‘Sewage’ is generated by residential, institutional, commercial and industrial establishments. Apart from wastewater generated by the process, it also includes waste liquid from toilets, baths, showers, kitchens, and sinks draining into sewers.
‘Sewage Treatment’ is the process of removing contaminants from wastewater using physical, chemical and biological processes. The output water or the recovered water is safe for the environment. What remains is a semi-solid waste called sewage sludge which is further treated to make it suitable for disposal.
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Sewage can be treated at the site where sewage is generated or at a centralized treatment plant.
Pretreatment is where large objects, grit, fat and grease are removed to ensure the downstream treatment stages operate at the desired efficiency levels. With the help of a bar screen, materials such as trash, tree limbs, leaves, branches, and other large objects are removed from the raw sewage to prevent any damage or clog the pumps and sewage lines. Grit such as sand, gravel, cinders, and organic materials are removed using a grit channel or chamber where the velocity of the incoming sewage is adjusted to allow settlement. Grit removal helps reduce formation of heavy deposits in aeration tanks, aerobic digesters, pipelines, channels and protect moving mechanical equipment from abrasion and accompanying abnormal wear.
Equalization tanks temporarily hold incoming sewage and serve as a means of diluting and distributing batch discharges of toxic or high-strength waste. This is to ensure uniform flow conditions so that the clarifiers and mechanized downstream treatment are efficient. Fat and grease are removed by passing the sewage through a small tank where skimmers collect the fat floating on the surface.
Sewage treatment generally involves three stages: primary, secondary and tertiary treatment.
Primary Sewage treatment consists of temporarily holding the sewage in a basin (“pre-settling basins”, “primary sedimentation tanks” or “primary clarifiers”) where heavy solids are allowed to settle to the bottom while oil, grease and lighter solids float to the surface. The settled and floating materials are removed and the remaining liquid is subjected to secondary treatment.
Secondary Sewage treatment removes dissolved and suspended biological matter. The biological content of the sewage is degraded by the indigenous, water-borne microorganisms in a managed habitat. The bacteria and protozoa consume biodegradable soluble organic contaminants and bind much of the less soluble fractions into floc.
Tertiary Sewage treatment is sometimes defined as anything more than primary and secondary treatment in order to allow ejection into the environment. It is also called “effluent polishing.”
Media Filtration removes residual suspended matter and residual toxins. Nitrogen is removed through the biological oxidation of nitrogen from ammonia to nitrate (nitrification), followed by denitrification, the reduction of nitrate to nitrogen gas. Phosphorus can be removed by enhanced biological phosphorus removal and also by chemical precipitation, usually with salts of iron (e.g. ferric chloride), aluminium (e.g. alum), or lime. Chlorination, Ozonation and Ultraviolet (UV) light are one of many methods to waste water disinfection.
Treated water is disinfected chemically or physically prior to discharge for the use of irrigation of a golf course, green way or park. If it is sufficiently clean, it can also be used for groundwater recharge or agricultural purposes.
Sludge treatment and Sewage disposal is an important stage in the sewage water treatment. The purpose of digestion is to reduce the amount of organic matter and the number of disease-causing microorganisms present in the solids. The most common treatment options include anaerobic digestion, aerobic digestion, and composting. Incineration is also used where necessary.