Study On The Purification Technology Of The Major Pollutants Nitrogen And Phosphorus Of Farmland Drainage

Purification techniques of nitrogen, phosphorus and other major pollutants in farmland drainage were studied in this paper. The fertilizer application and drainage law in the growth period of crops by farmers were gained through farmland fertilization and drainage surveys. The threat posed by agricultural water to surrounding water was analyzed through the study of water quality changes law within a certain period of time after fertilization. Plants which have a high efficient purification of nitrogen, phosphorus and other major pollutants and suit to local climate were screened out using agricultural water as test subjects. Absorption and utilization of the purification plants as the main means, combined with self-purification of water, and a combination of drains and oxidation pond purification system was used to purify water in the discharge channels and collection system, so that the farmland water was effectively purified before flowing into surrounding water.1.The results of the investigation in the amounts of the fertilizer and the farmland water quality after the fertilization are as follows:(1) Farmers generally use a small amount of urea or compound fertilizer when ploughing the soil, and used a large amount of ammonium bicarbonate in the second fertilization when transplanting.(2) The highest concentration of the ammonia nitrogen, total nitrogen, total phosphorus and COD were respectively up to 149 mg/L,165 mg/L,1.55 mg/L and 89 mg/L after fertilization, which poses a grave threat to the surrounding water. The concentration of ammonia and total nitrogen respectively fell below 2.00 mg/L in the 10th day and 13th day, the concentration of total phosphorus fell below 0.05 mg/L in the 11th-14th day, and the concentration of COD fell below 20 mg/L in the 5th-7th day.2.The results of the researches of the screening in farmland drainage are as follows:(1) Through plants screening tests of 14 days, the purification system without plants had an obvious purification effect, in which the removal efficiency of ammonia nitrogen, total nitrogen, total phosphorus and COD were 53.4%,50.5%,41.6% and 36.7%.(2) The purification system with plants had a higher efficient purification effect than the one without plants. Through plants screening tests of 14 days, Phragmites australis, Spiked Loosestrlfe, Myriophyllumverticillatum, Ceratophyllum demersum and Lemna minor had a better purification effect on ammonia nitrogen, the purification efficiency of which were all above 86.2%; Phragmites australis, Spiked Loosestrlfe, Myriophyllumverticillatum, Ceratophyllum demersum and Lemna minor had a better purification effect on total nitrogen, the purification efficiency of which were all above 83.2%; Typha angustifolia L., Canna indica, Phragmites australis, Ceratophyllum demersum, Myriophyllumverticillatum, Lemna minor had a better purification effect on total phosphorus, the purification efficiency of which were all above 81.6%; Phragmites australis, Typha angustifolia L., Spiked Loosestrlfe had a better purification effect on COD.3. Farmland drainage is purified by aquatic plants in the ditches, and then will be collected by oxidation and be purified, the researches of the tests are as follows:(1) The optimum flow rate for ditch purification was 3-3.5 m/min, and at this rate the removal efficiency of ammonia nitrogen, total nitrogen, total phosphorus and COD reach up to 13.4%,12.3%,12.5% and 26.2%. Ditches with a certain amount of Ceratophyllum demersum, Zizania aquatica and Canna indica planted had a significantly higher efficiency than ditches without plants. The purification recovery of ammonia nitrogen, total nitrogen and total phosphorus can be completed within 30 days after the ditch clean-up; the removal efficiency of COD was raised with the lapse of time within 90 days after the clean-up, and the process was quite slow.(2) In the oxidation pond with the aquatic plant-based ecological floating bed, the removal efficiency of ammonia nitrogen, total nitrogen and total phosphorus respectively reached 68.8%,67.4% and 47.4%, which were significantly higher than those in the oxidation pond without ecological floating bed (48.8%,51.8% and 32.5%). The removal efficiencies of organic matter in the two kinds of ponds were roughly equal.