前加氯處理優養化水體對消毒副產物之影響

Abstract

自來水廠在處理原水的程序上，除了最後清水中的加氯消毒之外，為因應原水中較高的有機物濃度，會採用前加氯的方法來氧化部份汙染物及其它有機物質，以減輕後續混凝、沉澱、過濾單元的處理負擔，並減少操作成本，目前台灣地區許多自來水廠皆採用此一方法。 前加氯的氧化作用固然可解決一部份水質問題，但當原水中含有藻類細胞，甚至是來自高優養化的水源時，可能反而會造成飲用水中消毒副產物(Disinfection By-Products, DBPs)前趨物質的增加，提高民眾健康危害的風險。 以往的文獻中大多著重在優養化原水中藻類細胞的去除，或是藻類本身所產生的消毒副產物生成潛能，鮮少有研究強調在前加氯處理後所產生的影響，以及藻類所產生的消毒副產物前趨物質在淨水單元中的流佈。本研究以各種實驗室培養的含藻原水進行淨水程序的模擬，並討論有無前加氯的差異、藻類細胞及胞外物質的貢獻量、培養溫度的影響、溴離子與氯離子的競爭，以及三鹵甲烷(Trihalomethanes, THMs)及部份含鹵乙酸(Haloacetic Acids, HAAs)的生成。 實驗結果表示，前加氯的氧化作用會增加過濾水中消毒副產物前趨物質的含量，且這部份前趨物質是由於藻類細胞破裂，釋出胞內物質而產生的；藻類細胞所產生的有機物大多為脂肪族的前趨物質，因此在形成消毒副產物時溴離子會增加其生成量1~2倍；而前加氯的氧化力對於低溫(17~20℃)環境培養下的藻類細胞作用較小。 此外，本研究採集金門地區太湖及榮湖淨水廠的原水進行相同的實驗，分析結果中由於水樣含高濃度非藻類的有機物質，以致於無法推論實驗室所模擬之結果，但是可確定前加氯處理的水樣在過濾出水中會導致得到較高的消毒副產物生成量。

Water treatment plants generally use chlorine as disinfectant to ensure the safety and health of drinking water. Besides, chlorine is also used as pre-oxidant when there is high level of organic matter in raw water, called pre-chlorination. Pre-chlorination could decompose a part of contaminants, like ammonia nitrogen and TOC, to alleviate the load of treatment units and diminish the operation cost of waterworks .It is a common process of water treatment in Taiwan. However, pre-chlorination might bring certain problems when dealing with anthropogenic pollutants. Reactions between pre-chlorination and algae from eutrophic water would increase the content of precursors of disinfection by-products (DBPs) in treated water and raise the risk of public health hazard. This study focus on the effects after pre-chlorination and the fate of algal cells and extracellular products (ECPs) in water treatment processes. Various kinds of simulation water samples with Microcystis aeruginosa (a species of blue-green algae) were used to proceed water treatment processes, including coagulation, flocuulation, settling, and filtration. Besides pre-chlorination, the contribution of cells and ECPs as DBPs precursors, cultivating temperature, competition between Br- and Cl-, formation of trihalomethanes(THMs) and haloacetic acids(HAAs) would also be discussed in the study. The results show that the oxidation of pre-chlorination did increase the content of DBPs precursors in treated water after filtration. These precursors were released into water because the breaking of algae cells. According to some literatures, many extracellular and intracellular products of Microcystis aeruginosa belong to aliphatic precursors so that Br- would increase the DBPs formation for 1~2 times. The aliphatic precursors tended to form Br-DBPs because of steric hindrance limit. Moreover, the algae cultivated in low temperature condition could resist the oxidation of pre-chlorination and release less precursors into water. Furthermore, this study also collected the raw water from two water treatment plants in Kinmen to proceed the water treatment processes. The results of DBPs analysis could not be inferred by the outcome from simulating experiments because the concentration of non-algal organic matter was too high. However, the treated water with pre-chlorination process revealed higher DBPs concentration than that one without pre-chlorination process, in both two plants.