Study On Nitrogen And Phosphorus Removal Of Rural Domestic Wastewater And Its Runoff In The Paddy Wetlands

Rural domestic wastewater discharge is the leading source of water quality impacts to rivers and lakes in many countries,and a lot of N and P discharged from rural domestic wastewater are closely related to eutrophication.The overall objectives of this research has been to investigate the degradation behavior of N and the N-removal mechanism during the treatment of rural domestic wastewater by the paddy wetlands,and study the floodwater P concentration character and P-removal process,and discover the features of N transference, transformation,distribution and balance,and analysis the runoff characteristics,production conditions and influencing factors of N and P in the paddy wetlands,and establish the linear regression equation among runoff concentration,rain-fertilization interval,floodwater depth, rainfall intensity and rainfall time,and verify it by the data from the field experiment in field scale and microscopic scale.The followings were the main results.1.The results from the field experiment of rural domestic wastewater treatment by the paddy wetlands showed that floodwater TN concentration decreased quickly after transplantation,due to the adsorption of the paddy soil,but increased greatly after topdressing, and the sequence was SW＞GW＞DW＞BW＞CK.After a growth period,N-removal efficiency was in the following sequence of BW(96.8%)＞DW(96.2%)＞GW(95.6%)＞SW(94.1%)＞CK(93.8%),and COD of the effluent was stabilized at about 20 mg L^-1,which met gradeâ…£of surface water discharge standard.N-removal rate from rural domestic wastewater (62.9%-69.3%) was higher significantly(P≤0.05) than that from urea(27.5%-32.7%),which showed that N from rural domestic wastewater was removed preferentially compared with that from urea by the paddy wetlands.TN load in the paddy wetlands floodwater had the similar variation characteristics with TN,and COD had a positive relation with TN and TN load.Moreover,the rice yields of GW,DW and BW were higher significantly than that of CK, SW,which indicated the paddy wetlands could not only remove effectively N from rural domestic wastewater,but also could improve the rice yield.2.Different from N fertilizer,COD concentration in the paddy wetlands floodwater changed little after P fertilizer application,but TP concentration rapidly rose,and gradually decreased,and was at about Oct.15.COD of CK,SW,GW and BW treatments were 13.54, 20.98,20.87,21.09,17.86 mg L^-1 respectively,and dephosphorization rates were in the following order at:GW(98.17%)＞DW(97.28%)＞BW(97.04%)＞SW(96.78%)＞CK(75.20%). Dephosphorization rate from rural domestic wastewater(60.3%-71.4%) was higher significantly(P≤0.05) than that from P fertilizer(27.5%-32.7%),which showed that P from rural domestic wastewater was removed preferentially.P transferred from roots to stems and leaves,as the rice grown,and accumulated in the grain lastly.TP and TP load in the paddy wetlands floodwater linearly decreased with time except CK treatment,and was stabilized on Oct.1.In addition,the rice yields of five treatments had not significant difference,which suggested irrigating the paddy wetlands by rural domestic wastewater instead of surface water could remove P and keep the rice yield simultaneously.3.The results of ¹⁵N isotopic tracer experiment showed NH₃ volatilization losses of CK, UR and ¹⁵NUR treatments was stabilized at 10.3 mg pot-1 until Oct.1,finally those of UR and ¹⁵NUR treatments account for 43.54%and 41.18%of the input total N,respectively. After fertilizer application,TN concentration of UR and ¹⁵NUR treatments rose quickly and reached the max value of 19.52,18.64 mg L^-1,respectively,but only 3.78 mg L^-1 for CK,and was stabilized on Oct.1 at the value of 0.32,1.05,0.93 mg L^-1,respectively.N concentration of the rice plant rose gradually at tillering stage,and reached the maximum value at the booting stage,absorptive N concentration of CK,UR and ¹⁵NUR treatments were 45.3,165.4, 173.6 mg pot^-1,respectively.N content of the rice plant was the lowest at the seedling stage, the highest at the booting stage and decreased in the mature stage,and followed the sequence of ¹⁵NUR＞UR＞CK.Soil residual N content of CK was the lowest of 0.064 g pot^-1,and residual N content and rate of UR were higher by 0.171 g pot^-1,1.4%than those of ¹⁵NUR, respectively,but N utilization efficiency and recovery rate were lower by 3.95%,2.55%, respectively.At the same time,nitrification and denitrification loss rates of UR and ¹⁵NUR were up to 5.81%,5.69%,respectively,and their rice yields increased by 33.69%,29.90%, respectively,which showed it was necessary for urea application to increase rice yield again.4.The results of indoor artificial rainfall experiment showed that the runoff of the paddy wetlands is a kind of opportunity runoff,which happened occurred accidentally.Producing runoff time decreased,as rainfall intensity increased,and runoff TN concentration had a positive correlation with rainfall intensity,and had a negative correlation with rain-fertilization interval and floodwater depth,and had no correlation with rainfall time. Rain-fertilization interval had the most important influence on runoff TN concentration, floodwater depth the second,rainfall intensity the third,rainfall time the least,under all tested conditions,runoff TN concentration was above the limited value(2 mg L^-1) of surface water environment quality standard,and could be simulated by the linear regression equation of y=-2.124x₁-1.147x₂+0.097x₃+0.001x₄+32.987 under the effect of four factors,which was verified well by the field experiment.NO₃^- and NH₄⁺ were the major N forms in the runoff of the paddy wetlands,and NO₃^- and NH₄⁺ rates fluctuated in the range of 32%-75%,24%-66%, respectively.TN loss load in the paddy wetlands decreased obviously,as rain-fertilization interval increased,and there had been a great NO₃^- runoff risk within 7 days,then the N loss risk would decrease greatly.5.The paddy wetlands is a natural weak alkaline system,DP rate was high(about 83%) in the early runoff period,and PP rate increased gradually(about 38%),and DP was the major P form in the runoff.TP had a positive correlation with rainfall intensity,rain-fertilization interval and rainfall time,and had a negative correlation with floodwater depth.Similar to TN concentration,rain-fertilization interval had the most important influence on runoff TP concentration,floodwater depth the second,rainfall intensity the third,rainfall time the least, and TP concentration was all above the limited value(0.4 mg L^-1) of surface water quality standard gradeâ…¤,and could be simulated by the linear regression equation of y=-0.548x₁-0.243x-2+0.014x₃-0.001x₄+7.386.Runoff P loss risk increased,mainly when rain-fertilizer interval was less than 10 days,after that it would decrease greatly.So,it will lead to great losses of N and P in the paddy wetlands,when runoff occurred shortly after fertilizer application.Runoff can be avoided by the adjustment of floodwater depth,and the paddy wetlands can be changed from output sources of N and P pools to the absorpted sinks, and highly-efficient denifrification and dephosphorization of rural domestic wastewater can be realized,and non-point source pollution of rural domestic wastewater was decreased.