Sewage

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.

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.[2]^: 21

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.[2][5][6] To be precise, wastewater is a broader term, because it refers to any water 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.

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 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).[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 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.[2]^: 77

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.[2]^: 43

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.[2]^: 45

Human feces in sewage may contain pathogens capable of transmitting diseases.[10]^: 9–38 The following four types of pathogens are found in sewage:[3][4]

• Bacteria like Salmonella, Shigella, Campylobacter, or Vibrio cholerae;
• Viruses like hepatitis A, rotavirus, coronavirus,[12] enteroviruses;
• Protozoa like Entamoeba histolytica, Giardia lamblia, Cryptosporidium parvum; and
• Helminths and their eggs including Ascaris (roundworm), Ancylostoma (hookworm), and Trichuris (whipworm)

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).[2]^: 52

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.[13] 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.[14] Trash and garbage in streets may be carried to combined sewers by stormwater runoff.

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[15] 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[16][17][18] 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,[19]^: 228 lubricants,[19]^: 228 adhesives, paint, solvents, detergents,[19]^: 228 and disinfectants into their sewer connections. This behavior can result in problems for the treatment plant operation and is thus discouraged.

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 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).[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 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.[5]^: 183

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.[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]

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.[22][23][24] 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.[25] General socioeconomic demographics may be inferred as well.[26]

Infiltration is groundwater entering sewer pipes through defective pipes, connections, joints or manholes.[2]^: 26[5]^: 164 Contaminated or saline 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 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]

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 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.

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 the pollutant 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 a water body (river, lakes, sea) or land, its relative impact will depend on the assimilative capacity of the water body or ecosystem.[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 treat partially 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]^: 152The 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 dissolved oxygen concentrations necessary to support organisms catabolizing 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]

Sewage may be discharged to an evaporation or infiltration basin..[10]^: 9–41 Groundwater recharge is used 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.[19]^{: 700–703}