wastewater treatment, also called sewage treatment, is the removal of impurities from wastewater, or sewage before it reaches aquifers or natural bodies of water such as rivers, lakes, estuaries, and oceans.
General types of water pollutants include pathogenic organisms, oxygen-demanding wastes, plant nutrients, synthetic organic chemicals, inorganic chemicals, microplastics, sediments, radioactive substances, oil, and heat. Sewage is the primary source of the first three types. Farms and industrial facilities are also sources of some of them. Sediment from eroded topsoil is considered a pollutant because it can damage aquatic ecosystems, and heat (particularly from power-plant cooling water) is considered a pollutant because of the adverse effect it has on dissolved oxygen levels and aquatic life in rivers and lakes.
There are three types of wastewater, or sewage: domestic sewage, industrial sewage, and storm sewage. Domestic sewage carries used water from houses and apartments; it is also called sanitary sewage. Industrial sewage is used water from manufacturing or chemical processes. Storm sewage, or stormwater, is runoff from precipitation that is collected in a system of pipes or open channels.
There are three levels of wastewater treatment: primary, secondary, and tertiary (or advanced). Primary treatment removes about 60 percent of total suspended solids and about 35 percent of BOD; dissolved impurities are not removed. It is usually used as a first step before secondary treatment. Secondary treatment removes more than 85 percent of both suspended solids and BOD. A level of secondary treatment is usually required in the United States and other developed countries. When more than 85 percent of total solids and BOD must be removed, or when dissolved nitrate and phosphate levels must be reduced, tertiary treatment methods are used. Tertiary processes can remove more than 99 percent of all the impurities from sewage, producing an effluent of almost drinking-water quality. Tertiary treatment can be very expensive, often doubling the cost of secondary treatment. It is used only under special circumstances.
For all levels of wastewater treatment, the last step prior to discharge of the sewage effluent into a body of surface water is disinfection, which destroys any remaining pathogens in the effluent and protects public health.
The four main stages of water treatment. In order, the stages are physical treatment, biological treatment, filtration, and disinfection. In the physical treatment stage, various materials are physically removed from wastewater. First, wastewater passes through wedge wire screens which catch large debris, like branches or rocks. Next, smaller pieces of materials-- such as sand and silt, stones, and facial tissues-- are trapped in grit chambers.
Filtration gives us the opportunity to constantly be protecting the environment, which is exactly what we're doing. It is very dynamic work in here, a process here. It's challenging in a way. It's not as simple as what sort of comes in and is discharged-- there's a lot to it. There are a lot of challenges.
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