Sewage treatment may also be referred to as wastewater treatment. However, the latter is a broader term which can also refer to industrial wastewater.
For most cities, the sewer system will also carry a proportion of industrial effluent to the sewage treatment plant which has usually received pre-treatment at the factories themselves to reduce the pollutant load. If the sewer system is a combined sewer then it will also carry urban runoff stormwater to the sewage treatment plant. Sewage water can travel towards treatment plants via piping and in a flow aided by gravity and pumps. The first part of filtration of sewage typically includes a bar screen to filter solids and large objects which are then collected in dumpsters and disposed of in landfills.
Fat and grease is also removed before the primary treatment of sewage. The term "sewage treatment plant" or "sewage treatment works" in some countries is nowadays often replaced with the term wastewater treatment plant or wastewater treatment station. Sewage can be treated close to where the sewage is created, which may be called a "decentralized" system or even an "on-site" system in septic tanks , biofilters or aerobic treatment systems.
Alternatively, sewage can be collected and transported by a network of pipes and pump stations to a municipal treatment plant. This is called a "centralized" system see also sewerage and pipes and infrastructure. Sewage is generated by residential, institutional, commercial and industrial establishments. It includes household waste liquid from toilets , baths , showers , kitchens , and sinks draining into sewers.
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In many areas, sewage also includes liquid waste from industry and commerce. The separation and draining of household waste into greywater and blackwater is becoming more common in the developed world, with treated greywater being permitted to be used for watering plants or recycled for flushing toilets. Sewage may include stormwater runoff or urban runoff.
Sewerage systems capable of handling storm water are known as combined sewer systems. This design was common when urban sewerage systems were first developed, in the late 19th and early 20th centuries. Heavy volumes of storm runoff may overwhelm the sewage treatment system, causing a spill or overflow.
Sanitary sewers are typically much smaller than combined sewers, and they are not designed to transport stormwater. Communities that have urbanized in the midth century or later generally have built separate systems for sewage sanitary sewers and stormwater, because precipitation causes widely varying flows, reducing sewage treatment plant efficiency.
As rainfall travels over roofs and the ground, it may pick up various contaminants including soil particles and other sediment , heavy metals , organic compounds , animal waste, and oil and grease. Some jurisdictions require stormwater to receive some level of treatment before being discharged directly into waterways. Examples of treatment processes used for stormwater include retention basins , wetlands , buried vaults with various kinds of media filters , and vortex separators to remove coarse solids.
In highly regulated developed countries, industrial effluent usually receives at least pretreatment if not full treatment at the factories themselves to reduce the pollutant load, before discharge to the sewer. This process is called industrial wastewater treatment or pretreatment. The same does not apply to many developing countries where industrial effluent is more likely to enter the sewer if it exists, or even the receiving water body, without pretreatment. Industrial wastewater may contain pollutants which cannot be removed by conventional sewage treatment.
Also, variable flow of industrial waste associated with production cycles may upset the population dynamics of biological treatment units, such as the activated sludge process. Sewage collection and treatment is typically subject to local, state and federal regulations and standards. Treating wastewater has the aim to produce an effluent that will do as little harm as possible when discharged to the surrounding environment, thereby preventing pollution compared to releasing untreated wastewater into the environment.
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Sewage treatment generally involves three stages, called primary, secondary and tertiary treatment. Pretreatment removes all materials that can be easily collected from the raw sewage before they 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 influent 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 not removed, they become entrained in pipes and moving parts of the treatment plant, and can cause substantial damage and inefficiency in the process. Grit consists of sand, gravel, cinders, and other heavy materials.
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 of 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 high 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.
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. Clarifiers and mechanized secondary treatment are more efficient under uniform flow conditions.
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 maintenance 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 septic tank pumpers. Flow equalization basins require variable discharge control, typically include provisions for bypass and cleaning, and may also include aerators.
Cleaning may be easier if the basin is downstream of screening and grit removal.
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In some larger plants, fat and grease are removed by passing the sewage through a small tank where skimmers collect the fat floating on the surface. Air blowers in the base of the tank may also be used to help recover the fat as a froth. Many plants, however, use primary clarifiers with mechanical surface skimmers for fat and grease removal. In the primary sedimentation stage, sewage flows through large tanks, commonly called "pre-settling basins", "primary sedimentation tanks" or "primary clarifiers ".
Primary settling tanks are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank where it is pumped to sludge treatment facilities. Secondary treatment is designed to substantially degrade the biological content of the sewage which are derived from human waste, food waste, soaps and detergent.
The majority of municipal plants treat the settled sewage liquor using aerobic biological processes. To be effective, the biota require both oxygen and food to live. The bacteria and protozoa consume biodegradable soluble organic contaminants e. Some secondary treatment methods include a secondary clarifier to settle out and separate biological floc or filter material grown in the secondary treatment bioreactor.
The purpose of tertiary treatment is to provide a final treatment stage to further improve the effluent quality before it is discharged to the receiving environment sea, river, lake, wet lands, ground, etc. More than one tertiary treatment process may be used at any treatment plant. If disinfection is practised, it is always the final process. It is also called "effluent polishing. Sand filtration removes much of the residual suspended matter. Lagoons or ponds provide settlement and further biological improvement through storage in large man-made ponds or lagoons.
These lagoons are highly aerobic and colonization by native macrophytes , especially reeds, is often encouraged. Small filter-feeding invertebrates such as Daphnia and species of Rotifera greatly assist in treatment by removing fine particulates.
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Biological nutrient removal BNR is regarded by some as a type of secondary treatment process,  and by others as a tertiary or "advanced" treatment process. Wastewater may contain high levels of the nutrients nitrogen and phosphorus. Excessive release to the environment can lead to a buildup of nutrients, called eutrophication , which can in turn encourage the overgrowth of weeds, algae , and cyanobacteria blue-green algae.
This may cause an algal bloom , a rapid growth in the population of algae. The algae numbers are unsustainable and eventually most of them die. The decomposition of the algae by bacteria uses up so much of the oxygen in the water that most or all of the animals die, which creates more organic matter for the bacteria to decompose. In addition to causing deoxygenation, some algal species produce toxins that contaminate drinking water supplies. Different treatment processes are required to remove nitrogen and phosphorus. Nitrogen is removed through the biological oxidation of nitrogen from ammonia to nitrate nitrification , followed by denitrification , the reduction of nitrate to nitrogen gas.
Nitrogen gas is released to the atmosphere and thus removed from the water. Nitrification itself is a two-step aerobic process, each step facilitated by a different type of bacteria. Denitrification requires anoxic conditions to encourage the appropriate biological communities to form. It is facilitated by a wide diversity of bacteria. Sand filters, lagooning and reed beds can all be used to reduce nitrogen, but the activated sludge process if designed well can do the job the most easily. This can be, depending on the waste water, organic matter from feces , sulfide , or an added donor like methanol.
The sludge in the anoxic tanks denitrification tanks must be mixed well mixture of recirculated mixed liquor, return activated sludge [RAS], and raw influent e. Over time, different treatment configurations have evolved as denitrification has become more sophisticated. The normal primary and secondary treatment processes of these wastewaters have been introduced in a growing number of places, in order to eliminate the easily settled materials and to oxidize the organic material present in wastewater.
The final result is a clear, apparently clean effluent which is discharged into natural water bodies. This secondary effluent is, however, loaded with inorganic nitrogen and phosphorus and causes eutrophication and more long-term problems because of refractory organics and heavy metals that are discharged. Microalgae culture offers an interesting step for wastewater treatments, because they provide a tertiary biotreatment coupled with the production of potentially valuable biomass, which can be used for several purposes.
Microalgae cultures offer an elegant solution to tertiary and quandary treatments due to the ability of microalgae to use inorganic nitrogen and phosphorus for their growth. The technologies discussed here include biosorption and bioaccumulation of heavy metals, cell immobilization of algae, and mathematical modelling of metal uptake by cells. This book is unique in that it takes a practical approach to the subject matter and is a useful reference both in and outside of the laboratory. Removal of Copper by Free and Immobilized Microalga.
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