含鹵乙酸於慢濾處理流程降解特性之探討

Abstract

加氯消毒由於使用技術成熟、成本低廉且於配水系統中保有餘氯等優點，目前仍是淨水廠主要之消毒方式。然而許多研究指出，含氯消毒劑會與水中天然背景有機物反應，而產生消毒副產物（Disinfection By-Products，DBPs），其中三鹵甲烷（Trihalomethanes，THMs）為主要副產物，其次則為含鹵乙酸（Haloacetic Acids，HAAs）。由於其對健康上之危害，因此美國環保署於1998年訂定消毒劑與消毒副產物法加以管制。過去由於HAA分析方法繁複及各物種於環境中濃度低，於管制上有其困難，但近年來由於分析方法進步，使得HAA逐漸被重視。 慢濾池為最古老淨水設備之一，由於其成本低廉及出水水質佳，至今於許多小型供水廠仍繼續使用。由於慢濾池濾程長，隨操作時間增加會於濾砂表層產生生物膜，因此慢濾池之去除機制除物理吸附過濾之外，另外還有生物作用。然而在探討兩種機制對於有機物去除之影響，相關研究甚少，尤其對於消毒副產物方面更為缺乏。 本研究之目的為透過管柱實驗、批次實驗及實場採樣分析以瞭解HAA於慢濾池中被移除之現象，分別觀察物理吸附過濾以及生物作用於其中所扮演之角色。另外比較慢濾池不同操作情況對於HAA去除之影響。 研究結果顯示，生物作用為慢濾池中造成HAA濃度大幅下降之主要機制，且降解速率與含鹵素多寡有關，含越多鹵素者，降解速率越慢；而單純過濾並無法有效去除親水性之HAA。透過實場濾砂批次實驗可知，濾砂上之微生物對於降解HAA貢獻較大，且能於短時間內降解一鹵及二鹵乙酸。另外利用活性碳之方式，於操作初期1個半月可有效去除HAA，去除率大於90％。

Chlorination is commonly used in water treatment processes to prevent the water from microbial contamination. However, many studies show that chlorination resulted in disinfection by-products（DBPs）, and some of DBPs could cause adverse health effects. In general, trihalomethanes（THMs）are the major group of DBPs and haloacetic acids（HAAs）are the second. Concerns regarding the health effects of HAAs led the USEPA to promulgate the Stage 1 D/DBP rule that regulate the HAA5 at a MCL at 60μg/L. Slow sand filtration（SSF） is one of the oldest water treatment processes that still been operated in small communities. Because of the long operation period, biofilm can be developed on the surface of the sand. For that reason, SSF has both physical-chemical and biological removal mechanisms for both particulate and organic removal. However, there are few studies focusing on organic matter removal in the SSF, especially for DBPs removal. The objectives of this study were to investigate the roles of physical-chemical and biological removal mechanisms for HAA removal through column study, batch study and field study, by using different experimental conditions to evaluate HAA removal in the SSF. The results indicate that biological process was the major mechanism for HAA removal, and biodegradation rates of HAA decreased as the number of halogen atoms increased. However, single filtration could not remove HAA efficiently. The batch sand studies revealed that microbial associated with filter sand could degrade monochloroacetic acid and dichloroacetic acid within 1 day. Furthermore, active carbon adsorption can efficiently remove HAA and no HAA breakthrough was observed after 6 weeks of operation.