Wastewater treatment is a process which removes and eliminatescontaminants fromwastewater. It thus converts it into aneffluent that can be returned to thewater cycle. Once back in the water cycle, the effluent creates an acceptable impact on the environment. It is also possible to reuse it. This process is calledwater reclamation.[1] The treatment process takes place in a wastewater treatment plant. There are several kinds of wastewater which are treated at the appropriate type of wastewater treatment plant. For domestic wastewater the treatment plant is called aSewage Treatment. Municipal wastewater orsewage are other names fordomestic wastewater. For industrial wastewater, treatment takes place in a separateIndustrial wastewater treatment, or in a sewage treatment plant. In the latter case it usually follows pre-treatment. Further types of wastewater treatment plants includeAgricultural wastewater treatment andleachate treatment plants.
One common process in wastewater treatment isphase separation, such as sedimentation. Biological and chemical processes such as oxidation are another example. Polishing is also an example. The main by-product from wastewater treatment plants is a type of sludge that is usually treated in the same or another wastewater treatment plant.[2]: Ch.14 Biogas can be another by-product if the process uses anaerobic treatment. Treated wastewater can be reused asreclaimed water.[3] The main purpose of wastewater treatment is for the treated wastewater to be able to be disposed or reused safely. However, before it is treated, the options for disposal or reuse must be considered so the correct treatment process is used on the wastewater.
The term "wastewater treatment" is often used to mean "sewage treatment".[4]
Wastewater treatment plants may be distinguished by the type of wastewater to be treated. There are numerous processes that can be used to treat wastewater depending on the type and extent of contamination. The treatment steps include physical, chemical and biological treatment processes.[citation needed]
Types of wastewater treatment plants include:
Sewage treatment is a type of wastewater treatment which aims to removecontaminants fromsewage to produce aneffluent that is suitable to discharge to the surrounding environment or an intended reuse application, thereby preventingwater pollution from raw sewage discharges.[5] Sewage containswastewater from households and businesses and possibly pre-treatedindustrial wastewater. There are a high number of sewage treatment processes to choose from. These can range fromdecentralized systems (including on-site treatment systems) to large centralized systems involving a network of pipes and pump stations (calledsewerage) which convey the sewage to a treatment plant. For cities that have acombined sewer, the sewers will also carryurban runoff (stormwater) to the sewage treatment plant. Sewage treatment often involves two main stages, called primary andsecondary treatment, while advanced treatment also incorporates a tertiary treatment stage with polishing processes and nutrient removal. Secondary treatment can reduce organic matter (measured asbiological oxygen demand) from sewage, using aerobic or anaerobic biological processes. A so-called quaternary treatment step (sometimes referred to as advanced treatment) can also be added for the removal of organicmicropollutants, such as pharmaceuticals. This has been implemented in full-scale for example in Sweden.[6]
A large number of sewage treatment technologies have been developed, mostly using biological treatment processes. Design engineers and decision makers need to take into account technical and economical criteria of each alternative when choosing a suitable technology.[7]: 215 Often, the main criteria for selection are: desired effluent quality, expected construction and operating costs, availability of land, energy requirements andsustainability aspects. Indeveloping countries and in rural areas with low population densities, sewage is often treated by variouson-site sanitation systems and not conveyed in sewers. These systems includeseptic tanks connected todrain fields,on-site sewage systems (OSS),vermifilter systems and many more. On the other hand, advanced and relatively expensive sewage treatment plants may include tertiary treatment with disinfection and possibly even afourth treatment stage to remove micropollutants.[6]
At the global level, an estimated 52% of sewage is treated.[8] 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%.[8].jpg)
Industrial wastewater treatment describes the processes used for treating wastewater that is produced by industries as an undesirable by-product. After treatment, the treated industrial wastewater (or effluent) may be reused or released to asanitary sewer or to asurface water in the environment. Some industrial facilities generate wastewater that can be treated insewage treatment plants. Most industrial processes, such aspetroleum refineries, chemical andpetrochemical plants have their own specialized facilities to treat their wastewaters so that the pollutant concentrations in the treated wastewater comply with the regulations regarding disposal of wastewaters intosewers or into rivers, lakes oroceans.[9]: 1412 This applies to industries that generate wastewater with high concentrations of organic matter (e.g. oil and grease), toxic pollutants (e.g. heavy metals,volatile organic compounds) or nutrients such asammonia.[10]: 180 Some industries install a pre-treatment system to remove some pollutants (e.g., toxic compounds), and then discharge the partially treated wastewater to the municipal sewer system.[11]: 60
Most industries produce somewastewater. Recent trends have been to minimize such production or to recycle treated wastewater within the production process. Some industries have been successful at redesigning their manufacturing processes to reduce or eliminate pollutants.[12] Sources of industrial wastewater include battery manufacturing, chemical manufacturing, electric power plants,food industry, iron and steel industry, metal working, mines and quarries, nuclear industry,oil and gas extraction,petroleum refining andpetrochemicals, pharmaceutical manufacturing,pulp and paper industry, smelters,textile mills, industrialoil contamination, water treatment andwood preserving. Treatment processes include brine treatment, solids removal (e.g. chemical precipitation, filtration), oils and grease removal, removal of biodegradable organics, removal of other organics, removal of acids and alkalis, and removal of toxic materials.
Agricultural wastewater treatment is afarm management agenda for controlling pollution fromconfined animal operations and fromsurface runoff that may be contaminated by chemicals infertilizer,pesticides,animal slurry,crop residues orirrigation water. Agricultural wastewater treatment is required for continuous confined animal operations like milk and egg production. It may be performed in plants using mechanized treatment units similar to those used forindustrial wastewater. Where land is available for ponds,settling basins andfacultative lagoons may have lower operational costs for seasonal use conditions from breeding or harvest cycles.[13]: 6–8 Animalslurries are usually treated by containment inanaerobic lagoons before disposal by spray or trickle application to grassland.Constructed wetlands are sometimes used to facilitate treatment of animal wastes.
Nonpoint source pollution includes sediment runoff, nutrient runoff and pesticides. Point source pollution includes animal wastes, silage liquor, milking parlour (dairy farming) wastes, slaughtering waste, vegetable washing water and firewater. Many farms generatenonpoint source pollution fromsurface runoff which is not controlled through a treatment plant.Leachate treatment plants are used to treat leachate fromlandfills. Treatment options include: biological treatment, mechanical treatment byultrafiltration, treatment withactive carbon filters, electrochemical treatment includingelectrocoagulation by various proprietary technologies and reverse osmosis membrane filtration using disc tube module technology.[14]
The unit processes involved in wastewater treatment include physical processes such as settlement or flotation and biological processes such oxidation or anaerobic treatment. Some wastewaters require specialized treatment methods. At the simplest level, treatment of most wastewaters is carried out through separation ofsolids fromliquids, usually bysedimentation. By progressively converting dissolved material into solids, usually a biological floc orbiofilm, which is then settled out or separated, an effluent stream of increasing purity is produced.[2][page needed][15]
Phase separation transfers impurities into anon-aqueous phase. Phase separation may occur at intermediate points in a treatment sequence to remove solids generated duringoxidation or polishing. Grease and oil may be recovered for fuel orsaponification. Solids often requiredewatering ofsludge in awastewater treatment plant. Disposal options for dried solids vary with the type and concentration of impurities removed from water.[16]
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Solids such asstones, grit, and sand may be removed from wastewater bygravity whendensity differences are sufficient to overcome dispersion byturbulence. This is typically achieved using a grit channel designed to produce an optimum flow rate that allows grit to settle and other less-dense solids to be carried forward to the next treatment stage.Gravity separation of solids is the primary treatment ofsewage, where the unit process is called "primary settling tanks" or "primary sedimentation tanks".[17] It is also widely used for the treatment of other types of wastewater. Solids that are denser than water will accumulate at the bottom of quiescentsettling basins. More complexclarifiers also have skimmers to simultaneously remove floating grease such as soap scum and solids such as feathers, wood chips, orcondoms. Containers like theAPI oil-water separator are specifically designed to separate non-polar liquids.[18]: 111–138
Oxidation reduces thebiochemical oxygen demand of wastewater, and may reduce the toxicity of some impurities.Secondary treatment converts organic compounds intocarbon dioxide,water, andbiosolids through oxidation and reduction reactions.[19] Chemical oxidation is widely used for disinfection.[20]
Advanced oxidation processes are used to remove somepersistent organic pollutants and concentrations remaining after biochemical oxidation.[18]: 363–408 Disinfection by chemical oxidation killsbacteria and microbialpathogens by addinghydroxyl radicals such asozone,chlorine or hypochlorite to wastewater.[2]: 1220 These hydroxyl radical then break down complex compounds in the organic pollutants into simple compounds such as water, carbon dioxide, andsalts.[22]
Anaerobic wastewater treatment processes (for exampleUASB,EGSB) are also widely applied in the treatment of industrial wastewaters and biological sludge.
Polishing refers to treatments made in further advanced treatment steps after the above methods (also called "fourth stage" treatment). These treatments may also be used independently for some industrial wastewater.Chemical reduction orpH adjustment minimizes chemical reactivity of wastewater following chemical oxidation.[18]: 439 Carbon filtering removes remaining contaminants and impurities by chemical absorption ontoactivated carbon.[2]: 1138 Filtration through sand (calcium carbonate) or fabric filters is the most common method used in municipal wastewater treatment.
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