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Agricultural wastewater treatment is afarm management agenda for controlling pollution fromconfined animal operations and fromsurface runoff that may be contaminated by chemicals or organisms infertilizer,pesticides,animal slurry,crop residues orirrigation water. Agricultural wastewater treatment is required for continuous confined animal operations like milk and egg production. It may be performed in plants using mechanized treatment units similar to those used forindustrial wastewater. Where land is available for ponds,settling basins andfacultative lagoons may have lower operational costs for seasonal use conditions from breeding or harvest cycles.[1]: 6–8 Animalslurries are usually treated by containment inanaerobic lagoons before disposal by spray or trickle application to grassland.Constructed wetlands are sometimes used to facilitate treatment of animal wastes.
Nonpoint source pollution includes sediment runoff, nutrient runoff and pesticides. Point source pollution includes animal wastes, silage liquor, milking parlour (dairy farming) wastes, slaughtering waste, vegetable washing water and firewater. Many farms generatenonpoint source pollution fromsurface runoff which is not controlled through a treatment plant.
Farmers can installerosion controls to reduce runoff flows and retain soil on their fields.[2][3]: pp. 4-95–4-96 Common techniques includecontour plowing, cropmulching,crop rotation, plantingperennial crops and installingriparian buffers.[4][3]: pp. 4-95–4-96 Farmers can also develop and implementnutrient management plans to reduce excess application of nutrients[4][3]: pp. 4-37–4-38 and reduce the potential fornutrient pollution. To minimize pesticide impacts, farmers may useIntegrated Pest Management (IPM) techniques (which can includebiological pest control) to maintain control over pests, reduce reliance on chemical pesticides, and protect water quality.[5]
Nonpoint source pollution from farms is caused bysurface runoff from fields during rain storms. Agricultural runoff is a major source ofpollution, in some cases the only source, in manywatersheds.[6]
Soil washed off fields is the largest source ofagricultural pollution in the United States. Excess sediment causes high levels ofturbidity in water bodies, which can inhibit growth ofaquatic plants, clogfishgills and smother animallarvae.[6]
Farmers may utilizeerosion controls to reduce runoff flows and retain soil on their fields. Common techniques include:
Nitrogen andphosphorus are key pollutants found inrunoff, and they are applied to farmland in several ways, such as in the form of commercialfertilizer, animalmanure, or municipal or industrial wastewater (effluent) or sludge. These chemicals may also enter runoff fromcrop residues,irrigation water,wildlife, andatmospheric deposition.[3]: p. 2–9
Farmers can develop and implementnutrient management plans to mitigate impacts on water quality by:
Pesticides are widely used by farmers to control plantpests and enhanceproduction, but chemical pesticides can also causewater quality problems. Pesticides may appear insurface water due to:
Some pesticides have also been detected ingroundwater.[3]: p.2–24
Farmers may useIntegrated Pest Management (IPM) techniques (which can includebiological pest control) to maintain control over pests, reduce reliance on chemical pesticides, and protect water quality.[10][11]
There are few safe ways of disposing of pesticide surpluses other than through containment in well managed landfills or byincineration. In some parts of the world, spraying on land is a permitted method of disposal.[12][13][citation needed]
Farms with largelivestock andpoultry operations, such asfactory farms, can be a major source ofpoint source wastewater. In the United States, these facilities are calledconcentrated animal feeding operations orconfined animal feeding operations and are being subject to increasing governmentregulation.[14]
Antibiotic-resistant bacteria have been found to infiltrate the water cycle from farms. Raising animals accounts for 73% of antibiotics use globally, and wastewater treatment facilities can transfer antibiotic-resistant bacteria to humans.[15][16]
The constituents of animal wastewater typically contain[17][18]
Animal wastes from cattle can be produced as solid or semisolid manure or as a liquidslurry. The production of slurry is especially common in housed dairy cattle.
Whilst solid manure heaps outdoors can give rise to polluting wastewaters from runoff, this type of waste is usually relatively easy to treat by containment and/or covering of the heap.
Animal slurries require special handling and are usually treated by containment in lagoons before disposal by spray or trickle application to grassland.Constructed wetlands are sometimes used to facilitate treatment of animal wastes, as areanaerobic lagoons. Excessive application or application to sodden land or insufficient land area can result in direct runoff to watercourses, with the potential for causing severepollution. Application of slurries to land overlyingaquifers can result in direct contamination or, more commonly, elevation of nitrogen levels asnitrite or nitrate.
The disposal of any wastewater containing animal waste upstream of a drinking water intake can pose serious health problems to those drinking the water because of the highly resistant spores present in many animals that are capable of causing disabling disease in humans. This risk exists even for very low-level seepage via shallow surface drains or from rainfall run-off.
Some animal slurries are treated by mixing with straws andcomposted at high temperature to produce a bacteriologically sterile and friable manure for soil improvement.
Piggery waste is comparable to other animal wastes and is processed as for general animal waste, except that many piggery wastes contain elevated levels ofcopper that can be toxic in the natural environment. The liquid fraction of the waste is frequently separated off and re-used in the piggery to avoid the prohibitively expensive costs of disposing of copper-rich liquid.Ascarid worms and their eggs are also common in piggery waste and can infect humans ifwastewater treatment is ineffective.
Fresh or wiltedgrass or other green crops can be made into a semi-fermented product calledsilage which can be stored and used as winter forage for cattle and sheep. The production of silage often involves the use of an acid conditioner such assulfuric acid orformic acid. The process of silage making frequently produces a yellow-brown strongly smelling liquid which is very rich in simplesugars,alcohol, short-chain organic acids and silage conditioner. This liquor is one of the most polluting organic substances known. The volume of silage liquor produced is generally in proportion to the moisture content of the ensiled material.
Silage liquor is best treated through prevention by wilting crops well before silage making. Any silage liquor that is produced can be used as part of the food for pigs. The most effective treatment is by containment in a slurry lagoon and by subsequent spreading on land following substantial dilution with slurry. Containment of silage liquor on its own can cause structural problems in concrete pits because of the acidic nature of silage liquor.
Although milk is an important food product, its presence in wastewaters is highly polluting because of its organic strength, which can lead to very rapidde-oxygenation of receiving waters.Milking parlour wastes also contain large volumes of wash-down water, some animal waste together with cleaning anddisinfection chemicals.
Milking parlour wastes are often treated inadmixture with human sewage in a localsewage treatment plant. This ensures that disinfectants and cleaning agents are sufficiently diluted and amenable to treatment. Running milking wastewaters into a farm slurry lagoon is a possible option although this tends to consume lagoon capacity very quickly. Land spreading is also a treatment option.[citation needed]
Wastewater from slaughtering activities is similar to milking parlour waste (see above) although considerably stronger in its organic composition and therefore potentially much more polluting.
Treatment is as for milking parlour waste.
Washing ofvegetables produces large volumes of water contaminated by soil and vegetable pieces. Low levels of pesticides used to treat the vegetables may also be present together with moderate levels of disinfectants such aschlorine.
Most vegetable washing waters are extensively recycled with the solids removed by settlement and filtration. The recovered soil can be returned to the land.
Although few farms plan for fires, fires are nevertheless more common on farms than on many other industrial premises. Stores of pesticides, herbicides, fuel oil for farm machinery and fertilizers can all help promote fire and can all be present in environmentally lethal quantities infirewater fromfire fighting at farms.
All farm environmental management plans should allow for containment of substantial quantities of firewater and for its subsequent recovery and disposal by specialist disposal companies.[19] The concentration and mixture of contaminants in firewater make them unsuited to any treatment method available on the farm. Even land spreading has produced severe taste and odour problems for downstream water supply companies in the past.
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