氯成分衰變對次氯酸鈉氧化混沉除藻效能及消毒副產物生成之影響

Effect of chlorine decay on algal removal and disinfection by-products formation by NaOCl oxidation-coagulation

張庭瑜(TING-YU CHANG)

指導教授 ： 林志麟

中原大學/工學院/環境工程學系/碩士(2023年)

https://doi.org/10.6840/cycu202301081

摘要

國內淨水場常用之氧化劑為次氯酸鈉，有些水場會保存一個月以上才使用完畢，然次氯酸鈉藥劑在保存期間易受時間、溫度、光線等因素影響導致氯成分發生衰變，改變其氧化與消毒效能，影響供水操作與水質，如消毒副產物(disinfection by-products, DBP)生成。本研究取用國內淨水場常用之兩種(T廠及C廠)商用次氯酸鈉藥劑，分別以恆溫(25℃、35℃及45℃)及室溫條件(25±2℃)保存於暗室中，定時分析氯成分(總氯(Clt)、自由有效氯(Clf)及結合氯(Clc)及總有機碳(Total organic carbon, TOC)隨保存時間之濃度變化，並根據餘氯結果建立總氯降解模式，以評估氯成分之降解或轉化機制，並分別使用無熟化(0%Clc)及熟化(11%Clc)次氯酸鈉，搭配聚氯化鋁(PACl)及硫酸鋁(Alum)混凝劑，探討混沉去除銅綠微囊藻(Microcystis aeruginosa, MA)效能及消毒副產物生成。
在探討氯成分衰變結果方面，研究結果顯示，溫度越高，氯及TOC降解速率越快，當儲存溫度>35℃時，氯會有強烈的分解反應，造成自由有效氯濃度衰減幅度突增，且藥劑中TOC濃度也會影響氯成分的衰變。另外，次氯酸鈉在室溫下降解過程存在氯分解及消毒(有機物與氯反應)兩段反應，儲存時間在30天內為快速分解反應及消毒反應同時並行，30天過後，僅剩慢速分解反應持續進行(無結合氯生成)。藉由餘氯降解模式中推估出次氯酸鈉殘餘自由有效氯濃度對於氯降解速率影響甚巨，消毒反應速率隨自由有效氯濃度增加而升高，而隨時間增加消毒反應速率則會逐漸下降。在銅綠微囊藻混凝方面，於最佳混凝劑量(4 mg/L as Al)下，無熟化次氯酸鈉(0%Clc)前氧化搭配PACl混沉程序，除藻效能可達約90%，在去除有機物方面，次氯酸鈉搭配Alum混沉程序，可有效降低DOC及螢光有機物。對於DBP生成之影響，使用無熟化次氯酸鈉(0%Clc)前氧化，在最佳加藥量(4 mg/L)下，搭配PACl混沉程序後生成之DBPs量較低，而使用熟化次氯酸鈉(11%Clc)搭配Alum加藥模式能有效降低混沉上澄液total DBPFP。

關鍵字

次氯酸鈉 ； 前氧化 ； 混凝 ； 氯衰變 ； 消毒副產物

並列摘要

Sodium hypochlorite (NaOCl) is a common oxidizing agent used in water treatment plants (WTPs), and NaOCl chemical would be stored for more than one month before running out in most of WTPs in Taiwan. During storage, the decay of chlorine in NaOCl with time, temperature, light, which may change its oxidation and disinfection ability. At such a condition, the stability of water supply and drinking water quality would be significantly influenced, and it could cause a critical concern on the formation of disinfection by-products (DBPs). In this study, two types of commercial NaOCl agents (Brand T and Brand C) used in WTPs were stored at a constant temperature condition (i.e., 25℃, 35℃ and 45℃) in oven and at ambient temperature (25±2℃) in a dark room, respectively. The chlorine constituents (total chlorine (Clt), free chlorine (Clf), combined chlorine (Clc) and total organic carbon (TOC) were analyzed regularly during aging. The chlorine consumption behaviors were used to establish the kinetic model in the degradation of total chlorine and evaluate the degradation or conversion mechanism of chlorine constituents. In addition, the NaOCl solution with and without aging were used as pre-oxidation for coagulation with polyaluminum chloride (PACl) and aluminum sulfate (Alum) to investigate the coagulation efficacy for Microcystis aeruginosa (MA) removal and the inhabitation of DBPs formation.
The results have shown that the free chlorine would decay strongly as the storage temperature is more than 35℃, and the varied TOC concentration would affect the decay of chlorine. Two pathways of chlorine decomposition and disinfection (organics react with chlorine) possibly occur during NaOCl aging. Within 30 days aging, fast chlorine decomposition and disinfection happen simultaneously, and afterwards slow chlorine decomposition dominates aging process. The disinfection rate increases with free chlorine concentration, while it decreases over time. For MA coagulation, at the optimum dosage (4 mg/L as Al), the cell removal by non-aged NaOCl (0% Clc) pre-oxidation assisted PACl coagulation can be reached as high as 90%. Moreover, NaOCl pre-oxidation with Alum coagulation can effectively reduce DOC and fluorescent organic matter. In the case of DBP formation potential (DBPFP), PACl coagulation with non-aged NaOCl (0% Clc) is effective to reduce the in-situ formed DBPs, whereas Alum coagulation with aged (11% Clc) NaOCl is effective to reduce total DBPFP.

並列關鍵字

NaOCl ； chlorine decay ； pre-oxidation ； coagulation ； disinfection by-products

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