Wastewater treatment works to treat wastewater from various industries with an acceptable environmental impact. Through various processes, wastewater and sewage can be treated to return to the water cycle or reused for other purposes (a process known as water reclamation). Treatment of wastewater occurs at wastewater treatment plants, also known as water resource recovery facilities or sewage treatment plants. Wastewater treatment is included in the field of sanitation as part of the management of human waste, solid waste, and stormwater management. Biogas can be a by-product of the wastewater treatment if anaerobic processes are used.
Wastewater is treated at plants. These include treatment plants designed for municipal sewage, industrial wastewater, agricultural wastewater, and plants specialized to treat leachate from landfills.
The treatment of wastewater includes physical treatment, chemical treatment, and polishing treatments. The most common method of processing wastewater follows a series of processes intended to seperate solids from liquids, usually by sedimentation. The progressively dissolved material into solids (biological floc) is settled out and an effluent stream of increasing purity is produced.
Through sedimentation, solids such as stones, grit, and sand are removed through gravity. This is achieved using a grit channel designed to produce an optimum flow rate to allow grit to settle and allow less-dense solids to be carried to the next treatment stage. The separation will happen in large settling basins. More complex basins, known as Clarifiers, include skimmers to remove floating grease such as soap scum and solids such as feathers, wood chips and other solids too light to settle.
Oxidation is a process to reduce the biochemical oxygen demand of wastewater and may reduce the toxicity of some impurities. Secondary treatment converts compounds into carbon dioxide, water, and biosolids. The process of chemical oxidation is used for disinfection. A secondary treatment of biochemical oxidation can be used for agricultural and industrial wastewater. Electrochemical oxidation can be used to remove some persistent organic pollutants after biochemical oxidation. Chemical oxidation will also be used to kill bacteria and microbial pathogens through the addition of ozone, chlorine or hypochlorite.
Polishing is often the last phase of treating wastewater and refers to methods of chemical reduction and pH adjustment to minimize the reactivity of the wastewater following chemical oxidation. Carbon filtering can be used to remove remaining contaminants and impurities followed by filtration through sand and fabric filters as a final stage.