Wastewater treatment is a process used to remove contaminants from wastewater or sewage 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), the treatment plant may be called a Water Resource Recovery Facility (WRRF) or a Sewage Treatment Plant (STP). 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 are designed to achieve phase separation, oxidation or polishing. Types of wastewater treatment plants include sewage treatment plants, industrial wastewater treatment plants, agricultural wastewater treatment plants and leachate treatment plants.
The treatment of wastewater is part of the overarching field of sanitation. Sanitation also includes the management of human waste and solid waste as well as stormwater (drainage) management. The main by-product from wastewater treatment plants is 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.
The processes involved in waste-water treatment include physical processes such as settlement or flotation and biological processes such as aerated lagoons, activated sludge, or bio-films in trickling filters. Other physical methods such as filtration through sieves may be used in specialized circumstances such as de-watering waste-water sludge.
To be effective, sewage must be conveyed to a treatment plant by appropriate pipes and infrastructure, and the process itself must be subject to regulation and controls. Some wastewaters require specialized treatment methods. At the simplest level, treatment of sewage and most wastewaters is carried out through separation of solids from liquids, usually by sedimentation. By progressively converting dissolved material into solids, usually a biological floc, which is then settled out, an effluent stream of increasing purity is produced.
Phase separation transfers impurities into a non-aqueous phase. Phase separation may occur at intermediate points in a treatment sequence to remove solids generated during oxidation or polishing. Grease and oil may be recovered for fuel or saponification. Solids often require dewatering of sludge in a wastewater treatment plant. Disposal options for dried solids vary with the type and concentration of impurities removed from water.
Solids such as stones, grit, and sand may be removed from wastewater by gravity when density differences are sufficient to overcome dispersion by turbulence. 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 of sewage, where the unit process is called "primary settling tanks" or "primary sedimentation tanks." 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 quiescent settling basins. More complex clarifiers also have skimmers to simultaneously remove floating grease such as soap scum and solids such as feathers, wood chips, or condoms. Containers like the API oil-water separator are specifically designed to separate non-polar liquids.
Oxidation reduces the biochemical oxygen demand of wastewater, and may reduce the toxicity of some impurities. Secondary treatment converts organic compounds into carbon dioxide, water, and biosolids through oxidation and reduction reactions. Chemical oxidation is widely used for disinfection.
Secondary treatment by biochemical oxidation of dissolved and colloidal organic compounds is widely used in sewage treatment and is applicable to some agricultural and industrial wastewaters. Biological oxidation will preferentially remove organic compounds useful as a food supply for the treatment ecosystem. Concentration of some less digestible compounds may be reduced by co-metabolism. Removal efficiency is limited by the minimum food concentration required to sustain the treatment ecosystem.
Chemical (including electrochemical) oxidation, also known as advanced chemical oxidation, is used to remove some persistent organic pollutants and concentrations remaining after biochemical oxidation. Disinfection by chemical oxidation kills bacteria and microbial pathogens by adding hydroxyl radicals such as ozone, chlorine or hypochlorite to wastewater.:1220 These hydroxyl radical then break down complex compounds in the organic pollutants into simple compounds such as water, carbon dioxide, and salts.
Polishing refers to treatments made following the above methods. These treatments may also be used independently for some industrial wastewater. Chemical reduction or pH adjustment minimizes chemical reactivity of wastewater following chemical oxidation. Carbon filtering removes remaining contaminants and impurities by chemical absorption onto activated carbon.:1138 Filtration through sand (calcium carbonate) or fabric filters is the most common method used in municipal wastewater treatment.
Types of treatment plants
Wastewater treatment plants may be distinguished by the type of wastewater to be treated, i.e. whether it is sewage, industrial wastewater, agricultural wastewater or leachate.
Sewage treatment plants
A typical municipal sewage treatment plant in an industrialized country may include primary treatment to remove solid material, secondary treatment to digest dissolved and suspended organic material as well as the nutrients nitrogen and phosphorus, and – sometimes but not always – disinfection to kill pathogenic bacteria. The sewage sludge that is produced in sewage treatment plants undergoes sludge treatment. Larger municipalities often include factories discharging industrial wastewater into the municipal sewer system. The term "sewage treatment plant" is sometimes replaced with the term "wastewater treatment plant". Sewage can also be treated by processes using "Nature-based solutions".
Tertiary treatment is a term applied to polishing methods used following a traditional sewage treatment sequence. Tertiary treatment is being increasingly applied in industrialized countries and most common technologies are micro filtration or synthetic membranes. After membrane filtration, the treated wastewater is nearly indistinguishable from waters of natural origin of drinking quality (without its minerals). Nitrates can be removed from wastewater by natural processes in wetlands but also via microbial denitrification. Ozone wastewater treatment is also growing in popularity, and requires the use of an ozone generator, which decontaminates the water as ozone bubbles percolate through the tank. The latest, and very promising treatment technology is the use aerobic granulation.
Industrial wastewater treatment plants
Disposal of wastewaters from an industrial plant is a difficult and costly problem. Most petroleum refineries, chemical and petrochemical plants:1412 have onsite facilities to treat their wastewaters so that the pollutant concentrations in the treated wastewater comply with the local and/or national regulations regarding disposal of wastewaters into community treatment plants or into rivers, lakes or oceans. Constructed wetlands are being used in an increasing number of cases as they provided high quality and productive on-site treatment. Other industrial processes that produce a lot of waste-waters such as paper and pulp production has created environmental concern, leading to development of processes to recycle water use within plants before they have to be cleaned and disposed.
Industrial wastewater treatment plants are required where municipal sewage treatment plants are unavailable or cannot adequately treat specific industrial wastewaters. Industrial wastewater plants may reduce raw water costs by converting selected wastewaters to reclaimed water used for different purposes. Industrial wastewater treatment plants may reduce wastewater treatment charges collected by municipal sewage treatment plants by pre-treating wastewaters to reduce concentrations of pollutants measured to determine user fees.
Although economies of scale may favor use of a large municipal sewage treatment plant for disposal of small volumes of industrial wastewater, industrial wastewater treatment and disposal may be less expensive than correctly apportioned costs for larger volumes of industrial wastewater not requiring the conventional sewage treatment sequence of a small municipal sewage treatment plant.
An industrial wastewater treatment plant may include one or more of the following rather than the conventional primary, secondary, and disinfection sequence of sewage treatment:
- An API oil-water separator, for removing separate phase oil from wastewater.
- A clarifier, for removing solids from wastewater.
- A roughing filter, to reduce the biochemical oxygen demand of wastewater.
- A carbon filtration plant, to remove toxic dissolved organic compounds from wastewater.
- An advanced electrodialysis reversal (EDR) system with ion-exchange membranes.
Agricultural wastewater treatment plants
Agricultural wastewater treatment for continuous confined animal operations like milk and egg production may be performed in plants using mechanized treatment units similar to those described under industrial wastewater; but where land is available for ponds, settling basins and facultative lagoons may have lower operational costs for seasonal use conditions from breeding or harvest cycles.
Many farms generate nonpoint source pollution from surface runoff which is not controlled through a treatment plant. Farmers can install erosion controls and implement nutrient management plans to control runoff pollution.:pp. 4–95–4–96
Leachate treatment plants
Leachate treatment plants are used to treat leachate from landfills. Treatment options include: biological treatment, mechanical treatment by ultrafiltration, treatment with active carbon filters, electrochemical treatment including electrocoagulation by various proprietary technologies and reverse osmosis membrane filtration using disc tube module technology.
Disposal or reuse
The main purpose of wastewater treatment is for the water to be able to be disposed or reused safely. However, before it is treated, the options for disposal or reuse must be considered so he correct treatment process is used on the wastewater.
For disposal into the ocean, environmental treaty requirements have to met. As international treaties often manage water over countries' borders, wastewater disposal is easier in bodies of water found entirely under the jurisdiction of one country.
Environmental impacts of wastewater disposal
In many developing countries, where water scarcity and water pollution is often a significant issue, wastewater treatment facilities are not apt to treat the amount of wastewater received, and it ends up being disposed in local waterways untreated. For example, in Afghanistan and Pakistan, the Kabul River receives about 1 m3/sec of untreated wastewater, and it has been proven to be a causing factor for the contamination of the river. Globally, water pollution from untreated wastewater comes from the presence of chemicals in detergents. The chemicals surfactants in detergent cannot be degraded unless they are in the presence of specific microorganisms. In more recent years, water pollution from detergent residue has become a more relevant problem.
Reuse of wastewater is a good solution to reduce water stress in cities, promote consciousness, and reduce pollutants into water bodies. Reused water can be used for multiple purposes such as “reclaimed and reused for crop and landscape irrigation, groundwater recharge, or recreational purposes.” There are two ways of wastewater reuse: direct and indirect reuse. Direct reuse entails using treated wastewater as it is, and indirect reused is diluting treated wastewater with another water source before further use.
At the global level, an estimated 52% of wastewater is treated. However, wastewater treatment rates are highly unequal around the world. For example, while high-income countries treat approximately 74% of their wastewater, low-income countries treat an average of just 4.2%. Improving wastewater treatment across the globe is crucial for reducing our pollution to the environment and achieve water quality improvements. For this reason, Sustainable Development Goal 6 has a Target 6.3 which is formulated as follows: "By 2030, improve water quality by reducing pollution, eliminating dumping and minimizing release of hazardous chemicals and materials, halving the proportion of untreated wastewater and substantially increasing recycling and safe reuse globally." The corresponding Indicator 6.3.1 is the "proportion of wastewater safely treated" (see map on the right for 2018). For example, Algeria, Senegal, Niger, Libya, Uganda, Somalia, and Iraq do not treat domestic wastewater safely yet. The World Health Organization reported that inadequate water, sanitation and hygiene conditions were responsible for 829,000 deaths from diarrhea in the world in 2016. Water and sanitation are a world problem, resulting from rapid global urbanization, and finding solutions in large cities is particularly urgent. Additionally there is the worsening water crisis in several regions of the world, as well as the evident ecological crisis, disasters and associated risks, such as the global warming in addition to the presence of polluting industries and technologies and their effects on water, soil, air and food, reducing the biodiversity and destroying ecosystems.
In Brazil, countrywide, 19.1% of households are connected to septic tanks but they are not connected to the general network. Approximately 9 million households (12.6%) had a ditch, rudimentary cesspit, river, lake or sea, in addition to other forms of waste disposal. The relevance of sanitation infrastructure in the health-disease process was highlighted in a study on hospitalizations due waterborne diseases. Overall, in 2015, diarrhea was estimated to be one of the main causes of death in all age groups (1.31 million). In terms of coverage, around 40% of the sewage generated in Brazil is treated, with an estimated number of treatment plants in the order of 2,800. National responsibility for wastewater and sewage treatment lies in the hands of the ministry of cities in Brazil. The ministry of cities consists of different ministries that each contribute to water supply and sanitation including wastewater and sewage treatment.
In Japan, wastewater treatment is managed by both the national government and local governments and municipalities. The governmental organizations that contribute to wastewater treatment policies at a national level in Japan are the Ministry of Health, Labor and Welfare,the Ministry of Land, Infrastructure, Transport and Tourism, and the Ministry of the Environment. Currently, Japan's methods of wastewater treatment include rural community sewers, wastewater facilities, and on-site treatment systems such as the Johkasou system to treat domestic wastewater. Larger wastewater facilities and sewer systems are generally used to treat wastewater in more urban areas with a larger population. Rural sewage systems are used to treat wastewater at smaller domestic wastewater treatment plants for a smaller population. Johkasou systems are on-site wastewater treatment systems tanks. They are used to treat the wastewater of a single household or to treat the wastewater of a small number of buildings in a more decentralized manner than a sewer system.
In Libya, wastewater treatment is managed by the general company for water and wastewater in Libya, which falls within the competence of the Housing and Utilities Government Ministry. There are approximately 200 wastewater treatment plants across the nation, but few plants are functioning. In fact, the 36 larger plants are in the major cities; however, only nine of them are operational, and the rest of them are under repair.The largest operating wastewater treatment plants are situated in Sirte, Tripoli, and Misurata, with a design capacity of 21,000, 110,000, and 24,000 m3/day, respectively. Moreover, a majority of the remaining wastewater facilities are small and medium-sized plants with a design capacity of approximately 370 to 6700 m3/day. Therefore, 145,800 m3/day or 11 percent of the wastewater is actually treated, and the remaining others are released into the ocean and artificial lagoons although they are untreated. In fact, nonoperational wastewater treatment plants in Tripoli lead to a spill of over 1,275, 000 cubic meters of unprocessed water into the ocean every day. 
In the United States over 75 percent of the population is served by over 16,000 municipal sewage treatment plants. Most plants are required to meet national secondary treatment standards.
The United States Environmental Protection Agency (EPA) and state environmental agencies set wastewater standards under the Clean Water Act. Point sources must obtain surface water discharge permits through the National Pollutant Discharge Elimination System (NPDES). Point sources include industrial facilities, municipal governments (sewage treatment plants and storm sewer systems), other government facilities such as military bases, and some agricultural facilities, such as animal feedlots. EPA sets basic national wastewater standards: The "Secondary Treatment Regulation" applies to sewage treatment plants, and the "Effluent guidelines" which are regulations for categories of industrial facilities.
- Algae fuel
- Decentralized wastewater system
- List of largest wastewater treatment plants
- List of waste-water treatment technologies
- List of water supply and sanitation by country
- Sanitary engineering
- Water treatment
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