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A study on the biological stability of drinking water in the distribution system
本研究的主要研究目的為藉由生物培養技術，探討影響自來水配水管線生物穩定性之因子與有效控制技術。研究中針對配水管網常見的無機營養源 (氮、磷)、有機營養源 (醋酸、草酸、腐植酸) 以及常用的消毒劑 (自由餘氯、二氧化氯、一氯胺) 作為研究對象，以模場配水管線與批次實驗同時並行的方式，來探討不同的營養源與消毒劑對配水管線中的微生物再生長的變化情形。同時配合水質模式，來模擬管線內微生物、營養源及消毒劑之間的消長情形，以做為未來自來水廠控制微生物再生長之主要參考依據。
添加無機營養源的結果顯示，添加濃度低於0.1 mg N/L的銨鹽、硝酸鹽或添加濃度低於0.005 mg P/L的磷酸鹽，對管線中的生物膜生長並沒有明顯的助益。當添加氮源濃度增加至0.5 mg N/L或磷源濃度增加至0.01 mg P/L時，管線生物膜中的異營菌數目有明顯增加的趨勢。比較添加氮源與磷源對管線中生物膜的影響發現，添加磷源對管線中微生物再生長的影響效應明顯大於氮源，而實場批次實驗結果亦顯示台灣中部地區的管線微生物生長有受到磷酸鹽限制的情形。
水質模式的模擬結果顯示，模式所模擬之營養源與微生物濃度均與實驗值十分吻合。探討模式參數對模擬結果(靈敏度分析)則顯示，隨著微生物對基質的轉換率 (kconv) 愈小、自然死亡速率 (kx) 愈快及附著性細菌對氯殺菌力的阻抗作用 (kr1) 愈小，附著性細菌能夠有效抑制，微生物生長至穩定所需的時間也愈短。
Given proper water treatment, finished water meets the standard. However, consumers are still unsatisfied with the quality of drinking water. The main reason is that drinking water deteriorates while being transported through the distribution system to the consumer. Biological pollution has long been the major concern. The growth of microbes and the accumulation of biofilms have been found in the drinking water pipes by many investigators. According to our field observations on the water quality in the distribution system, the biological stability of drinking water is strongly related to nutrients and disinfectant levels. The purposes of this research are to study the effects of different inorganic nutrients (nitrate, ammonium, and phosphate), organic nutrients (acetic acid, oxalic acid, and humic acid), and disinfectants (free chlorine, chlorine dioxide, and monochloramine) on the growth of heterotrophic bacteria in the drinking water distribution system. Effective control strategies to maintain a high biological stability of drinking water in the distribution system are thus proposed to the experimental observations and theoretical results.
Results from the effects of adding ammonium or nitrate on the biofilm formation were insignificant for adding levels below 0.1 mg N/L. Analogous results were observed for adding phosphate at levels below 0.005 mg P/L However, as the addition of ammonium level increased to 0.5 mg N/L, significant effects on the biofilm formation could be observed. Similar results were obtained for adding phosphate at levels above 0.01 mg P/L. For addition of same level nutrient, biofilm formation was significantly greater for phosphorous than for nitrogen. The results of batch test indicated that the bacterial growth in the drinking water distribution system of central Taiwan was stimulated by adding phosphate.
The results of adding organic nutrients to distributed water showed that the addition of three organic nutrients, acetate, oxalate and humic acid, to treated water has positive effect on the AOC content of drinking water and bacterial regrowth in the water pipe. The effect is enhanced with addition of high-level organic nutrients. The comparisons of AOC results showed the AOC/TOC ratio is the highest with existence of acetic acid, followed by oxalic acid, and then humic acid. The HPCs of free bacteria has a linear relationship with the HPCs of attached bacteria indicating that the increase in HPCs of free bacteria could be attributed to detachment of biofilm caused by fluid shear stress. The blank tests of batch system indicated that the biological stability of distributed drinking water is the highest in treatment plant effluent, the moderate in the commercial area and mixed area, and the lowest in the residential area.
The experimental results with addition of different-type disinfectants in the distributed drinking water indicated that most bacterial regrowth was inhibited by free chlorine, followed by chlorine dioxide, and then by monochloramine. The inhibition was enhanced by addition of high-level disinfectants.
The theoretical results showed that the water quality model could predict the experimental data very well. The results of sensitivity analysis demonstrated that the growth of attached bacteria could be effectively inhibited under conditions of lower utilized rate of substrates (kconv), higher natural decayed rate (kx) and lower resistive ability to chlorine (kr1), and then took less time to reach steady-state conditions.
According to the BRP results, the bacterial regrowth potential were stimulated with adding inorganic and organic nutrients. Therefore, the biological statibility of the distributed water was good in central Taiwan. The strategy for using disinfectant did not only consider the disinfection efficiency, but also needed to evaluate the impact of biological statibility.
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