Effect of alternate dry-wet patterns on the performance of bioretention units for nitrogen removal

The removal rate of NO3–-N is a key indicator for the performance evaluation of nitrogen removal in bioretention units. At present, most studies show that setting the submerged area and adding carbon (C) source can improve denitrification effect and removal rate of NO3–-N in bioretention units. However, experiments show that the dissimilatory nitrate reduction to ammonium (DNRA) has a significant impact on the N cycle and C/N ratio determines the reduction pathway of NO3–-N. Based on earlier findings, current work aimed to investigate the DNRA process in bioretention system by alternate wetting and drying operation mode, considering Total Nitrogen, NO3–-N and NH4+-N as assessment indices. Results show that after a prolonged drought period, bioretention unit was able to remove NO3–-N mainly through DNRA with addition of C source. NH4+-N accumulation occurs in dry period and the removal rate of NH4+-N was lower than that of with C source. This phenomenon indicates that carbon is the main factor to determine the reduction pathway of NH4+-N. Results of the microbial sequencing analysis revealed the presence of six common DNRA bacteria in bioretention: Pseudomonas (Pseudomonas), Bacillus (Bacillus), Thiobacillus (Thiobacillus), E. coli (Escherichia), phosphorus Vibrio (Desulfovibrio) and Desulfuvibibrio, which proves the existence of DNRA process in bioretention units.