| Summary: | 博士 === 國立交通大學 === 環境工程系所 === 105 === The industrial wastewater often contain low concentration of organic carbon and high concentration of nitrate, thus is a challenge for the scientist community in finding an appropriate solution regarding the removal efficiency and economic aspects of nitrogen nutrient. Until now, there are many nitrate removal methods, including abiotic and biotic processes. Abiotic methods such as ion exchange, reverse osmosis, or electrodialysis have been widely used; however, the production of waste brine, membrane fouling, low efficiency, and high capital and operating costs hindered their widespread use. As a result, biological processes using both autotrophic and heterotrophic bacteria were selected. Since the industrial wastewater contains a low concentration of organic carbon, there is disadvantage using heterotrophic denitrifying bacteria when external organic carbon is needed during the treatment process. Recently, there are numerous studies focusing on the nitrate removal using autotrophic bacteria. However, little information is available on autohydrogenotrophic bacteria, which utilizes hydrogen gas as electron donor and nitrate as electron acceptor. This study was to investigate the possibility and applicability of autohydrogenotrophic bacteria in the treatment of nitrate from nitrate containing wastewater of low C/N ratio. The experiment was conducted in a 5-L bioreactor coupled with a clarifier for long-term operation. The initial nitrate concentration was kept at around 100 mg NO3--N/L, while hydrogen gas was controlled at a flow rate of 50 mL/min. Batch experiments were also conducted during this period using 50 mL serum bottles placed in an orbital shaking incubator at 100 rpm and 30oC for 36 h. Besides, 100 mL serum bottle was modified and used to perform the electricity tests for 100 min. Results showed that autohydrogenotrophic bacteria was a good choice for treating nitrate from the low C/N ratio of nitrate containing wastewater under the external supply of hydrogen gas. Specifically, long-term operation showed a stable nitrate removal by autohydrogenotrophic bacteria in a hydrogen-fed bioreactor with a removal rate of higher than 95% during 450-day test with the nitrate loading rate of 0.1314 kgN/m3.day. Analysis of microbial community in the bioreactor found that the phyla of Proteobacteria strongly adapted to environment of low C/N ratio and hydrogen gas supplied as an electron donor and played an important role in treating nitrate from the wastewater. Moreover, the class of Alphaproteobacteria and Betaproteobacteria dominated the total classes of microorganism in the hydrogen-fed bioreactor. Investigation on the effect of operational conditions on the performance of autohydrogenotrophic bacteria found that the hydrogen flow rate higher than 70 mL/min showed an excellent treatment efficiency with 100% of initial nitrate being converted to nitrogenous gas, while 30oC was the suitable temperature for the autohydrogenotrophic bacteria. In addition, the C/N ratio higher than 5 showed the best treatment efficiency in the batch experiment, compared to lower C/N ratio and the suitable pH value for the growth and performance of autohydrogenotrophic bacteria was at 7.5. Initial nitrate concentration of up to 1000 mg/L did not affect the removal efficiency. Investigation on the effect of heavy metals found that copper ion (Cu2+) directly affected the performance of autohydrogenotrophic bacteria, in which the concentration of copper ion of higher 5 mg/L significantly inhibited the reduction of nitrate, causing the low nitrogen removal rate. Furthermore, Cu2+ presented stronger effect to autohydrogenotrophic bacteria than other heavy metals (Mg2+, Mn2+, Fe2+, Co2+, and Zn2+). The co-existence of Cu2+ and Ni2+ showed the higher impact on the performance of autohydrogenotrophic bacteria than that of individual ones. Moreover, electricity could be applied into the autohydrogenotrophic reactor to treat nitrate containing wastewater by in situ generating hydrogen gas.
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