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.

TerminologyEdit

Sewage vs. wastewaterEdit

Sewage (or domestic wastewater) consists of wastewater 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's water supply), human excreta (feces and urine), used water from bathrooms, food preparation wastes, laundry wastewater, and other waste products of normal living. The term "sewage" is nowadays used interchangeably with "wastewater" in many textbooks and in the literature.[2][5]

Sewage from municipalities contains "commercial wastewater" which is wastewater discharged from restaurants, laundries, hospitals, schools, prisons, offices, stores and establishments serving the local area of larger communities..[2]: 21 A community's commercial facilities may increase the volume or waste concentration of sewage if these facilities are used by populations from areas not served by the sewer system.[6]

BlackwaterEdit

Blackwater in a sanitation context denotes wastewater from toilets, which likely contains pathogens. Blackwater can contain feces, urine, water and toilet paper from flush toilets. Blackwater is distinguished from greywater, which comes from sinks, baths, washing machines, and other kitchen appliances apart from toilets. Greywater results from washing food, clothing, dishes, as well as from showering or bathing.[7]

Blackwater and greywater are separated in "ecological buildings", such as autonomous buildings. Recreational vehicles often have separate holding tanks for greywater from showers and sinks, and blackwater from the toilet.

GreywaterEdit

Greywater (or grey water, sullage, also spelled gray water in the United States) refers to domestic 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 include sinks, showers, baths, washing machines or dishwashers. As greywater contains fewer pathogens than domestic wastewater, it is generally safer to handle and easier to treat and reuse onsite for toilet flushing, landscape or crop irrigation, and other non-potable uses. Greywater may still have some pathogen content from laundering soiled clothing or cleaning the anal area in the shower or bath.

The application of greywater reuse in urban water systems provides substantial benefits for both the water supply subsystem, by reducing the demand for fresh clean water, and the wastewater subsystems by reducing the amount of wastewater required to be conveyed and treated.[8] Treated greywater has many uses, such as toilet flushing or irrigation.[9]
 
Pumping station lifting sewage to the treatment plant in Bujumbura, Burundi
 
Greywater (a type of wastewater) in a settling tank

Liquid and solid fractionsEdit

Sewage consists primarily of water and usually contains less than one part solid matter per thousand parts of water. 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.[10]: 9–38

The suspended and dissolved solids include organic and inorganic matter plus microorganisms.[2]: 28 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 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 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

Sewage contains urine and feces. Mass of feces varies with dietary fiber intake. An average person is estimated to produce 128 grams of wet feces per day, or a median dry mass of 29 g/person/day.[11] The same research found that the median urine generation rate is about 1.42 L/person/day.[11]

Overall appearanceEdit

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. 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.[10]: 9–38 Sewage can have high turbidity from suspended solids.

Wastewater analysesEdit

The pH value of sewage is usually near neutral, and can be in the range of 6.7–8.0.[2]: 57 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.[2]: 33

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.

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.[12][13][14] Sequencing DNA from an environmental sample is known as metagenomics.

Sewage has also been analyzed to determine relative rates of use of prescription and illegal drugs among municipal populations.[15] General socioeconomic demographics may be inferred as well.[16]

PollutantsEdit

Organic matterEdit

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.[2]: 35 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).[2]: 35 In order to quantify this organic matter, indirect methods are commonly used: mainly the Biochemical Oxygen Demand (BOD) and the Chemical Oxygen Demand (COD).[2]: 36 A United States source published in 1972 estimated that the daily dry weight of solid wastes per 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 (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.[17]: 234

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 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

This compares with the following values for households in the United States, 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.[5]: 183

NutrientsEdit

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 found in raw sewage.[2]: 43

Total phosphorus is 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. Or they are organic phosphate, where the source is organic compounds to which the organic phosphate is bound.[2]: 45

If sewage is discharged untreated, its nitrogen and phosphorus content can lead to pollution of lakes and reservoirs via a process called eutrophication.[2]: 77

PathogensEdit

Human feces make blackwater a unique environmental threat because of the likelihood it may contain pathogenic organisms that can transmit disease to humans and animals.[10]: 9–38 The following four types of pathogens are found in sewage:[3][4]

Solid wasteEdit

 
Screening of the sewage with bar screens at a sewage treatment plant to remove larger objects in Norton, Zimbabwe
 
Screening of sewage at a sewage treatment plant in Bujumbura, Burundi

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.[19] 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.[20] Trash and garbage in streets may be carried to combined sewers by stormwater runoff.

Micro-pollutantsEdit

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[21] 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[22][23][24] 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,[17]: 228 lubricants,[17]: 228 adhesives, paint, solvents, detergents,[17]: 228 and disinfectants into their sewer connections.

FlowratesEdit

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 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 production of sewage generally corresponds to the water consumption. However water used for landscape irrigation will not enter the sewer system, while groundwater and stormwater may enter the sewer system in addition to sewage.[2]: 22 There are usually two peak flowrates of sewage arriving at a treatment plant: 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 estimated water consumption in China as 80 L per person per day, Africa as 15-35 L per person per day, and Latin America and Caribbean as 70-190 L per person per day.[5]: 163

For comparison, typical sewage flowrates from urban residential sources in the United 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).

CollectionEdit

Sewage is commonly collected and transported in gravity sewers, either in a sanitary sewer or in a combined sewer. The latter also conveys urban runoff (stormwater) which means the sewage gets diluted during rain events.[2]: 9

Sanitary sewerEdit

A sanitary sewer or foul sewer is an underground pipe or tunnel system for transporting sewage from houses and commercial buildings (but not stormwater) to a sewage treatment plant or disposal. Sanitary sewers are a type of gravity sewer and are part of an overall system called a "sewage system" or sewerage. Sanitary sewers serving industrial areas also carry industrial wastewater. In municipalities served by sanitary sewers, separate storm drains may convey surface runoff directly to surface waters. An advantage of sanitary sewer systems is that they avoid combined sewer overflows. Sanitary sewers are typically much smaller in diameter than combined sewers which also transport urban runoff. Backups of raw sewage can occur if excessive stormwater inflow or groundwater infiltration occurs due to leaking joints, defective pipes etc. in aging infrastructure.

Combined sewerEdit

A combined sewer is type of gravity sewer with a system of pipes, tunnels, pump stations etc. to transport sewage and urban runoff together to a sewage treatment plant or disposal site. This means that during rain events, the sewage gets diluted, resulting in higher flowrates at the treatment site. Uncontaminated stormwater simply dilutes sewage, but runoff may dissolve or suspend virtually anything it contacts on roofs, streets, and storage yards.[25]: 296 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. Combined sewers may also receive dry weather drainage from landscape irrigation, construction dewatering, and washing buildings and sidewalks.

Dilution by mixingEdit

Dilution reduces the concentration of pollutants, but does not reduce the amount of pollutants. Dilution remained the most common method of sewage disposal into the late 20th century. Most sewage 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.[26] The larger suspended or floating solids may be removed rather than released to the receiving water.[27]: 573

One method of dilution is by mixing of sewage with groundwater, stormwater, or industrial wastewater in the sewer system during the collection process:

GroundwaterEdit

Infiltration is groundwater entering sewer pipes through defective pipes, connections, joints or manholes.[2]: 26[5]: 164 Contaminated or saline groundwater may introduce additional wastes 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 local groundwater 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 to groundwater pollution.[28]

StormwaterEdit

Combined sewers are designed to dilute sewage with stormwater. Inflow similarly dilutes sewage with 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

Industrial wastewaterEdit

Sewage from communities with industrial facilities may include some industrial 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 be toxic or non-biodegradable waste including pharmaceuticals,[29] biocides, heavy metals, radionuclides, or thermal pollution. Industrial wastewater may receive pre-treatment at the factories to reduce the pollutant load.[2]: 27 Dilution with industrial wastewater reduces the concentrations of remaining industrial pollutants while reducing the concentration of sewage pollutants.

TreatmentEdit

Sewage treatment is beneficial in reducing environmental pollution. Bar screens can remove large solid debris from sewage,[17]: 274–275 and primary treatment can remove floating and settleable matter.[17]: 446 The remaining liquid usually contains less than half of the original solids content and approximately two-thirds of the BOD in the form of colloids and dissolved organic compounds.[30] Secondary treatment can reduce the BOD of organic waste in undiluted sewage,[27]: 575 but is less effective for dilute sewage.[31] 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.[17]: 359

There is also the possibility of resource recovery representing untapped potential worldwide to make agriculture more sustainable by using nutrients, biomass and water recovered from sewage[32]. Technologies are available to recover carbon, nitrogen, phosphorus, water and energy[32]. Benefits of rapid scale up of this technology include climate change mitigation, reversing land degradation, water resources protection and improved urban sanitation, and public health[32].

DisposalEdit

 
Ocean outfall pipes in Cape May, New Jersey, United States - pipes exposed after the sand was removed by severe storm

Sewage may be discharged to an evaporation or infiltration basin, or to a stream, lake, or ocean.[10]: 9–41 Receiving water bodies can to some extent assimilate the pollutants in sewage, provided the volume of sewage matches the size and capabilities of the aquatic ecosystem for "self purification". This depends on the ability of the receiving water to sustain dissolved oxygen concentrations necessary to support organisms catabolizing organic waste.[17]: 9&673 Fish may die if dissolved oxygen levels are depressed below 5 mg/l.[27]: 573

Application of sewage to land can be considered as a form of final disposal or of treatment, or both.[2]: 189 Groundwater recharge is a method of treated sewage disposal to reduce saltwater intrusion, or replenish aquifers used for agricultural irrigation. Treatment is usually required to sustain percolation capacity of infiltration basins, and more extensive treatment may be required for aquifers used as drinking water supplies.[17]: 700–703 Land disposal alternatives require consideration of land availability, groundwater quality, and possible soil deterioration.[33]

A marine outfall (or ocean outfall) is a pipeline or tunnel that discharges municipal or industrial wastewater, stormwater, combined sewer overflows (CSOs), cooling water, or brine effluents from water 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 primary treatment, with the intention of using the assimilative capacity of the sea for further treatment. Submarine outfalls 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.[34] More than 200 outfalls alone have been listed in a single international database maintained by the Institute for Hydromechanics at Karlsruhe University for the International Association of Hydraulic Engineering and Research (IAHR) / International Water Association (IWA) Committee on Marine Outfall Systems.[35]

Sewage farmEdit

Sewage farms use sewage for irrigation and fertilizing agricultural land. The practice is common in warm, arid climates where irrigation is valuable while sources of fresh water are scarce. Suspended solids may be converted to humus by microbes and bacteria in order to supply nitrogen, phosphorus and other plant nutrients for crop growth. Many industrialized nations use conventional sewage treatment plants nowadays instead of sewage farms. These reduce vector and odor problems; but sewage farming remains a low-cost option for some developing countries. Sewage farming should not be confused with sewage disposal through infiltration basins or subsurface drains.

RegulationsEdit

 
Sewage overflowing from a manhole into the street of an informal settlement near Cape Town, South Africa.

Sewage can cause water pollution when discharged to the environment. Management of sewage may include collection and transport for release to surface water or reuse applications, after either advanced, basic or no sewage treatment.[2]: 156 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. In the US and EU, 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 US, see Clean Water Act.) Ships at sea are forbidden from discharging their sewage overboard unless three miles or more from shore.[36]

See alsoEdit

ReferencesEdit

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  36. ^ "Evolutions in U.S. Navy Shipboard Sewage and Graywater Programs" (PDF). 19 August 2018. CiteSeerX 10.1.1.387.9132. S2CID 5967529. Archived from the original (PDF) on 19 August 2018.