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◉ Introduction to Agricultural Wastewater Test
Agricultural sewage refers to sewage or liquid substances that are discharged from the processes of crop cultivation, livestock raising, and agricultural product processing, and affect human health and environmental quality. Its main sources are farmland runoff, feedlot sewage, and agricultural product processing sewage.
Sewage contains various pathogens, suspended solids, fertilizers, pesticides, insoluble solids and salts. The amount of agricultural sewage is large and its influence 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 large-scale soil pollution and damage the ecology System balance. In order to prevent and control agricultural sewage, it is mainly to reduce farmland runoff at present.
◉ Agricultural Wastewater Sources
(1) Farmland runoff
The discharge of rainwater or irrigation water over the surface of farmland is the main source of agricultural sewage. Farmland runoff mainly contains nitrogen, phosphorus, pesticides and other pollutants.
(2) Feeding farm sewage
The manure and urine of livestock and poultry is the second source of agricultural sewage. Feedlot sewage can be used as manure, but it is often discarded in industrially developed countries, causing environmental problems. As a manure, the method of surface application is mostly adopted. If a large amount of soluble carbon, nitrogen and phosphorus compounds in the manure have not fully interacted with the soil before runoff occurs, it can also cause more serious pollution than chemical fertilizers. At present, there is no perfect detection method for manure to determine the release rate of its nutrient elements, so as to calculate the reasonable dosage and time.
Therefore, such runoff pollution is unavoidable. The excretion of livestock manure in the farm is large. Watering vegetable fields and farmland with manure and urine that is not fully disinfected and sterilized will cause soil pollution. With feedlots close to river banks and winter land freezes, this sewage has a greater impact on the surrounding aquatic and terrestrial ecosystems.
(3) Wastewater from agricultural product processing
The processing of fruit, meat, grain and dairy products, as well as effluents from industries such as cotton basic dyeing, papermaking, and wood processing, are the third source of agricultural wastewater. In developed countries, the amount of waste water from agricultural product processing is quite large.
◉ Test items
(1) Nitrogen: Nitrogen fertilizer applied to farmland but not absorbed and utilized by plants or fixed by microorganisms and soil is the main source of nitrogen in farmland runoff. Fertilizers are especially easily carried away by runoff when they are in the form of nitrate and nitrite (see 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%, and the total nitrogen content per hectare is 1500-6000 kg per hectare, and the annual mineralized nitrogen is about 30-60 kg per hectare. The nitrogen content of farmland runoff varies greatly in different regions and soils.
(2) Phosphorus: The total phosphorus content in the soil is 0.01-0.13%, and the water-soluble phosphorus is 0.01-0.1ppm. The organic phosphorus in the soil is inactive, and the inorganic phosphorus is also easily fixed by the soil. Phosphorus is about 0.06 mg/L in runoff from marine sedimentary clay farmland in the Netherlands, about 0.04 mg/L in runoff from clay farmland in river sediments, and about 0.7 in runoff from soil rich in organic matter excavated from peat excavation. mg/L, the amount of soluble phosphorus in the soil can increase due to waterlogging in paddy fields, and the annual phosphorus loss is more, about 0.53 kg per hectare.
Nutrient elements such as nitrogen and phosphorus in soil can be lost with water and soil particles in runoff. Most cultivated land contains 0.1% phosphorus, 0.1-0.2% nitrogen, and 1-2% carbon. Therefore, if the soil erosion of farmland is 1 mm, there are 10 kg of phosphorus, 10 to 20 kg of nitrogen and 100 to 200 kg of carbon in the runoff per hectare of soil. .
(3) Pesticides: The content of pesticides in farmland runoff is generally not high, and the loss is about 5% of the application amount. If heavy rain or torrential rain occurs in a short period of time after spraying, the pesticide content in the first runoff is relatively high. 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 soil particles and suspended in water with soil particles in runoff.
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