N-亞硝基二甲胺之生物降解

Abstract

水處理廠為維持出水品質會在淨水流程中進行消毒以去除致病菌活性，而經由成本以及便利性考量，含氯消毒劑皆優於其他消毒程序，因此多數淨水廠與廢水廠多以含氯消毒劑進行消毒。然而含氯消毒劑的使用可能與天然有機物(Nature Organic Matters, NOMs)反應生成多樣的消毒副產物(Disinfection by-products, DBPs)，其中亞硝基二甲基胺(N-Nitrosodimethylamine, NDMA)即為一新興之消毒副產物。NDMA易在以含氯消毒劑進行消毒之水處理廠中被發現，特別是在含氮量較高的情形下，並且經由文獻指出，NDMA具有造成人體致癌的風險而使得NDMA的去除逐漸被受重視。目前已知NDMA可經由物化程序進行有效之去除，然而考量其去除成本較高，現轉而尋求較具經濟效益之微生物處理方法。目前雖有文獻指出NDMA具有微生物可降解性，但對於可降解NDMA的菌株卻所知甚少，因此本研究期從甲烷氧化菌以及水處理廠中找出可降解NDMA之菌株。 為尋找可有效降解NDMA之菌株，本研究將三株甲烷氧化菌Methylosinus trichosporium OB3b、Methylocystis parvus OBBP和 05A-I-S2以及內湖污水廠之活性污泥和經由四甲基氫氧化銨(Tetramethylammonium Hydroxide, TMAH)馴養過後之活性污泥，進行NDMA降解實驗。 研究結果顯示，甲烷氧化菌在表現單氧化酵素時，可依共代謝作用降解NDMA，而當甲烷氧化菌M. trichosporium OB3b 和 05A-I-S2在分泌sMMO之情形下降解NDMA的效率大於M. parvus OBBP分泌pMMO時。此外兩種試驗之污泥皆被發現具有降解NDMA之能力，然而唯有以TMAH馴養過後之污泥中所分離出之一菌株，Uncultured alpha proteobacterium UP2，具有降解NDMA並同時以其作為碳源之能力。

Chlorination is the most common disinfection method to reduce the activities of pathogens in water treatment process due to its low-cost and effectiveness. However, with the existence of nature organic matters (NOMs), the application of chlorine-contained disinfectants may result in the formation of disinfection byproducts (DBPs) which are of particular concern considering their potential impact on people health. One of the emerging DBPs, N-Nitrosodimethylamine (NDMA), is considered as a potent carcinogen. It is often found in finished water with chlorine or chloramine applied, especially in water with high nitrogen contents. Thus the removal of NDMA in water treatment process is crucial to ensure the safety of aquatic environment. The removal of NDMA by physical-chemical process has been proved to be effective; however, with consideration of cost, physical-chemical technology may not be the best available strategy, promoting biodegradation as an alternative. Previous literature indicated that NDMA is biodegradable, but with limited information regarding functional bacteria provided. The objective of this study is to identify the bacteria with capability to degrade NDMA. Three methanotrophs, i.e., Methylosinus trichosporium OB3b, Methylocystis parvus OBBP and isolated strain 05A-I-S2, as well as two activated sludges including one from Neihu municipal wastewater treatment plant and the other cultivated by tetramethylammonium hydroxide (TMAH) were chosen to examine their biodegradability of NDMA. The result showed that NDMA was degradable by three methanotrophs through cometabolism along with expression of methane monooxygenase (MMO). Note that the efficiency of degradation was higher to methanotrophs with inducement of the soluble MMO (sMMO) than those with particulate MMO (pMMO) produced. In addition, NDMA can also be biodegraded by both sources of sludge adopted in this study with only one pure strain isolated from the TMAH-cultivated sludge, Uncultured alpha proteobacterium UP2, being capable to utilize NDMA as the carbon source while biodegrading.