Enhancement Of Low C/N Wastewater Treatment In A Microbial Electrolysis Cell

Traditional nitrification and denitrification face various problems in application. Apart from these two reactions occuring in the two separate stages, namely aerobic process and anaerobic process, a major problem limiting the efficiency of the traditional denitrification is the lack of carbon source. In many of municipal wastewater plants, extra carbon source is required to compensate into the wastewater due to the shortage of electron donor. It is of scientific significancy to improve the proportion of carbon source utilization for denitrification and even to realize the nitrification and denitrification in a single anaerobic reactor. Microbial electrolysis cell ?MEC?, a combined equipment of anodically oxidizing organics with electrogens ?such as iron reducing bacteria, IRB? and cathodic reduction, is capable of obtaining energy or processing biosynthesis along with the contamination removal. Currently, cathodic denitrification has been extensively explored in MEC. However, there are few studies to investigate whether the dependence of denitrification on carbon source is alleviated and whether NH₄⁺can be anodically oxidized into NO₂- that is cathodically reduced or involved in the anammox reaction. To address these problems, the microbial anodic oxidation in MECs were investigated ?1? to improve organics oxidation in anode coupling with the denitrification in cathode with the aim to concentrate the carbon source for denitrification, ?2? to anodically oxide NH₄⁺ to NO₂- that was used for the cathodical denitrication or anammox in one reactor. Based on these considerations, we also attempted to further improve nitrogen removal via shortening hydraulic retention time to decrease carbon resource consumption by other heterotrophic processes or via strengthening anodic oxidation by enriching the IRB. The results are as follows:?1? Inserting a pair of C-C ?graphite? electrodes into an up-flow anaerobic sludge blanket ?UASB? reactor could significantly improve the performance of denitrification at low C/N. Addition of electrodes might provide an efficient pathway to transfer the electrons for the denitrification by the combination of anodic oxidation and cathodic reduction, decreasing the proportion of organics used for other heterotrophism. The results demonstrated that addition of C-C electrodes could improve the COD removal efficiency by about 12% and the nitrate removal efficiency by about 14%. The average anodic coulombic efficiency of this reactor with C-C electrodes was about 18% by the current caculation. This chemical measurement was consistent with the improved nitrate removal. Addition of electrodes could induce the conversion of propionate and butyrate to acetate. As compared with the control reactor, the extra electrons from the conversion of propionate and butyrate in the C-C electrode reactor was 15.5 mmo/L. Therefore, at least 80% of the electrons were used for the cathodic denitrification ?the rest for methaeation?. Zero-valent iron ?ZVI? as anodic electrode could further enhance the performance of denitrification at low C/N. Nitrate removal efficiency was improved by about 26% in Fe-C electrode reactor.The average anodic Coulombic efficiency of this Fe-C electrode reactor was about 25%. Addtion of Fe-C electrodes further accelerated the decomposition of propionate/butyrate to acetate. As compared with the control reactor, the extra electrons from the conversion of propionate and butyrate in the Fe-C reactor was 26.4 mmo/L. Therefore, at least 85% of the electrons were used for the cathodic denitrification. ZVI as electron donor directly taking part in the denitrification could be negligible in the Fe-C reactor ?less thanl% of the total nitrogen removal?.The higher nitrate removal in Fe-C reactor was mainly attributed that more exoelectrogenic bacteria were enriched in the presence of ZVI. ZVI could enhance the anodic oxidation and better drive the cathodic denitrification to carry out. DGGE and FISH analysis showed that the electrodes especially the ZVI as anodic electrode enhanced the abundance of exoelectrogens.?2? Although addition of electrodes realized the utilization of more organics for denitrification, carbon source inevitably was used for methanogenesis. We attempted to further improve the nitrogen removal via declining the hydraulic retention time to decrease consumption the carbon resource for other heterotrophic processes. The results showed that the nitrate removal rate in the hydrolysis acidification reactor was 69.0 mg/?Lh? higher than that in the control reactor. With the decrease of HRT from 4 to 2 h, addition of electrodes improved the nitrate removal. The carbon sources used for denitrification was increased from 19% to 31% in the hydrolysis acidification. FISH results showed that the electrodes relieved the influence of HRTs on the growth of denitrifying bacteria.?3? To realize the higher nitrogen removal, anodic oxidation of NH₄⁺ to NO₂- was used for the cathodic denitrification or anammox in a single-chamber reactor. Anodic oxidation of NH₄⁺ to NO₂- was realized in a single-chamber bioelectrochemical system with the anodic potential of -0.5 V. At the influent NH₄⁺ of 100 mg/L, the TN removal efficiency was increased by about 12% in the reactor with applied potential. Anodic oxidation contributed to 18% of the NH₄⁺ removal. With the influent NH₄⁺ increasing from 100 to 150 and 200 mg/L, the gap of NH₄⁺ removal in the two reactors narrowed, which was decreased from 20% to 18%. The reason was likely related to the inadequate electron acceptors in the cathode. The anodic oxidation of NH₄⁺ needed to couple with the cathodic reduction that captured the electrons produced from the anodic oxidation. The contribution of nitrite reduction by cathodic denitrification and Anammox process was 19% and 81%, respectively. FISH and 16S rDNA analysis showed that the abundance of nitrosomonas was obviously higher in the reactor with applied potential. It was reported that nitrosomonas belonged to the ammonium oxidizing bacterium which could oxidize ammonia and transfer electrons to the anode. These bacteria were likely to oxidize NH₄⁺ to NO₂- leading to accelerate the Aanmmox process and cathodic denitrification.?4? To improve the efficiency of anodic oxidation, different iron oxides were dosed into the UASB reactors to improve the NH₄⁺ removal. Different kinds of iron oxides such as Fe₂O₃, Fe₃O₄, Fe?OH?₃ and ferric citrate were added into the UASB reactors. The NH₄1⁺ removal in the Fe₂O₃-supplemented reactor was higher than others about 46%. Addition of Fe₂O₃ improved the NH₄⁺ removal by 48%. However, the presence of organics inhibited the ammonia nitrogen removal in the Fe₂O₃-supplemented reactor. The results showed that the NH₄⁺ removal was reduced by 28% when the reactor was fed with organics. To further improve the NH₄⁺removal, the experiment was conducted in the single-chamber MEC. It showed that the NH₄⁺ removal was higher than the control reactor by 29% and increased by 12%, compared to the addition of electrodes reactor. FISH analysis indicated that Fe₂O₃ increased the abundance of iron-reducing bacteria in order to increase the NH₄⁺ removal. Meanwhile, applying 0.5 V to the MEC could enrich the abundance of AOB bacteria which promoted the ammonium oxidation.