General Principles of Sewage Treatment
Raw sewage and septic wastewaters contain a variety of contaminants. Many technologies are available to render the sewage suitable for safe discharge to the environment. These include those used in the municipal treatment works that receive sewage discharged to public sewers in the nation’s developed areas; conventional on-site sewage treatment that uses a septic tank and soil absorption field commonly used in rural areas; and the alternative on-site technologies that form the focus of this publication. Most sewage treatment technologies operate by combining basic physical, chemical, and biological processes
1. Primary treatment removes solid chunks and particles from raw sewage through gravity separation and/or screening. A septic tank is the most common primary treatment device in on-site systems. In alternative systems, the septic tank is commonly outfitted with an outlet filter, to capture solid particles that are too small or too light to settle. When used with conventional septic systems, an outlet filter will extend system longevity and improve performance. The partially-treated liquid discharged from primary treatment is called primary effluent.
2. Secondary-treatment processes (also called microbial digestion) receive primary effluent. Most secondary-treatment processes move the effluent through an aeration process environment that is favorable to aerobic microorganisms, those that thrive in atmospheric oxygen (O2) environments. The following wastewater renovation processes occur during this treatment:
Pathogenic microorganism populations are reduced. The vast majority of microorganisms found in sewage thrive within the human digestive system, an environment where oxygen does not occur as O2. Consequently, these organisms are not well adapted to aerated environments. Within secondary-treatment devices, some microorganisms (including most pathogens) perish as a result of exposure to O2.
Other organisms, including predators that consume pathogens, do thrive in an aerobic environment, sustained by the rich mix of O2 with H2O, biodegradable organic compounds, and essential nutrients that comprises sewage. Where the effluent passes through secondary treatment media with small pores (such as a sand filter, or natural soils), pathogen numbers are also reduced via physical straining.
Biodegradable organic contaminants, such as dissolved organic substances, and organic particles, remaining in the effluent after primary treatment are removed.
The microorganisms in the aerated secondary-treatment medium consume and metabolize biodegradable organic compounds, deriving energy by breaking the carbon-carbon bonds and converting the organic carbon to carbon dioxide (CO2).
Small particulate contaminants are removed. Where the filtration media are comprised of mineral particles with small pores (such as a natural soil or a sand filter), particulate contaminants are removed via physical screening; biodegradable components of the particles captured in the fine pores are consumed by the resident aerobic bacteria.
The partially-treated liquid discharged from secondary treatment is called secondary effluent.
3. Advanced treatments are optional processes that may be applied to remove additional contaminants from secondary effluent prior to dispersal. Advanced treatment is usually included only in systems intended to discharge directly to the land surface, or to surface-water streams. Advanced treatment processes designed to remove additional nitrogen and phosphorous from the effluent are sometimes necessary to protect water quality in streams receiving treated effluent discharges.
4. Disinfection systems often rely on chlorination, ozonation, or ultraviolet light. Systems that discharge treated effluent where there is a potential for direct human exposure (i.e., discharge to surface waters or the soil surface) are often required to disinfect the effluent so as to eliminate potential hazards due to human exposure.
Effluent that has been disinfected, and has received advanced treatment, is called tertiary effluent.
Treated effluent must be discharged to (or dispersed in) the environment. Secondary effluent is commonly dispersed in soils below the surface, while tertiary effluent may be discharged to flowing waters (such as a surface-water stream) or on the soil surface. Surface discharge or dispersal typically requires a permit from an agency responsible for protecting surface-water quality as well as an on-site septic system permit. Credit:https://www.slideshare.net/ricardo0hodges91/ onsite-sewage-treatment-alternatives-publications-and-educational-resources