Agricultural Sewage Detection

1. Introduction to Agricultural Sewage Detection

Agricultural sewage refers to wastewater or liquid substances discharged during crop cultivation, livestock rearing, and agricultural product processing that affect human health and environmental quality. Its main sources include farmland runoff, livestock farm wastewater, and agricultural product processing wastewater.

The sewage contains various pathogens, suspended solids, chemical fertilizers, pesticides, insoluble solid matter, and salts. Agricultural sewage is vast in quantity and has a wide-ranging impact. Nutrients such as nitrogen and phosphorus entering rivers, lakes, and inland seas can cause eutrophication; pesticides, pathogens, and other toxic substances can contaminate drinking water sources, endangering human health; and it can lead to extensive soil pollution, disrupting ecosystem balance. To control agricultural sewage, the current focus is primarily on reducing farmland runoff.

2. Sources of Agricultural Sewage

(1) Farmland Runoff

Runoff from rainwater or irrigation water flowing over farmland surfaces is the primary source of agricultural sewage. Farmland runoff mainly contains pollutants such as nitrogen, phosphorus, and pesticides.

(2) Livestock Farm Wastewater

Wastewater from livestock and poultry manure is the second source of agricultural sewage. While this wastewater can be used as manure, industrialized countries often discard it, leading to environmental issues. When used as manure, it is typically applied superficially. If runoff occurs before the soluble carbon, nitrogen, and phosphorus compounds in the manure have fully interacted with the soil, it can cause more severe pollution than chemical fertilizers. Currently, there is no comprehensive testing method to determine the release rate of nutrients in manure to calculate appropriate application amounts and timing.

As a result, such runoff pollution is difficult to avoid. The large volume of manure excreted by livestock, if used to irrigate vegetable plots and farmland without proper disinfection and sterilization, can contaminate the soil. If washed into rivers, streams, ponds, and ditches by rainwater runoff, it can pollute drinking water sources. The impact of this wastewater on surrounding aquatic and terrestrial ecosystems is even greater when livestock farms are near riverbanks or during winter when the ground is frozen.

(3) Agricultural Product Processing Wastewater

Wastewater from processing fruits, meat, grains, and dairy products, as well as from industries like cotton dyeing, papermaking, and wood processing, is the third source of agricultural sewage. In developed countries, the volume of agricultural product processing wastewater is substantial.

3. Testing Parameters

(1) Nitrogen: Nitrogen fertilizers applied to farmland but not absorbed by plants or fixed by microorganisms and soil are the main source of nitrogen in farmland runoff. When nitrogen exists in the form of nitrate and nitrite, it is particularly prone to being carried away by runoff (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 soil ranges from 0.075% to 0.3%. For a topsoil layer of 15 cm, the total nitrogen content per hectare is 1,500–6,000 kg, with approximately 30–60 kg of nitrogen mineralized per hectare annually. The nitrogen content in farmland runoff varies significantly across different regions and soil types.

(2) Phosphorus: The total phosphorus content in soil ranges from 0.01% to 0.13%, while water-soluble phosphorus is between 0.01 and 0.1 ppm. Organic phosphorus in soil is immobile, and inorganic phosphorus is easily fixed by the soil. In Dutch marine sedimentary clay farmland, runoff contains about 0.06 mg/L of phosphorus; in river sedimentary clay farmland, runoff contains about 0.04 mg/L; and in runoff from soils rich in organic matter, such as excavated peatlands, phosphorus levels can reach about 0.7 mg/L. In paddy fields, waterlogging increases the soluble phosphorus content in the soil, leading to significant phosphorus loss, approximately 0.53 kg per hectare annually.

Nutrients such as nitrogen and phosphorus in the soil can be lost with water and soil particles in runoff. Most farmland contains about 0.1% phosphorus, 0.1–0.2% nitrogen, and 1–2% carbon. Therefore, for every 1 mm of soil erosion per hectare, runoff carries away about 10 kg of phosphorus, 10–20 kg of nitrogen, and 100–200 kg of carbon.

(3) Pesticides: The pesticide content in farmland runoff is generally low, with loss amounting to about 5% of the applied quantity. If heavy rain or storms occur shortly after pesticide application, the first runoff may contain higher pesticide levels. Water-soluble pesticides are primarily found in the aqueous phase of runoff, while strongly adsorbent pesticides (such as 2,4-D and triazines) can adsorb onto soil particles and remain suspended in the water with runoff.