- 學位論文
前氧化結合聚矽酸鐵混凝程序除藻及降低消毒副產物生成之研究
Algal removal and disinfection by-products reduction by chlorination-coagulation with poly-silicate iron
摘要
當水庫出現藻華或優養化現象時，以水庫為主要水源之淨水場原水中藻類劇增(>104 cells/mL)，而傳統無機混凝劑(如聚氯化鋁及氯化鐵)無法有效藉由混沉程序移除藻類，導致後續沙濾單元操作負荷過大而影響出水穩定性。前加氯能降低原水藻類活性及增加除藻效能，但氯會與原水有機物反應生成消毒副產物(三鹵甲烷及鹵化乙酸)，影響飲用水供水品質。淨水處理藥劑聚矽酸鐵混凝劑(polysilicate iron, PSI)可有效提升水處理混沉程序除藻效能，但PSI混凝劑中鐵型態物種分佈會因製備條件矽/鐵比不同而產生差異，進而影響PSI混凝行為及混沉除藻成效。因此，本研究目標主要探討前氧化結合聚矽酸鐵混凝程序除藻效能及消毒副產物生成。 本研究培養水庫水優勢藻種，即小球藻(Chlorella sp.)及銅綠微囊藻(Microcystis aeruginosa)，製備綠藻/藍綠藻混合水樣，再進行NaOCl及ClO2前氧化搭配PSI混凝實驗，評估兩種PSI(PSI0.25(Si/Fe=0.25)與PSI0.5(Si/Fe=0.5))對兩種藻類之混沉移除效能，以及探討溶解性有機碳(dissolved organic carbon, DOC)去除效能與藻類有機物(algogenic organic matter, AOM)螢光物質成分變化，最後分析水中消毒副產物生成量。 研究結果顯示，矽鐵比會影響PSI混凝劑鐵型態及混凝機制，低矽鐵比PSI混凝劑高聚態鐵(Fec)居多，混凝劑分子較大，其混凝架橋作用強，除藻效率佳；高矽鐵比中低聚鐵(Feb)居多，混凝劑分子較小，其混凝架橋作用弱，電性中和能力強。此外，搭配PSI混沉程序下，二氧化氯前氧化對DOC去除優於次氯酸鈉前氧化，而次氯酸鈉前氧化對螢光有機物去除則較佳，尤其是對類磺酸(FAL)及類腐植酸(HAL)最為顯著。在DBP生成潛能降低上，次氯酸鈉前氧化搭配PSI混凝程序中，低矽鐵比(PSI0.25)混凝劑可有效降低水中DBP即時生成量，降低幅度優於高矽鐵比(PSI0.5)混凝劑即時生成量將近1.7倍。然而，PSI0.25及PSI0.5混凝劑均無法有效降低混沉上澄液total DBPFP及specific DBPFP。
並列摘要
When algae blooming or eutrophication occurs in drinking water reservoirs, the algae suddenly increase to extremely high level (>104 cells/mL) in the raw water from reservoirs for water treatment plants (WTPs), which results in ineffective algal removal by coagulation-sedimentation with traditional coagulants (e.g., polyaluminum chloride and ferric chloride). This frequently influence the stability of water supply due to increased burden on the operation of filter for WTPs. Pre-chlorination lowers the viability of algae and increases algal removal effectively, but chlorine would react with organics to form disinfection by-products (e.g., Trihalomethanes(THMs) and Haloacetic acids(HAAs)) in chlorinated water, which substantially affect the quality of drinking water supply. Polysilicate iron (PSI) coagulants, as water clarification chemical, can effectively enhance the performance of algae removal through coagulation-sedimentation. However, the coagulation performance of PSI could be dominated by Fe species distribution in synthesis process, which influences PSI coagulation behaviors and its performance for algae removal by coagulation-sedimentation. Thus, this study aimed to investigate algal removal and disinfection by-products reduction by chlorination-coagulation with poly-silicate iron. In this study, micro algae species (i.e., Chlorella sp. and Microcystis aeruginosa) were cultured by shaking flask system to prepare the mixed algae suspensions. Then, NaOCl and ClO2 pre-oxidation tests were carried out followed by coagulation. For pre-oxidation-assisted coagulation, two PSI coagulants, namely PSI0.25 (Si/Fe=0.25) and PSI0.5 (Si/Fe=0.5), were used to evaluate the algae removal efficiency by oxidation-coagulation in addition to investigation into dissolved organic matter (DOC) removal as well as the variation in fluorescent organics fraction. Finally, disinfection by-products formation for tested samples were determined. The results have shown that silicon-to-iron ratio would affect the Fe species distribution of PSI coagulants and their coagulation mechanisms. At low silicon-to-iron ratio (Si/Fe=0.25), it would mostly form highly polymeric Fe (Fec) with large molecule in the PSI coagulant, which makes strong inter-particle bridging effects for effective algal removal. At high silicon-to-iron ratio (Si/Fe=0.5), it would mostly form polymeric Fe (Feb) with small molecule in the PSI coagulant, which results in weak inter-particle bridging effects for algal removal. In addition, for oxidation-assisted PSI coagulation, the DOC removal by ClO2 pre-oxidation is superior to NaOCl pre-oxidation; however, the removal of fluorescent organics by NaOCl pre-oxidation is more effective, especially for the removal in fulvic acid-like and humic acid-like substances. In the case of DBP formation potential (DBPFP), the PSI coagulant with low silicon-to-iron ratio is able to effectively reduce the in-situ formation of DBPs during coagulation-sedimentation process, in which the reduction ratio of DBPs formation is nearly 1.7 times higher than that by coagulation with high silicon-to-iron ratio PSI. In contrast, both PSI0.25 and PSI0.5 coagulants is ineffective to reduce total DBPFP and specific DBPFP of supernatant after coagulation-sedimentation.