Effect Of Iron Input On Removal Of Nitrogen And Phosphorus In Drainage Ditches And Intensive Techniques

Eutrophication is one of the important environmental problems facing the world.The main reason is the excessive input of nutrients（such as nitrogen and phosphorus）in water,and agricultural sources are important sources of nutrients.At present,agricultural non-point source has become the main source of water pollution in China,and the prevention and control of agricultural non-point source pollution has become the focus of improving the quality of water environment.Northeast China is an important grain production base in China.Irrigation agriculture is large-scale,intensive and agricultural non-point source pollution is serious.Drainage system is the channel of agricultural drainage and non-point source pollution output,and is the main place of agricultural non-point source pollution’middle interception’.Ecological ditch technology and ecological wetland treatment technology have high nitrogen and phosphorus removal effect and good landscape effect.The concentration of iron ion in groundwater in Sanjiang Plain of Northeast China is high,and groundwater is the main source of paddy field irrigation.Ferrous ions in water can not only act as electron donors to promote the denitrification process,but also promote the deposition and precipitation of phosphate,which can simultaneously remove nitrate and phosphate in water.Therefore,in view of the major national demand for the prevention and control of agricultural non-point source pollution in the large-scale farmlandization process in the Sanjiang Plain,this study takes farmland drainage ditches as the research object to study the dynamic distribution and output load of nitrogen and phosphorus in the ditches in the Sanjiang Plain,explore the removal effect and influencing factors of nitrogen and phosphorus in the ditches by iron input,and develop the strengthening measures for midway interception,so as to provide scientific and technological support for the prevention of regional agricultural non-point source pollution.The main conclusions of this study are as follows:（1）TN and NH₄⁺-N in the ditches of Sanjiang Plain had obvious seasonal variations, while the seasonal variations of NO₃^--N and TP were small.The concentrations of NH₄⁺-N,NO₃^--N,TN and TP were 0.05-3.01 mg/L,0.19-2.36 mg/L,0.41-8.86 mg/L and 0.015-1.02 mg/L,respectively.In space,with the increase of ditch scale,the concentrations of NH₄⁺-N,NO₃^--N,TN and TP decreased.In addition to NH₄⁺-N,the NO₃^--N,TN and TP loads from the lateral infiltration in surface water irrigation area were higher than those in groundwater irrigation area in Sanjiang Plain.The NH₄⁺-N,NO₃^--N,TN and TP loads of artificial drainage and storm runoff were higher than those of groundwater irrigation area in Sanjiang Plain.（2）The addition of Fe²⁺can promote the removal of nitrogen and phosphorus in ecological ditches to varying degrees.The removal rates of NH₄⁺-N,NO₃^--N and TP are 62.54%-88.66%,72.94%-99.61%and 57.69%-88.68%,respectively.Adding10 mg/L Fe²⁺had the highest nitrogen removal rate（88.66%for NH₄⁺-N and 97.69%for NO₃^--N）,and adding 5 mg/L Fe²⁺had the highest TP removal rate（88.68%）.After adding different concentrations of Fe²⁺,the abundances of Firmicutes,Carnobacteriaceae and Trichococcus in ecological ditches were significantly higher than those in the control group（p<0.05）.Trichococcus showed obvious enrichment.Trichococcus could reduce Fe³⁺to Fe²⁺and continue to provide electrons for nitrate,indicating that Trichococcus was involved in nitrogen biodegradation.（3）Carbon addition can significantly improve the removal rate of pollutants.In the ecological ditch with Fe²⁺concentration of 5 mg/L,different concentrations of carbon sources can improve the removal rates of NH₄⁺-N and TP,and can significantly improve the removal rates of NO₃^--N and TN（p<0.05）.When the C/N ratio is 6,the comprehensive removal rates of NH₄⁺-N,NO₃^--N,TN and TP in the ecological ditch are the best,and the removal rates in 72 h(when the Fe²⁺concentration is 5 mg/L)are more than 80.56%.The addition of carbon source increased the abundance of Trichococcus and Toluomonas and decreased the relative abundance of Pseudomonas and Acidovorax bacteria in ecological ditches.（4）Different iron substrates（pyrite,iron sponge and iron filing）can significantly improve the removal efficiency of NO₃^--N,TN and TP in ecological ditches.Moreover,the removal rate of nitrogen and phosphorus can be improved on the basis of shortening HRT,and NO₃^--N can increase the removal rate by up to 31.31%.TN removal rate increased by up to 25.17%;TP increased the removal rate by up to 34.64%.The addition of sponge iron is beneficial to enrich Dechloromonas,which not only has the ability of autotrophic denitrification,but also has the ability of phosphate accumulation.Combined removal efficiency and economic cost,iron sponge and iron filing can be used as iron matrix of ecological ditch.The iron filing and iron sponge with different mass ratios can reduce the nitrogen and phosphorus concentration in the device.The iron filing and gravels are put into the ditch in the form of matrix dam.The NH₄⁺-N removal rate is the highest when the mass ratio of iron filing and gravels in the matrix is 1:5,and the NO₃^--N,TN and TP removal rates are the highest when the mass ratio of iron sponge and gravels is 1:5.The NO₃^--N removal rate of iron scraps with different mass ratios and iron sponge for 24 h ranged from 65.60%to 90.69%.（5）As an external solid carbon source,agricultural and forestry waste can slow release carbon source,and its decomposition consumes DO in the device,resulting in an anaerobic environment.At the same time,it can also be used as a carrier of microorganisms.Adding different agricultural and forestry waste（natural corn cob and rice straw,alkali corn cob and rice straw,corn cob biochar and rice straw biochar）can strengthen the removal of nitrogen in the ditch by iron addition,and improve the removal efficiency of NH₄⁺-N,NO₃^--N and TN.The removal efficiency of TP in ecological ditches by solid carbon source was not significantly improved,and TP removal was mainly co-precipitation with Fe³⁺and matrix adsorption.According to the comprehensive removal efficiency,alkali rice straw is the best external solid carbon source.When HRT is 8 h,the removal rate of NO₃^--N is up to 76.65%by filling alkali rice straw+gravel in the matrix dam,which significantly shortens the HRT and improves the removal efficiency.In short,the nitrogen and phosphorus in some ditches in Sanjiang Plain exceeded class V of Surface Water Environmental Quality Standards of China（GB3838-2002）.The lateral seepage output of surface water irrigation area will increase the concentration of nitrogen and phosphorus in ditches.The main factor affecting the effect of iron input in ditches is the concentration of Fe²⁺.The optimum Fe²⁺ concentration is 5 mg/L,the optimum C/N is 6.The application of iron matrix（sponge iron and iron scraps）and agricultural and forestry waste（alkali rice straw）in ecological ditches improved the removal efficiency of nitrogen and phosphorus,and shortened the treatment time.Both measures can be used as practical strengthening technical measures.The research results can provide theoretical and technical support for the prevention and control of agricultural non-point source pollution in paddy drainage in the Sanjiang Plain,which is of great significance to ensure the ecological safety of water bodies and improve the environmental quality.