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  • 學位論文

慢濾池降低水中亞硝基二甲胺濃度之研究

N-Nitrosodimethylamine Reduction by Slow Sand Filtration during Drinking Water Treatment

指導教授 : 王根樹
共同指導教授 : 童心欣(Hsin-Hsin Tung)
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摘要


許多研究證實淨水廠或廢水處理廠的加氯消毒會生成包括亞硝基二甲胺 (N-Nitrosodimethylamine;NDMA)等不同類別的消毒副產物。在動物實驗上,亞硝基二甲胺具有致基因變異的特性,因此美國環保署 (USEPA) 將其分類為可能的人類致癌物質 (Group B2),且其致癌性遠遠高於傳統消毒副產物,如三鹵甲烷 (Trichloromethanes;THMs) 與含鹵乙酸 (Haloacetic acids;HAAs)。為了確保公眾健康,避免飲用水中出現亞硝基二甲胺為十分重要的議題。過去研究顯示紫外光可以有效地光解亞硝基二甲胺,然而其所需的成本相當高,且水中可能進一步生成亞硝基二甲胺的前驅物質。近來陸續有研究針對亞硝基二甲胺的生物降解進行探討,包括培養環境微生物與實場調查研究。然而,嘗試在實場樣品中純化出可以降解亞硝基二甲胺的微生物時卻總是未能成功,因此無法進一步了解這些微生物分解亞硝基二甲胺的機制與特性。 本研究針對實場能減少水中亞硝基二甲胺的微生物進行物種鑑定,瞭解會影響亞硝基二甲胺進行生物轉換的環境因素,以及調查這些微生物在實場的分佈比例情形。首先,以塗抹與劃碟法於淨水場慢濾單元中純化出不同的微生物,再進行生物處理亞硝基二甲胺的測試。接著針對具有去除液態基質中亞硝基二甲胺能力的微生物進行動力學實驗,並且改變不同環境因子檢視其對生物處理效率的影響;包括亞硝基二甲胺起始濃度、溫度、實場進流水中基質、以及混菌狀態的影響等等。同時於實驗室建立模擬濾砂管柱系統,以實場慢濾池進流水添加亞硝基二甲胺進行微生物馴養與處理效率的探討。最後利用末端限制片段長度多型性 (Terminal restriction fragment length polymorphism;T-RFLP) 分析冬季與夏季時,減少水中亞硝基二甲胺濃度的微生物實場分佈,同時也探討其於模擬管柱的空間分布情形。 本研究於慢濾池濾砂生物膜中純化出具有降低水中亞硝基二甲胺能力的微生物,鑑定結果為甲基桿菌屬(Methylobacterium sp.) 的物種。此菌種並非以亞硝基二甲胺作為生長營養物質,而是經由共代謝的形式將其分解,測試後發現可催化共代謝的物質為一複合培養基,R2A。動力學結果顯示,在25℃和15℃時,亞硝基二甲胺起始濃度越高,反應速率常數與十小時去除百分比越低,如起始濃度為600

並列摘要


N-Nitrosodimethylamine (NDMA) was recently found as one of the emerging disinfection by-products (DBPs) in the wastewater and drinking water treatments. The toxicity of NDMA, which has been classified by USEPA as Group B2 of probable human carcinogen, was much higher than that of conventional DBPs such as THMs and HAAs. Thus, the control of NDMA in drinking water is an important issue for public health protection. Previous studies showed that NDMA can be destructed effectively by UV irradiation; however, it requires high UV dosage and NDMA precursors are likely to be formed again after UV treatment. Recently, microbial degradation of NDMA was studied extensively in laboratory and field investigations. However, isolation of NDMA-degrading bacteria from the field was not successful. Hence, the information regarding to the NDMA biotransformation mechanisms in the field was limited. The objectives of this study were to identify the organisms responsible for the bio-reduction of NDMA, to provide information concerning environmental factors affecting the efficiency of NDMA biotransformation, and to investigate the distribution of NDMA-reducing bacteria in the field. First, the candidates of NDMA-reducing bacteria were isolated from the biofilm attached on the slow sand filter in a water treatment plant by traditional plating methods. Followed by NDMA bio-reduction capability tests, kinetic experiments were conducted to demonstrate the roles of initial NDMA concentrations and environmental factors on NDMA removal. Third, a laboratory simulated sand column system was installed to establish a natural distribution of microbes along different depths of the column through continuous acclimation. Finally, the distribution of NDMA-reducing bacteria in the slow sand filter obtained in different seasons and which from simulated sand column was analyzed by terminal restriction fragment length polymorphism (T-RFLP). The Methylobacterium sp. was isolated from the slow sand filter, and the ability of NDMA bio-reduction was observed after growth on a low nutrient medium, R2A. The bacterial isolate was unable to grow in the presence of NDMA, but it performed NDMA biotransformation through possible cometabolism when incubated with R2A medium. In the kinetic experiments, first-order and second-order reaction equations were used to fit the kinetic data from the reduction curve. The results demonstrated that 1st and 2nd order kinetic reaction rate constants (hour-1) and percent removal after 10 hours of contact time increased as the initial concentration decreased at 25℃ and 15℃, but no significant difference was found at 35℃. For the same NDMA initial concentration, the rate constant and extent of NDMA removal were higher at 25℃ than 15℃, however, biotransformation of NDMA seemed unfavorable to the bacterial isolate for a higher temperature at 35℃. Also, the enzyme’s maximum rate, Vmax, was the highest at 25℃. It was suggested that the optimal temperature for NDMA biotransformation was approximately at room temperature for the isolated Methylobacterium sp.. Different proportions of sterile influents from slow sand filter were spiked into the reaction matrix, and the percentage of NDMA removal in 100% of field scale influent water was only 5.4%, revealing that the compositions of organic or inorganic compounds in the filter influent might affect the efficiency of NDMA bio-reduction in the slow sand filter. In addition, mixed bacteria might have no direct influence on NDMA bio-reduction, but it could affect the growth of the Methylobacterium sp.. T-RFLP showed that NDMA-reducing bacteria like Methylobacterium sp. was detected in the slow sand filter, and its relative proportion as total community were two-fold increased in the winter than in the summer. The laboratory simulated sand column fed with influents was spiked with 100

參考文獻


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