Removal effect of nitrate nitrogen for sandy loam filtration system under different water supply conditions

Abstract: Reclaimed water has been widely used to restore rivers and lakes in water scarce areas as well as in Beijing City, China. However, refilling the rivers and lakes with reclaimed water may result in groundwater pollution. Purification is necessary prior to utilization in order to minimize the pollution and human-health risk. To study the long-term NO3-N removal efficiency during land filtration system under different water supply conditions, a long-term filtration system was built using soil columns supplied with reclaimed water from reclaimed wastewater treatment plant. Four sets of soil columns (100 cm in length, 20 cm in diameter) repacked with sandy loam soil were utilized to simulate the land filtration system. Four water supply conditions were considered including continuous wetting, alternating wetting/drying, constant flow rate and lateral injection. The results showed that NO3-N removal efficiency decreased with the increase of hydraulic loading rate when the hydraulic loading rate varied from 0.25 to 2.65 cm/d. Different NO3-N removal efficiencies were mainly attributed to the different hydraulic retention time under different water supply conditions. For the constant flow rate and lateral injection condition, longer hydraulic retention time was conducive to efficient denitrification. Removal efficiency of NO3-N reached higher than 90% in the constant flow rate and lateral injection column. The highest removal efficiency of NO3-N was obtained in the column recharged with lateral injection, with an average removal efficiency of 96.1%. NO3-N concentrations in the effluents in the alternating wetting/drying, constant flow rate and lateral injection columns were all lower than the drinkable water standard (<10 mg/L). The transport and removal of NO3-N varied with soil depth. The variation trend of NO3-N removal along soil depth was different under different water supply conditions. For the continuous wetting and lateral injection condition, NO3-N concentration decreased rapidly in the top 20 cm depth. Top 20 cm layer of the sandy loam columns (0-20 cm for the continuous wetting condition, 40-60 cm for the lateral injection condition) was the main zone for NO3-N removal. For the alternating wetting/drying and constant flow rate condition, NO3-N concentration decreased gradually along the soil depth. For the alternating wetting/drying condition, NO3-N removal efficiency increased from 23.2% in 20 cm to 76.2% in 100 cm. For the constant flow rate condition, NO3-N removal efficiency increased from 20.8% in 20 cm to 94.1% in 100 cm. The results indicated that the NO3-N removal efficiency in different soil depths was correlated to the number of denitrifying bacteria in 4 soil columns. The results also showed that the NO3-N removal efficiency presented an exponential relationship with the temperature ranging from 15 to 32 ℃ under the continuous wetting condition. For the alternating wetting/drying condition, NO3-N removal efficiency showed a power function relationship with the temperature, while temperature had no significant influences on NO3-N removal under low hydraulic loading rate condition, i.e., constant flow rate and lateral injection condition. The results confirmed that the NO3-N removal performance under constant flow rate and lateral injection condition could both be relatively stable during the land filtration system's long-term running. The land filtration system could be used for further purification for reclaimed water. The result can provide important information for the reuse of reclaimed water in refilling rivers and lakes.