Wastewater treatment is a process which removes contaminants from wastewater. The resulting effluent, discharged to a water body, has an acceptable impact on the environment. Domestic wastewater, also called municipal wastewater or sewage, is processed at a sewage treatment plant. Industrial wastewater is often processed at a specially-designed industrial wastewater treatment facility, or at a sewage treatment plant. In the latter case the industry typically performs on-site pretreatment of the waste, before it is sent to the municipal plant. Other types of wastewater treatment plants include agricultural wastewater treatment and leachate treatment plants.
The term "wastewater treatment" is often used to mean "sewage treatment".
Common processes in wastewater treatment include phase separation, such as sedimentation, various biological and chemical processes, such as oxidation, and 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. Biogas can be another by-product if the process uses anaerobic treatment.
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.
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:
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.
The unit processes involved in wastewater treatment include physical processes such as settlement or flotation and biological processes such as oxidation or anaerobic treatment. Some wastewaters require specialized treatment methods. At the simplest level, treatment of 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 or biofilm, which is then settled out or separated, 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 lowers the biochemical oxygen demand of wastewater, and may detoxify 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.
Advanced oxidation processes are 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. These hydroxyl radicals then break down complex compounds in the organic pollutants into simple compounds such as water, carbon dioxide, and salts.
Anaerobic wastewater treatment processes are also widely applied in the treatment of industrial wastewaters and biological sludge. Anaerobic treatment occurs in a rectangular or cylindrical reactor, containing a porous packing medium lined with biofilm or biomass, that filters the solids out of the wastewater that enters it. The biomass interacts with the wastewater's organic compounds and turns them into methane and then into carbon dioxide. Upflow anaerobic sludge blanket digestion (UASB) submerges the packing medium, whereas downflow digestion may or may not. There is a variant of the UASB process called expanded granular sludge bed digestion (EGSB) that moves the wastewater faster through the reactor. Anaerobic processes produce biogas from 70-90%, and microbial biomass from 5-15%, of the wastewater's organic matter; the remainder does not decompose. Anaerobic processes are cheaper to set up; require less energy, nutrients and land; create methane gas; and produce only a fifth to a third of the solid byproducts that aerobic processes do. Anaerobic processes, however, do not do well at removing pathogens, nitrogen and phosphorus. They also produce foul odours, require post-treatment and can be slow to start up. Many compounds, like ammonia and sulfide, can disrupt anaerobic processes.
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 or pH adjustment minimizes chemical reactivity of wastewater following chemical oxidation. Carbon filtering removes remaining contaminants and impurities by chemical adsorption onto activated carbon. Filtration through sand (calcium carbonate) or fabric filters is the most common method used in municipal wastewater treatment.