آب و فاضلاب (Sep 2020)
Simultaneous Nitrification and Denitrification in Aerobic Moving-Bed Sequencing Batch Reactor under the Effect of COD/N
Abstract
Nitrogen compounds such as ammonium and nitrate are among the most common pollutants in wastewater and water resources. Removal of these substances is essential for reducing their adverse environmental and health effects. One of the biological methods for eliminating nitrogen is the simultaneous nitrification and denitrification process (SND) which, due to simplicity and cost effectiveness has attracted much interest. The SND can be performed by using bacterial attachment growth. The COD/N ratio is one of the effective parameters on the SND process. It can influence the process performance by changing the biofilm mass and the bacterial population. For this reason, the purpose of this study was to investigate the effect of COD/N ratio on simultaneous nitrification and denitrification in aerobic moving-bed sequencing batch reactor (MBSBR). For this purpose, an aerobic MBSBR reactor was operated in two phases. The selected values for COD/N in phases 1 and 2 were 10 and 20, respectively. According to the results, doubling of COD/N significantly increased the biofilm mass from 15 mg/media to 25 mg/media and increased the effluent COD from 22 mg/L to 192 mg/L as well, for phases 1 and 2, respectively. However, it had trivial effect on the concentration of suspended solids. The ammonium concentration in effluent of phase 2 was around 30 mg N/L which was about 33% more than that of phase 1 (10 mg N/L). This indicates a decrease in the rate of ammonium oxidation in phase 2 which most probably was attributed to increase of the population of heterotrophic bacteria and inhibition of the growth of autotrophs as a result of the higher abundance of organic matter in this phase compared with phase 1. In phase 2, in spite of increasing biofilm mass and part of the organic matter remaining until the end of the cycle (the factors that make the conditions more desirable for denitrification), denitrification decreased and from about 39% in phase 1 reached 12 % in this phase. The reason for this observation was the lack of proper nitrification and low concentration of nitrate in phase 2. Therefore, it can be said that by increasing COD/N ratio, the nitrate concentration, or in the other words the nitrification process, has become the controlling factor of the denitrification process. In general, in phase 1 (COD/N of 10) simultaneous nitrification and denitrification took place much better so that more than three-fold nitrogen removal efficiency was achieved compared with phase 2 (COD/N of 20).
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