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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... which alternates the operation mode between an aerobic and(facultative)anaerobic environment,thus . employing a more diverse mix of microorganisms. The unique operation mode and the rich diversity of microorganisms colonizing the filling medium of a CRI are favorable for efficient removal of pollutants from sewage [2]. The common practice,with photographs shown in Figure 1,results in satisfying removal performances of organic pollutants and suspended solids(SS); in particular removal rates of chemical oxygen demand(COD)of 85%can be established,while ammonia nitrogen(NH4+-N)removal rates can be as high as 90%. In contrast,removal of total nitrogen(TN) is relatively poor,only reaching 10-30% [4]. In a simulation experiment,Wang et al. [5] found that the upper part of the CRI simulation column contained higher amounts of denitrifying bacteria,related to presence of organic carbon sources,better oxygen transmission and aeration,while these were lacking in the anaerobic environment of the lower part of the CRI simulation column.Due to the limitation of denitrifying bacteria and organic carbon sources in that lower part, the overall removal of TN from the effluent was poor[5]. Removal of TN, the sum of NH4+-N,NO3--N,NO2--N and organic nitrogen,is difficult due to fluctuations between these various nitrogen forms. Insufficient removal and excessive discharge of TN into the environment can lead to eutrophication of rivers and lakes, which poses a world-wide environmental problem [6]. Thus,various methods have been employed to improve the nitrogen removal efficiency of a CRI. It was shown that the main factors determining nitrogen removal in a CRI are the concentration of dissolved oxygen, the ratio of C/N in the water, and the residence time of NO3 -N in the anaerobic section [7]. To improve nitrogen removal performance, a sub-section of the effluent can be fed back into the CRI,which in simulated columns resulted in a 64.8%increase of TN removal efficiency [8]. Matsumoto [9] observed that denitrification by CRI could be improved when the C/N ratio in the influent was increased to 2. Moreover,Chen et al. [10] found that the TN removal rate could reach over 60%by adding 7 mg/L Fe 3+to the water-saturated section of CRI. Lastly, Song et al. [i I] added corncob wastelo the water-saturated section of CRI,which increased the nitrogen removal rate to above 79%. Thus, various methods exist to improve the nitrogen removal efficiency of CRI systems, but few studies determined the optimal C/N ratio in influent of the water-saturated section, or the variation in microbial biomass related to changes in C/N ratio in that section. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6069465/ 2/18