The main sources are runoff from farmland, sewage from breeding farms and sewage from agricultural products processing. The sewage contains various pathogens, suspended solids, chemical fertilizers, pesticides, insoluble solids and salt. The amount of agricultural sewage is large and the impact is wide. Nutrient elements such as nitrogen and phosphorus in sewage enter rivers, lakes, inland seas and other waters, which can cause eutrophication; pesticides, pathogens and other toxic substances can pollute drinking water sources and endanger human health; cause extensive soil pollution and destroy ecosystems balance. In order to prevent and control agricultural sewage, the main reason is to reduce farmland runoff.

Source summary

Farmland runoff
Rainwater or irrigation water is the main source of agricultural wastewater after it flows through the farmland surface. Farmland runoff mainly contains nitrogen, phosphorus, pesticides and other pollutants.

① Nitrogen: Nitrogen fertilizer applied to farmland but not absorbed by plants or fixed by microorganisms and soil is the main source of nitrogen in farmland runoff. When chemical fertilizers exist in the form of nitrate nitrogen and nitrite nitrogen, they are particularly easily taken away by runoff (see Chemical Fertilizer Pollution). Nitrogen in farmland runoff also comes from soil organic matter, plant residues and manure applied to farmland. Generally, the total nitrogen content in the soil is 0.075-0.3%. Based on the thickness of the topsoil layer of 15 cm, the total nitrogen content is 1500-6000 kg per hectare. The annual mineralized nitrogen is about 30-60 kg per hectare. The nitrogen content of farmland runoff in different regions and on different soils is quite different. For example, the field drainage in the UK contains 0.5 mg/l of ammonium nitrogen and 17 mg/l of nitrate nitrogen. The annual runoff is 100 mm. The ammonium nitrogen is 0.5 kg per hectare and the nitrate nitrogen is 17 kg. Swedish farmland runoff contains 0.09 mg/L ammonium nitrogen and 4.1 mg/L nitrate nitrogen. The nitrate nitrogen in some areas is 20-40 mg/L, even reaching 81.6 mg/L.

② Phosphorus: The total phosphorus in the soil is 0.01～0.13%, and the water-soluble phosphorus is 0.01～0.1ppm. Organic phosphorus in the soil is inactive, and inorganic phosphorus is also easily fixed by the soil. The runoff of marine sedimentary clay farmland in the Netherlands contains about 0.06 mg/L phosphorus, the runoff of river sediment clay farmland contains about 0.04 mg/L phosphorus, and the runoff from the soil rich in organic matter excavated peat contains about 0.7 Mg/L, because of the waterlogging in the paddy field, the amount of soluble phosphorus in the soil can be increased, and the amount of phosphorus lost every year is about 0.53 kg per hectare.
Nutrient elements such as nitrogen and phosphorus in the soil can be lost with water and soil particles in the runoff. Most of the cultivated land contains 0.1% phosphorus, 0.1-0.2% nitrogen, and 1-2% carbon. Therefore, the soil erosion of the farmland is 1 mm, and 10 kg of phosphorus, 10-20 kg of nitrogen, and 100-200 kg of carbon in the runoff per hectare of soil. .

③ Pesticide: The content of pesticide in farmland runoff is generally not high, and the loss is about 5% of the amount of pesticide applied. If heavy rain or torrential rain occurs shortly after application, the pesticide content in the first runoff is higher. Pesticides with strong water solubility are mainly in the water phase of runoff; pesticides with strong adsorption capacity (such as 2,4-D, triazine, etc.) can be adsorbed on the soil particles, and the soil particles in the runoff are suspended in the water.

Sewage from feedlot
Manure and sewage from livestock and poultry are the second source of agricultural sewage. Sewage from breeding farms can be used as manure, but industrially developed countries often discard them, causing environmental pollution problems. As stable manure, most of them are applied by surface application. If a large amount of soluble carbon, nitrogen, and phosphorus compounds in stable manure have runoff before they fully interact with the soil, it will also cause more serious pollution than chemical fertilizer. At present, there is no perfect detection method for stable manure to determine the release rate of its nutrient elements in order to calculate a reasonable amount and time. Therefore, this type of runoff pollution is inevitable.

The excretion of livestock manure and urine in the farm is large. Irrigation of vegetable fields and farmland with inadequately sterilized manure and urine will cause soil pollution; manure and urine will be washed into rivers, rivers and pits by rainwater, which will cause pollution of drinking water sources. When the breeding ground is near the river bank and the land is frozen in winter, this kind of sewage has a greater impact on the surrounding aquatic and terrestrial ecosystems.

Agricultural product processing wastewater
The processing of fruits, meat, cereals and dairy products, as well as the basic sewage of cotton, papermaking, wood processing and other industrial wastewater is the third source of agricultural wastewater. The volume of agricultural product processing wastewater in developed countries is quite large. For example, the US food industry discharges about 2.5 billion tons of wastewater each year, ranking fifth among various types of wastewater.

Related influence
Nutrient elements such as nitrogen and phosphorus in agricultural sewage enter rivers, lakes, inland seas and other waters, which can cause eutrophication; pesticides, pathogens and other toxic substances in agricultural sewage can pollute drinking water sources and endanger human health; agricultural sewage can also It causes large-scale soil pollution, destroys the natural ecosystem, and makes the species in the ecosystem unbalanced. The measures to prevent agricultural sewage pollution are currently mainly to reduce farmland runoff.