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Sewage treatment - Wikipedia

The digestion process is speeded up by heating the sludge to a temperature where naturally occurring bacteria respond to these comfortable conditions and feed on other bacteria.

On cooling, the well-fed bacteria die off, and the sludge is suitable for use on agricultural land. A by-product of the sludge digestion process is methane gas, which can be burned to produce electricity.

The electricity can be used to heat more sludge or to provide heat and light for the treatment works. Further treatments are available to reduce pollutants from sewage sludge, particularly PTE, due to their persistence and connected health risk during land application.

For a review of the various methodologies available for PTE removal from sewage sludge, see Babel and del Mundo Dacera Ascaris roundwormAncylostoma hookwormTrichuris whipworm ; It can also contain non-pathogenic bacteria and animals such as insectsarthropodssmall fish.

Wastewater quality indicators Since all natural waterways contain bacteria and nutrients, almost any waste compounds introduced into such waterways will initiate biochemical reactions such as shown above. Those biochemical reactions create what is measured in the laboratory as the biochemical oxygen demand BOD.

Such chemicals are also liable to be broken down using strong oxidizing agents and these chemical reactions create what is measured in the laboratory as the chemical oxygen demand COD. Both have been widely adopted as a measure of pollution effect. The BOD test measures municipal wastewater treatment oxygen demand of biodegradable pollutants whereas the COD test measures the oxygen demand of oxidizable pollutants.

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Tertiary treatment is sometimes defined as anything more than primary and secondary treatment in order to allow ejection into a highly sensitive municipal wastewater treatment fragile ecosystem estuaries, low-flow rivers, coral reefs, Treated water is sometimes disinfected chemically or physically for example, by lagoons and microfiltration prior to discharge into a streamriverbaylagoon or wetlandor it can be used for the 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.

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Simplified process flow diagram for a typical large-scale treatment plant Process flow diagram for a typical treatment plant via subsurface flow constructed wetlands SFCW Pretreatment[ edit ] Pretreatment removes all materials that can be easily collected from the raw sewage before municipal wastewater treatment damage or clog the pumps and sewage lines of primary treatment clarifiers.

Objects commonly removed during pretreatment include trash, tree limbs, leaves, branches, and other large objects. The municipal wastewater treatment in sewage water passes through a bar screen to remove all large objects like cans, rags, sticks, plastic packets etc. The solids are collected and later disposed in a landfill, or incinerated.

Bar screens or mesh screens of varying sizes may be used to optimize solids removal. If gross solids are municipal wastewater treatment removed, they become entrained in pipes and moving parts of the treatment plant, and can cause substantial damage and inefficiency in the process.


It also includes organic matter such as eggshells, bone chips, seeds, and coffee grounds. Pretreatment may include a sand or grit channel or chamber, where the velocity of the incoming sewage is adjusted to allow the settlement of sand and grit.

Grit removal is necessary to 1 reduce formation municipal wastewater treatment heavy deposits in aeration tanks, aerobic digesters, pipelines, channels, and conduits; 2 reduce the frequency of digester cleaning caused by excessive accumulations of grit; and 3 protect moving mechanical equipment from abrasion and accompanying abnormal wear.


The removal of grit is essential for equipment with closely machined metal surfaces such as comminutors, fine screens, centrifuges, heat exchangers, and municipal wastewater treatment pressure diaphram pumps.

Grit chambers come in 3 types: Vortex type grit chambers include mechanically induced vortex, hydraulically induced vortex, and multi-tray vortex separators.

Wastewater treatment

Given that traditionally, grit removal systems have been designed to remove clean inorganic particles that are greater than 0. During periods of high flow deposited grit is resuspended and the quantity of grit reaching the treatment plant increases substantially.

It is, therefore important that the grit removal system not only operate efficiently during normal flow conditions but also under sustained peak flows when the greatest volume of grit reaches the plant.

Equalization basins may be used for temporary storage of diurnal or wet-weather flow peaks. Basins provide a place to temporarily hold incoming sewage during plant municipal wastewater treatment and a means of diluting and distributing batch discharges of toxic or high-strength waste which might otherwise inhibit biological secondary treatment including portable toilet waste, vehicle holding tanks, and municipal wastewater treatment tank pumpers.

Flow equalization basins require variable discharge control, typically include provisions for bypass and cleaning, and may also include aerators.