or manufacture of goods. Volumes of industrial wastewater vary widely with the type of industry.[2]: 27 Industrial wastewater may contain very different pollutants at much higher concentrations than what is typically found in sewage.[5]: 188 Pollutants may betoxic or non-biodegradable waste includingpharmaceuticals,[29]biocides,heavy metals,radionuclides, orthermal pollution.
An industry may treat its wastewater and discharge it into the environment (or even use the treated wastewater for specific applications), or, in case it is located in the urban area, it may discharge the wastewater into the public sewerage system. In the latter case, industrial wastewater may receive pre-treatment at the factories to reduce thepollutant load.[2]: 27 Mixing industrial wastewater with sewage does nothing to reduce the mass of pollutants to be treated, but the volume of sewage lowers the concentration of pollutants unique to industrial wastewater, and the volume of industrial wastewater lowers the concentration of pollutants unique to sewage.
When wastewater is discharged into awater body (river, lakes, sea) or land, its relative impact will depend on theassimilative capacity of the water body orecosystem.[2]: 78 Water bodies have a self-purification capacity, so that the concentration of a pollutant may decrease along the distance from the discharge point. Furthermore, water bodies provide a dilution to the pollutants concentrations discharged, although it does not decrease their mass. In principle, the higher the dilution capacity (ratio of volume or flow of the receiving water and volume or flow of sewage discharged), the lower will be the concentration of pollutants in the receiving water, and probably the lower will be the negative impacts. But if the water body already arrives very polluted at the point of discharge, the dilution will be of limited value.[30]
In several cases, a community may partially treat its sewage, and still count on the assimilative capacity of the water body. However, this needs to be analyzed very carefully, taking into account the quality of the water in the receiving body before it receives the discharge of sewage, the resulting water quality after the discharge and the impact on the intended water uses after discharge. There are also specific legal requirements in each country. Different countries have different regulations regarding the specifications of the quality of the sewage being discharged and the quality to be maintained in the receiving water body.[2]: 152 The combination of treatment and disposal must comply with existing local regulations.
The assimilative capacity depends – among several factors – on the ability of the receiving water to sustain dissolvedoxygen concentrations necessary to support organismscatabolizing organic waste.[19]: 9, 673 For example, fish may die if dissolved oxygen levels are depressed below 5 mg/L.[31]: 573
Application of sewage to land can be considered as a form of final disposal or of treatment, or both.[2]: 189 Land disposal alternatives require consideration of land availability, groundwater quality, and possible soil deterioration.[32]
Amarine outfall (or ocean outfall) is a pipeline or tunnel that discharges municipal or industrialwastewater,stormwater,combined sewer overflows (CSOs),cooling water, orbrine effluents fromwater desalination plants to the sea. Usually they discharge under the sea's surface (submarine outfall). In the case of municipal wastewater, effluent is often being discharged after having undergone no or only primarytreatment, with the intention of using theassimilative capacity of the sea for further treatment. Submarineoutfalls are common throughout the world and probably number in the thousands. The light intensity and salinity in natural sea water disinfects the wastewater to ocean outfall system significantly.[33] More than 200 outfalls alone have been listed in a single international database maintained by the Institute for Hydromechanics atKarlsruhe University for theInternational Association of Hydraulic Engineering and Research (IAHR) /International Water Association (IWA) Committee on Marine Outfall Systems.[34]
Before the 20th century in Europe, sewers usually discharged into abody of water such as a river, lake, or ocean. There was no treatment, so the breakdown of thehuman waste was left to theecosystem. This could lead to satisfactory results if theassimilative capacity of the ecosystem is sufficient which is nowadays not often the case due to increasing population density.[35]: 78
Today, the situation in urban areas ofindustrialized countries is usually that sewers route their contents to a sewage treatment plant rather than directly to a body of water. In manydeveloping countries, however, the bulk of municipal and industrial wastewater is discharged to rivers and theocean without any treatment or after preliminary treatment or primary treatment only. Doing so can lead towater pollution. Few reliable figures exist on the share of the wastewater collected in sewers that is being treated worldwide. A global estimate byUNDP andUN-Habitat in 2010 was that 90% of all wastewater generated is released into the environment untreated.[36] A more recent study in 2021 estimated that globally, about 52% of sewage is treated.[37] However, sewage treatment rates are highly unequal for different countries around the world. For example, whilehigh-income countries treat approximately 74% of their sewage,developing countries treat an average of just 4.2%.[37] As of 2022, without sufficient treatment, more than 80% of all wastewater generated globally is released into the environment. High-income nations treat, on average, 70% of the wastewater they produce, according to UN Water.[38][39][40] Only 8% of wastewater produced in low-income nations receives any sort of treatment.[38][41][42]
Sewage treatment is beneficial in reducing environmental pollution.Bar screens can remove large solid debris from sewage,[19]: 274–275 andprimary treatment can remove floating andsettleable matter.[19]: 446 Primary treated sewage usually contains less than half of the original solids content and approximately two-thirds of the BOD in the form ofcolloids anddissolved organic compounds.[43]Secondary treatment can reduce the BOD of organic waste in undiluted sewage,[31]: 575 but is less effective for dilute sewage.[44]Water disinfection may be attempted to kill pathogens prior to disposal, and is increasingly effective after more elements of the foregoing treatment sequence have been completed.[19]: 359
An alternative to discharge into the environment is toreuse the sewage in a productive way (for agricultural, urban or industrial uses), in compliance with local regulations and requirements for each specific reuse application. Public health risks of sewage reuse in agriculture can be minimized by following a "multiple barrier approach" according to guidelines by theWorld Health Organization.[45]
Sewage management includes collection and transport for release into the environment after atreatment level that is compatible with the local requirements for discharge into water bodies, onto soil, or for reuse applications.[2]: 156 In most countries, uncontrolled discharges of wastewater to the environment are not permitted under law, and strict water quality requirements are to be met. For requirements in the United States,seeClean Water Act.
^abcdefghijklWastewater engineering: treatment and reuse. George Tchobanoglous, Franklin L. Burton, H. David Stensel, Metcalf & Eddy (4th ed.). Boston: McGraw-Hill. 2003.ISBN0-07-041878-0.OCLC48053912.{{cite book}}: CS1 maint: others (link)
^Norton, John F.; Maxcy, Kenneth F.; Pirnie, Malcolm (1947).Standard Methods for the Examination of Water and Sewage (Ninth ed.). New York:American Public Health Association. pp. 145–146.
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