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Sewage treatment often involves two main stages, called primary and secondary treatment, while advanced treatment also incorporates a tertiary treatment stage with polishing processes. [13] Different types of sewage treatment may utilize some or all of the process steps listed below.
Primary treatment settling removes about half of the solids and a third of the BOD from raw sewage. [7] Secondary treatment is defined as the "removal of biodegradable organic matter (in solution or suspension) and suspended solids. Disinfection is also typically included in the definition of conventional secondary treatment."
Sewage treatment plant (a type of wastewater treatment plant) in La Crosse, Wisconsin. Wastewater treatment is a process which removes and eliminates contaminants from wastewater. It thus converts it into an effluent that can be returned to the water cycle. Once back in the water cycle, the effluent creates an acceptable impact on the environment.
Secondary treatment domestic vermifilter for wastewater showing ventilation around basket and pine bark media. Secondary and tertiary treatment vermifilters can be underneath the primary vermifilter in a single tower, but are typically single reactors, where several reactors can be chained in series as sequential vermifilters.
Facultative lagoon (polishing pond) providing tertiary treatment after a constructed wetland in Hamburg-Allermöhe, Germany. Overflow from the facultative lagoon may be routed through one or more polishing ponds supporting lower populations of anaerobic micro-organisms and a higher proportion of aerobic organisms adapted to survival in lower concentrations of organic material.
Constructed subsurface flow wetlands are meant as secondary treatment systems which means that the effluent needs to first pass a primary treatment which effectively removes solids. Such a primary treatment can consist of sand and grit removal, grease trap, compost filter, septic tank , Imhoff tank , anaerobic baffled reactor or upflow ...
Additionally, AOPs can be used to treat effluent of secondary treated wastewater which is then called tertiary treatment. [6] The contaminant materials are largely converted into stable inorganic compounds such as water, carbon dioxide and salts, i.e. they undergo mineralization.
Preventive healthcare strategies are described as taking place at the primal, [2] primary, [13] secondary, and tertiary prevention levels. Although advocated as preventive medicine in the early twentieth century by Sara Josephine Baker, [14] in the 1940s, Hugh R. Leavell and E. Gurney Clark coined the term primary prevention.