wastewater treatment wastewater treatment wastewater treatment is a process which removes and eliminates contaminants from wastewater and converts this 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. there are several kinds of wastewater which are treated at the appropriate type of wastewater treatment plant. for domestic wastewater (also called municipal wastewater or sewage), the treatment plant is called a sewage treatment. for industrial wastewater, treatment either takes place in a separate industrial wastewater treatment, or in a sewage treatment plant (usually after some form of pre-treatment). further types of wastewater treatment plants include agricultural wastewater treatment and leachate treatment plants. this article is about the treatment of any type of wastewater from a range of sources. for treatment of wastewater from households or municipalities (domestic wastewater), see sewage treatment. sewage treatment plant (a type of wastewater treatment plant) in cuxhaven, germany. processes commonly used in wastewater treatment include phase separation (such as sedimentation), biological and chemical processes (such as oxidation) or polishing. 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.: ch.14 biogas can be another by-product if anaerobic treatment processes are used. treated wastewater can be reused as reclaimed water. 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". types of treatment plants 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. types of wastewater treatment plants include: sewage treatment plants industrial wastewater treatment plants agricultural wastewater treatment plants leachate treatment plants sewage treatment plants aeration tank of an activated sludge process at the wastewater treatment plant in dresden-kaditz, germanythis section is an excerpt from sewage treatment. sewage treatment (or domestic wastewater treatment, municipal wastewater treatment) is a type of wastewater treatment which aims to remove contaminants from sewage to produce an effluent that is suitable to discharge to the surrounding environment or an intended reuse application, thereby preventing water pollution from raw sewage discharges. sewage contains wastewater from households and businesses and possibly pre-treated industrial wastewater. there are a high number of sewage treatment processes to choose from. these can range from decentralized systems (including on-site treatment systems) to large centralized systems involving a network of pipes and pump stations (called sewerage) which convey the sewage to a treatment plant. for cities that have a combined sewer, the sewers will also carry urban runoff (stormwater) to the sewage treatment plant. sewage treatment often involves two main stages, called primary and secondary treatment, while advanced treatment also incorporates a tertiary treatment stage with polishing processes and nutrient removal. secondary treatment can reduce organic matter (measured as biological oxygen demand) from sewage, using aerobic or anaerobic biological processes. 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.: 215 often, the main criteria for selection are: desired effluent quality, expected construction and operating costs, availability of land, energy requirements and sustainability aspects. in developing countries and in rural areas with low population densities, sewage is often treated by various on-site sanitation systems and not conveyed in sewers. these systems include septic tanks connected to drain fields, on-site sewage systems (oss), vermifilter systems and many more. on the other hand, advanced and relatively expensive sewage treatment plants in cities that can afford them may include tertiary treatment with disinfection and possibly even a fourth treatment stage to remove micropollutants. at the global level, an estimated 52% of sewage is treated. however, sewage treatment rates are highly unequal for different countries around the world. for example, while high-income countries treat approximately 74% of their sewage, developing countries treat an average of just 4.2%. industrial wastewater treatment plants this section is an excerpt from industrial wastewater treatment. wastewater from an industrial process can be converted at a treatment plant to solids and treated water for reuse. 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 a sanitary sewer or to a surface water in the environment. some industrial facilities generate wastewater that can be treated in sewage treatment plants. most industrial processes, such as petroleum refineries, chemical and petrochemical 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 into sewers or into rivers, lakes or oceans.: 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 as ammonia.: 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.: 60 (most industries produce some wastewater. 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. 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 and petrochemicals, pharmaceutical manufacturing, pulp and paper industry, smelters, textile mills, industrial oil contamination, water treatment and wood 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 plants this section is an excerpt from agricultural wastewater treatment. agricultural wastewater treatment is a farm management agenda for controlling pollution from confined animal operations and from surface runoff that may be contaminated by chemicals in fertilizer, pesticides, animal slurry, crop residues or irrigation 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 for industrial wastewater. 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.: 6–8 animal slurries are usually treated by containment in anaerobic 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 generate nonpoint source pollution from surface runoff which is not controlled through a treatment plant. leachate treatment plants main article: leachate § treatment 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. unit processes diagram of a typical surface-aerated basin for wastewater treatment.