Study On Natural Purification Effect Of Nitrogen And Phosphorus In Ditches In North Irrigation Area

Nitrogen and phosphorus are essential nutrients for the normal growth of crops,but also an important component of pollution in farmland water ecosystem.Irrigation ditches,as irrigation and water conservancy infrastructure,are part of the agricultural ecosystem and have the characteristics of linear wetlands.Drainage ditch not only can regulate water balance,but also can reduce and affect the migration and transformation of pollutants by adsorption of the ditch sediment,plant absorption and microbial degradation.In this paper,the drainage ditches in Hunbei Irrigation District of Shenyang city were taken as the research object,through the outdoor test and laboratory test for the following investigations – the migration and transformation of nitrogen and phosphorus in the drainage ditch;the adsorption of nitrogen and phosphorus in water sediment interception;the removal efficiency of nitrogen and phosphorus by the aquatic plants in the ditch;the degradation of the microorganisms;and the factors affecting the sediment's adsorption of nitrogen and phosphorous.The conclusions drawn from the experimental study are as follows:The concentrations of total nitrogen,total phosphorus and ammonia nitrogen were easily influenced by fertilization and rainfall,with the strong fluctuation of time.They peaked in June and August and reached a minimum in July.Ditch system had strong interception and purification effect on nitrogen and phosphorus,especially had high removal rate of ammonia nitrogen and total phosphorus.Total nitrogen removal rate was 15.79% to 36.11%.The removal rate of ammonia nitrogen was the highest in June,reaching 62.5%.Total phosphorus removal rate was 46.24% to 63.44%.The different forms of nitrogen content in all levels of ditch system accumulated in the downstream,and the peaks gradually shifted from the furrow to the farm,branch,and trunk.Aquatic plants can enhance the retention effect: the presence of aquatic plants increased the TN and TP interception rates by 11.5% and 7.8%,respectively.Aquaticplants can accelerate the migration velocity of nitrogen and phosphorus from the water into the sediment,thereby enhancing the removal effect.Among three plants,phragmites australis had the strongest interception ability on ammonia;acorus calamus had the strongest absorption capacity of phosphorus.Sediment microorganisms showed obvious removal of nitrogen,but not so for phosphorous.The interception of ammonia nitrogen in the sediment with microorganisms was0.028mg/g higher than without microorganisms.The adsorption capacity of nitrogen(phosphorus)is related to the amount of nitrogen(phosphorus)in the sample.The more nitrogen(phosphorus)the sediment has,the weaker the ability to adsorb ammonia nitrogen(phosphorus).The isothermal adsorption curves of nitrogen and phosphorus were in accordance with Langmuir equation and Freundlich equation.The adsorption kinetics of nitrogen and phosphorus were in accordance with quasi-second-order kinetics equation.Higher temperature facilitated phosphorous adsorption but hinders ammonia nitrogen adsorption.The equilibrium adsorption capacity of branch,trunk and farm sediments increased with the raise of p H value.At p H=7,the equilibrium adsorption capacity of phosphorus in branch,trunk and farm sediments peaked at 91.56,39.86 and 41.56 mg/kg respectively.The adsorption capacity decreased under other conditions.With the increase of p H,the adsorption rate of sediment increased.The adsorption amount of ammonia nitrogen decreased with the increase of COD concentration.When the concentration of COD was 50 mg/L,the equilibrium adsorption capacity of phosphorus reached the maximum,which was 89.72,48.72 and52.75 mg/kg,respectively.Under other conditions,the adsorption capacity decreased.