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". Types of wastewater include: domestic wastewater from households, municipal wastewater from communities (also called sewage) and industrial wastewater. Wastewater can contain physical, chemical and biological pollutants. Its characteristics vary depending on the source.
Management of wastewater includes its collection, treatment, reuse or disposal. Wastewater that is produced by activities within a municipality is usually 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 wastewater treatment plant, treated wastewater (also called effluent) is discharged to a receiving water body. The terms "wastewater reuse" and "water reclamation" apply if the treated wastewater is used for another purpose. Wastewater that is discharged to the environment without suitable treatment can cause water pollution.
Sources of wastewater include households, municipalities, industries, urban runoff and agriculture.
When the source is from households, the wastewater is called sewage. It can come from the following domestic or household activities:
- 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 (drinks, cooking oil, pesticides, lubricating oil, paint, cleaning detergents, etc.)
Activities producing industrial wastewater include:
- Industrial site drainage (silt, sand, alkali, oil, chemical residues);
- Industrial cooling waters (biocides, heat, slimes, silt)
- 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
- Extreme pH waste from acid and alkali manufacturing
- Toxic waste from metal plating, cyanide production, pesticide manufacturing, etc.
- Solids and emulsions from paper mills, factories producing lubricants or hydraulic oils, foodstuffs, etc.
- Water used in hydraulic fracturing
- Produced water from oil & natural gas production
Other related activities or events:
- 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.)
- Agricultural pollution, direct and diffuse
Dilution and mixing
Wastewater 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
The composition of wastewater varies widely. This is a partial list of pollutants that may be contained in wastewater:
Chemical or physical pollutants
- Heavy metals, including mercury, lead, and chromium
- Organic particles such as feces, hairs, food waste, vomit, paper fibers, plant material, humus, etc.;
- Soluble organic material such as urea, fruit sugars, soluble proteins, drugs, pharmaceuticals, 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.;
- Gases such as hydrogen sulfide, carbon dioxide, methane, etc.;
- Emulsions such as paints, adhesives, mayonnaise, hair colorants, emulsified oils, 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
- Bacteria (for example Salmonella, Shigella, Campylobacter, Vibrio cholerae),
- Viruses (for example hepatitis A, rotavirus, enteroviruses, coronaviruses),
- Protozoa (for example Entamoeba histolytica, Giardia lamblia, Cryptosporidium parvum) and
- Parasites such as helminths and their eggs (e.g. Ascaris (roundworm), Ancylostoma (hookworm) and Trichuris (whipworm));
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.
Management of wastewater 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.
Wastewater from factories, power plants and other industrial activities is extensively regulated in developed nations, and treatment is required before discharge to surface waters. (See Industrial wastewater treatment.)
In many cities, municipal wastewater is carried together with stormwater, in a combined sewer system, to a sewage treatment plant. In some urban areas, municipal wastewater is carried separately in sanitary sewers and runoff from streets is carried in storm drains. Access to these systems, for maintenance purposes, is typically through a manhole.
During high precipitation periods a combined sewer system may experience a combined sewer overflow event, which forces untreated sewage to flow directly to receiving waters. This can pose a serious threat to public health and the surrounding environment.
In less-developed or rural regions, sewage may drain directly into major watersheds with minimal or no treatment. This usually has serious impacts on the quality of an environment and on human health. Pathogens can cause a variety of illnesses. Some chemicals pose risks even at very low concentrations and can remain a threat for long periods of time because of bioaccumulation in animal or human tissue.
Wastewater treatment is a process used to remove contaminants from wastewater and convert it into an effluent that can be returned to the water cycle. Once returned to the water cycle, the effluent creates an acceptable impact on the environment or is reused for various purposes (called water reclamation). The treatment process takes place in a wastewater treatment plant (WWTP).There are two kinds of wastewater: domestic and industrial; both types of wastewater are treated at the appropriate wastewater treatment plant. For domestic wastewater (also called municipal wastewater or sewage), the treatment plant is called a sewage treatment plant. For industrial wastewater, treatment either takes place in a separate industrial wastewater treatment plant, or in a sewage treatment plant (usually after some form of pre-treatment).Processes commonly used include phase separation (such as sedimentation), biological and chemical processes (such as oxidation) or polishing. Further types of wastewater treatment plants include agricultural wastewater treatment plants and leachate treatment plants. The main by-product from wastewater treatment plants is a type of sludge (for example sewage sludge) which is usually treated in the same or another wastewater treatment plant.:Ch.14 Biogas can be another by-product if anaerobic treatment processes are used.
Water reclamation (also called wastewater reuse) is the process of converting wastewater into water that can be reused for other purposes. Types of reuse include: urban reuse, agricultural reuse (irrigation), environmental reuse, industrial reuse, planned potable reuse, de facto wastewater reuse (unplanned potable reuse). For example, reuse may include irrigation of gardens and agricultural fields or replenishing surface water and groundwater (i.e., groundwater recharge). Reused water may also be directed toward fulfilling certain needs in residences (e.g. toilet flushing), businesses, and industry, and could even be treated to reach drinking water standards. Treated wastewater reuse for irrigation is a long-established practice, especially in arid countries. Reusing wastewater as part of sustainable water management allows water to remain as an alternative water source for human activities. This can reduce scarcity and alleviate pressures on groundwater and other natural water bodies.
There are several technologies used to treat wastewater for reuse. A combination of these technologies can meet strict treatment standards and make sure that the processed water is hygienically safe, meaning free from pathogens. The following are some of the typical technologies: Ozonation, ultrafiltration, aerobic treatment (membrane bioreactor), forward osmosis, reverse osmosis, advanced oxidation. Some water demanding activities do not require high grade water. In this case, wastewater can be reused with little or no treatment.The cost of reclaimed water exceeds that of potable water in many regions of the world, where a fresh water supply is plentiful. The costs of water reclamation options might be compared to the costs of alternatives options which also achieve similar effects of freshwater savings, namely greywater reuse systems, rainwater harvesting and stormwater recovery, or seawater desalination.
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- Fecal sludge management
- List of waste-water treatment technologies
- Reuse of excreta
- Water management
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