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4/2/2019 Effect of C/N Ratio on the Removal of Nitrogen and Microbial Characteristics in the Water Saturated Denitrifying Section of a Two-Stage C... The mean concentration of ammonium in the second stage effluent was lower in the control T 1 (0.3 mg/L),than in T2 and T3 (0.4 mg/L and 0.5 mg/L,respectively). The removal percentages of NH4+-N reached up to 99.1%with only minor differences between the three experiments, indicating that the modeled CRI resulted in good overall ammonium removal and an increased C/N ratio in the second- stage influent contributed little to improve this. The removal of NH4+-N in this CRI mainly occurs through adsorption and nitrification,which take place in the first stage, so that alterations in conditions of the second stage have little effect. Moreover,as a result of the used setup, the denitrifying column provides a constant submerged environment,which restricts the nitrification reaction [5] that would otherwise convert ammonium to nitrate(Equation(1)). As a result,varying the carbon to nitrogen ratio in the influent of the second phase has no effect on the overall removal rate of ammonium. The nitrate in the effluent is mainly generated by nitrifying bacteria that oxidize ammonium during the dry phase [25]. As shown in Figure 4b, the mean concentration of NO3--N in the first-stage influent was only around 2.7 mg/L,but it was much higher in the effluent of the second stage. In the controrthe mean nitrate concentration reached 45 mg/L in second-stage effluent, accounting for 9,3% of the TN concentration. Thus,the nitrifying section of the CRI resulted in nearly complete nitrification of the available nitrogen, as a result of the employed dry-wet alternating operation mode. Since negatively charged nitrate is not adsorbed by dielectric particles that are mostly also negatively charged [26], it remained in solution and was efficiently flushed out with the water flow,resulting in high concentration of nitrate and TN in the effluent. Wang et al. [25] showed that an extension of the residence time of the denitrifying section could improve the denitrification capacity of a CRI, as it would allow bacteria more time for nitrogen conversion. We extended the retention time to 6.5 h in the second stage by constructing a permanent water-saturated environment. However,this did not significantly increase the removal efficiency of nitrate in second-stage effluent of T1. Hou et al. [2 7] reported that a C/N ratio less than 2 in influent provided insufficient amounts of carbon for denitrification,resulting in low TN removal in effluent. We found that an increase in C/N to 2:1 inesulted in average nitrate concentrations in the second-stage effluent o ArrigAt,which represents a decrease of 6.4 mg/L compared to the control T1. With the highest tested C/N ratio of 5:1 (in T3)the nitrate concentration decreased b compared to TI, so that only12.7 mg/L NO3--N remained in the second stage effluent. Thus,we confirmed that the denitrifying capacity of a CRI can be improved by adding an external carbon source to the water-saturated denitrifying section, in order to meet the energy demand of denitrifying bacteria. 3.2. Effect of C/N Ratio on Removal Efficiency of Total Nitrogen CRI has been shown to perform well for removal of COD,NH4+-N and SS,but under standard conditions the removal of TN is typically only around 10-30% [4]. This can be improved by an increased C/N ratio in the second stage influent, as shown in Figure 5. https://www.ncbi.nIm.nih.gov/pmc/articles/PMC6069465/ 9/18