Agricultural wastewater treatment

Agricultural wastewater treatment relates to the treatment of wastewaters produced in the course of agricultural activities.

As agriculture is a highly intensified industry in many parts of the world, the range of wastewaters requiring treatment can encompass at least the following:

• Animals wastes - both liquid and solid
• Silage liquor
• Pesticide run off and surpluses
• Milking parlour wastes including milk
• Slaughtering waste
• Vegetable washing water
• Fire water

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Animal wastes

The constituents of animal wastewater typically contain

• Strong organic content—much stronger than human sewage
• High solids concentration
• High nitrate and phosphorus content
• Antibiotics
• Synthetic hormones
• Often high concentrations of parasites and their eggs
• Spore of cryptosporidum - a bacterium resistant to drinking water treatment processes
• Spore of Giardia
• Human pathogenic bacteria such as Brucella and Salmonella

Animal wastes from cattle can be as produced as solid or semisolid manure or as a liquid slurry. The production of slurry is especially common in housed dairy cattle.

Treatment

Whilst solid manure heaps outdoors can give rise to polluting wastewaters from rain washing, 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 are anaerobic lagoons. Excessive application or application to sodden land or insufficient land area can result in direct runoff to watercourses with the potential for causing severe pollution. Application of slurries to land overlying aquifers can result in direct contamination or, more commonly, elevation of nitrogen levels as nitrite 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 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 and composted at high temperature to produce a bacteriologically sterile and friable manure for soil improvement.

Piggery waste

Piggery waste is comparable to other animal wastes except that many piggery wastes contain elevated levels of copper that can be toxic in the natural environment. Ascarid worms and their eggs are also common and can infect humans if wastewater treatment is ineffective.

Treatment

As for general animal waste although 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 a copper rich liquor.

Silage liquor

Fresh or wilted grass or other green crops can be made into the semi fermented product called silage 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 as sulphuric acid or formic acid. The process of silage making frequently produces a yellow-brown strongly smelling liquid which is very rich in simple sugars, 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.

Treatment

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 subsequently spread 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.

Pesticide runoff and surpluses

Inappropriate use of pesticides so that pesticide-containing wastewaters enter the environment can give rise to severe and long-lasting ecological damage. This is particularly true for insecticides used in sheep dips because of the volumes of pesticide-containing wastewater requiring disposal and because of the persistent and damaging nature of the pesticides.

Treatment

There are few safe ways of disposing of pesticide surpluses other than through containment in well managed landfills or by incineration. In some parts of the world, spraying on land is a permitted method of disposal.

Milking parlour wastes including milk

Although milk has a deserved reputation as an important and valuable food product, its presence in wastewaters is highly polluting because of its organic strength, which can lead to very rapid de-oxygenation of receiving waters. Milking parlour wastes also contain large volumes of wash-down water, some animal waste together with cleaning and disinfection chemicals.

Treatment

Milking parlour waste are often treated in admixture with human sewage in a local sewage 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.

Slaughtering waste

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

As for milking parlour waste (see above).

Vegetable washing water

Washing of vegetables 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 as chlorine.

Treatment

Most vegetable washing waters are extensively recycled with the solids removed by settlement and filtration. The recovered soil can be returned to the land

Fire water

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 fertilisers can all help promote fire and can all be present in environmentally lethal quantities in wastewater from firefighting at farms.

Treatment

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. The concentration and mixture of contaminants in fire-water 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.

See also

• Ecological sanitation