Removal And Transformation Of Pollutant During Reclaimed Water Refilling Rivers And Lakes Process

Reclaimed water has been widely used to restore rivers and lakes in water scarce areas as well as in Beijing municipality,China.However,refilling the rivers with reclaimed water may result in groundwater pollution.Four sets of soil columns repacked with sandy loam soil were utilized to simulate river bank filtration（RBF）system to investigate the removal and transport of nitrate（NO₃-N）,ammonia（NH4-N）,chemical oxygen demand（COD）and total nitrogen（TN）with reclaimed water.Four water supply conditions were considered including continuous wetting,alternating wetting/drying,constant flow rate and lateral injection.A three-year field monitoring program was conducted to assess the effect of riverbank filtration system on the removal of nitrogen and organics of the river water at the Qingyang River in Beijing area,which was replenished by reclaimed water.In addition,one-dimensional and two-dimensional soil water,nitrogen transport model was developed.The model was used to simulate the soil water and NO₃-N dynamic under different hydraulic conditions.The main conclusions were as follows:（1）The results showed that the N03-N removal efficiency decreased with the increase of hydraulic loading rate when the hydraulic loading rate varied from 0.25 cm d-1 to 2.65 cm d-1.The highest removal efficiency of NO₃-N was obtained in the column recharged with lateral injection,with an average removal efficiency of 96%.The transport and removal of N03-N varied with soil depth in soil columns.The results indicated that the NO₃-N removal efficiency was positively correlated with the number of denitrifying bacteria in four soil columns.The results also found that the NO₃-N removal efficiency showed an exponential relationship with increasing of temperature under the continuous wetting condition（15～32℃）.NO₃-N removal efficiency showed a power function relationship with increasing of temperature under the alternating wetting/drying condition.Longer retention time and aerobic condition improved NH4-N removal efficiency with low NH4-N concentration in the influent.（2）The COD removal efficiency under the alternating wetting/drying condition（51%）was higher than that under the continuous wetting condition（41%）.COD removal primarily occurred at the upper part of the soil columns（0-40 cm）,where there was high dissolved oxygen concentration and a large amount of microorganism.The much higher removal efficiency of TN was obtained under the alternating wetting/drying condition（84%）than that under the continuous wetting condition（51%）.Removal efficiency of TN increased with increasing of temperature（15～32℃）.TN removal efficiency could reach higher than 90%as temperature was higher than 30℃.（3）Results indicated that the NO₃-N and TN were the major factors in the Qingyang River and the NO₃-N and TN concentrations in river decreased with distance from the source of reclaimed water.In contrast,the COD and BOD5 concentration increased with the distance from the source of reclaimed water.Sediment was the major position for NO₃-N removal,about 85%of NO₃-N was removed by sediment.Attenuation of NO₃-N through the RBF to groundwater varied among seasons,and the NO₃-N removal efficiency was strongly related to groundwater temperature.Approximate 92%of NH4-N was removed during the infiltration from the sediment to groundwater.Temperature and organic content was realted to NH4-N concentration variation in the groundwater.On average,54%of COD was removed by RBF system.Temperature did not show significant effect on COD removal by the RBF,whereas COD removal increased with the retention time and/or travel distance in the RBF system.These results suggested that RBF system was an effective barrier against NO₃-N,NH4-N and COD in the Qingyang River as well as those rivers refilled by reclaimed water with similar geological and climatic conditions.（4）The model was calibrated and validated by soil water and NO₃-N concentration obtained from the soil column and field experiment.The simulation results of water and NO₃-N concentration were in good agreement with the observed values,which indicated that the model can be used to simulate the soil water and nitrogen transport under different hydraulic conditions.In addition,a modify relation was build between temperature and denitrification rate constant.It was found that denitrification rate constant and temperature had a positive exponential correlation.After calibration,the model was used to predict the NO₃-N tratnsport for different temperatures and NO₃-N concentrations scenarios.Results found that NO₃-N removal efficieny increased by 0.5～1.6 times with increasing temperature from 5℃to 25℃.Considering the period of reclaimed water refilling（March-November）,NO₃-N concentration in reclaimed water should be less than 30 mg L-1.And the optimum time for reclaimed water refilling river was summer,when temperature was higner than 20℃.