以氫氣生物反應器處理低碳氮比的硝酸鹽廢水之研究

Abstract

來自工業活動之廢水除了含有高含量之硝酸鹽外尚有少量之有機碳，因此，對於近年來尋找可有效處理硝酸鹽且低成本之技術便成為一大挑戰。目前為止，雖有許多可藉生物或非生物處理技術可有效去除硝酸鹽，但因其過程中具產生難以處理之廢滷水、薄膜易積垢、低處理效率及高營運成本等缺點往往阻礙了諸類技術之發展，故使用自營及異營菌之生物處理方法便成為主流技術。此外，工業廢水中因有機物濃度不足，故處理過程中外加有機碳便成為異營脫氮菌處理硝酸鹽技術之缺點。本研究是以自營性嗜氫菌於低碳氮比之硝酸鹽廢水中其處理硝酸鹽之可行性，實驗為於5L之生物反應器中結合沉澱池進行長時間運作，氫氣流速為50 mL/min，起始之硝酸鹽濃度維持在100 mg NO3--N/L，36小時控溫30℃期間之批次實驗為使用50 mL血清瓶以100 rpm轉速進行震盪；對於結合電解之實驗則採用100 mL血清瓶並縮短反應時間至100分鐘。結果顯示，在外部供應氫氣條件下，低C/N比廢水之硝酸鹽可使用自營性嗜氫菌處理之；於450天長時間試驗中，在硝酸鹽負荷率為0.1314 kgN/m3.day之氫氣生物反應器中其硝酸鹽去除率可達95%以上。此外，根據菌相分析可知Proteobacteria門菌種可適應低C/N比廢水並以氫氣為電子提供者之環境，其對於處理含硝酸鹽廢水中佔有重要角色，且Alphaproteobacteria 及Betaproteobacteria 類為氫氣生物反應器中之主要菌種。在操作條件對自營性嗜氫菌之實驗中可知當氫氣流速大於70 mL/min且溫度控制在30oC時，硝酸鹽幾乎可被完全轉換為氮氣；在pH為7.5且當碳氮比大於5時，相較於低碳氮比，自營性嗜氫菌具有較好之生長情形及較高之硝酸鹽處理效率；當硝酸鹽濃度高達1000 mg/L時結果亦同。於重金屬對處理效能之影響試驗中發現當銅離子濃度大於5 mg/L時，將抑制自營性嗜氫菌且降低氨氮去除率；且銅離子較其他重金屬如鎂、錳、亞鐵、鈷或鋅離子具有明顯影響。此外，當銅及鎳離子同時存在時，則較單純銅離子對自營嗜酸菌之影響性低。故生物反應器結合電化學系統因可直接於水中生成氫氣，本研究開發之自營性嗜酸菌反應器可作為水中硝酸鹽之處理程序。

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.