| 英文摘要: | Disinfection by-products are chemicals which are formed during drinking water treatment and are usually associated with the process of disinfection. Disinfection by-products have been a major focus of research in the US and around the world for the past 40 years. This research project is significant because it will demonstrate that biofilms which coat the water distribution pipes are an important source of a particular class of disinfection by products known as N-nitrosamines. This family of disinfection by products, at least some of them, are known human carcinogens. A novel N-nitrosamine formation pathway will be investigated involving a previously unidentified intermediates in water treatment systems which use chloramine disinfection. Further, strategies to control biofilm growth and facilitate biofilm detachment will be systematically evaluated to determine the conditions required to achieve bacterial stable water, while controlling formation of N-nitrosamines.
The research objective of this proposal is to test the hypothesis that biofilm extracellular polymeric substances (biofilms) in chloraminated drinking water distribution systems are important N-nitrosamine precursors. N-nitrosamines are a highly toxic, non-halogenated group of disinfection by-products formed primarily in drinking water distribution systems which use chloramine, a common secondary disinfectant used to curb formation of regulated disinfection by products. A recent study using EPA Method 521, which can quantitatively determine seven different N-nitrosamines, were shown to comprise only approximately 5% of the total N-nitrosamines formed in drinking water distribution systems. This result implies that other important N-nitrosamines may have been overlooked. Biofilm extracellular polymeric substances contains functional groups such as secondary amines, known to react with chloramines to form N-nitrosodimethylamine (NDMA), the most prevalent of the EPA Method 521 species. This study will utilize a total N-nitrosamines assay to assess the contribution of biofilm-derived materials to the overall precursor pool. Preliminary data demonstrate that chloramination of exopolysaccharide isolates yielded total N-nitrosamines with a direct dose-response relationship. Pure and mixed culture biofilms, relevant to drinking water distribution systems, will be characterized using a combination of traditional methodologies and next generation sequencing methodology. These systems will then be chloraminated to elucidate the role of extracellular polymeric substances in the formation of an expanded list of formation products in the N-nitrosamine family. The proposed research will evaluate the following three objectives: 1.) Characterize biofilm dynamics in simulated drinking water distribution systems. Pure and mixed culture biofilm will be grown in batch and annular reactors, respectively. The biofilm EPS composition (e.g., exopolysaccharides and extra-cellular proteins) and bacteria species will be characterized. 2.) Measure N-nitrosamine yields from chloramination of biofilm-derived materials and precursor isolates. Attached and detached pure and mixed culture biofilm will be chloraminated to assess the formation of the EPA Method 521 species and total N-nitrosamines. Biofilm isolates and known NDMA precursors will be assessed similarly and a series of abiotic experiments will be completed to help reveal the underlying reaction mechanism. 3.) Assess the impact of free chlorine on biofilm characteristics and formation of N-nitrosamines. Free chlorine burns will be simulated in biofilm annular reactors at different disinfectant concentrations, durations, and temperatures. Biofilm characteristics and N-nitrosamine precursors will be assessed before, during, and after the free chlorine addition. This research marks the first investigation into the role of biofilm, ubiquitous in all drinking water distribution systems, as N-nitrosamine precursors. Results from this study will form a scientific basis to justify a nationwide occurrence survey of total N-nitrosamines in chloraminated drinking water distribution systems. The objective of the education and outreach plan is to engage graduate students and high-performing engineering undergraduates from underrepresented groups in the research activities, create scientifically accurate animations of the elucidated reaction mechanisms, and disseminate the research findings to a wide range of stakeholders. |