不同氧化劑對混沉程序移除藻類及錳之影響

Abstract

淨水場普遍使用前加氯搭配混沉程序移除水中藻類，然而前加氯會破壞藻體細胞而生成總三鹵甲烷等消毒副產物，此外，若原水中含有溶解錳將會消耗前加氯量。當原水中同時含有大量藻類及溶解錳時，以次氯酸鈉為前氧化劑無法有效氧化水中溶解錳，因此，部分淨水場會使用二氧化氯或高錳酸鉀作為替代氧化劑以增進溶解錳氧化效率，故在氧化程序中，氧化劑之選擇將顯著地影響混沉程序移除藻類及錳顆粒物之效能。 本研究主要探討原水中同時含有藻類及溶解錳時，不同氧化劑(次氯酸鈉、二氧化氯及高錳酸鉀)對水中藻類及溶解錳之氧化效果，及對後續搭配混沉程序移除藻類及錳之效能。實驗原水以掃流超過濾薄膜模組對寶山水庫表層水進行濃縮，並以人工添加溶解錳(0.3 mg/L)方式配置而得，再以此原水進行前氧化搭配混沉試驗。在氧化試驗中，將改變次氯酸鈉、二氧化氯及高錳酸鉀氧化劑量及氧化攪拌時間，以流式細胞儀分析水中藻類活性並分析氧化後水中溶解錳(錳離子及有機錳)含量，以了解不同氧化劑對藻體細胞活性及溶解錳氧化程度之影響。 研究結果顯示，以次氯酸鈉及二氧化氯氧化可破壞藻體表面結構而大幅降低藻體活性，而以高錳酸鉀氧化則不易降低藻體活性，但可誘使藻類釋出胞外物，其中又以二氧化氯氧化降低細胞活性之反應動力常數值(k_(t=30s))為最大。當溶解錳存在於含藻原水時，次氯酸鈉氧化可有效降低藻類活性但無法氧化水中錳離子及有機錳，而高錳酸鉀可有效氧化溶解錳形成錳顆粒物，但添加過量會造成水中錳離子含量升高，僅二氧化氯氧化可同時有效降低藻體活性並氧化錳離子及有機錳。使用硫酸鋁混凝劑，以高錳酸鉀氧化提升藻類去穩定之效能最佳，而二氧化氯前氧化同時提升混沉除藻及錳之效能最佳。

Prechlorination with NaOCl is usually used to improve the algae removal by coagulation/sedimentation process for water treatment plants (WTPs), but prechlorination may cause cell lysis and increase disinfection by-products such as trihalomethanes. In addition, the majority of soluble manganese forms in the raw water, it would increase the dosage of prechlorination. When the raw water contains a great amount of algae in the presence of soluble manganese, NaOCl chlorination is not effectively to oxidize soluble manganese. Therefore, ClO2 and KMnO4 are used instead to improve the formation of oxidized manganese. The oxidants in oxidation process prior to coagulation could strongly affect the algae and Mn oxides removal through coagulation/sedimentation process. This study aims to investigate the effect of different oxidants (NaOCl, ClO2 and KMnO4) on the removal of algae and Mn oxides by coagulation/sedimentation. The raw water collected from Bao-San reservoir was first concentrated through ultrafiltration process, and then the prepared soluble manganese was added into the concentrated water to a desire concentration of 0.3 mg/L. After that, preoxidation with NaOCl, ClO2 and KMnO4 oxidants were conducted at various oxidant dosage and mixing time. After oxidation, algae viability was determined by flow cytometer, and residual algae and soluble manganese (i.e., Mn ions and complexed Mn) were analyzed. The addition of various oxidants in preoxidation process was carried out to evaluate the removal in algae and manganese through coagulation/sedimentation process. The results showed that NaOCl and ClO2 oxidation lower the viability of algae cells due to the occurrence of intact cells. However, KMnO4 oxidation is not effectively to reduce the viability of algae cells, and extracellular organic matter (EOM) releases from cells. ClO2 oxidation has a higher rate constants (k_(t=30s)) in lowering the cell viability more than NaOCl and KMnO4 oxidation. In the presence of soluble manganese, NaOCl oxidation is effectively to reduce the viability of algae cells instead of oxidizing Mn ions and complexed Mn. The majority of particulate manganese was formed with KMnO4 oxidation, accompanying the formation of Mn ions. ClO2 oxidation is effectively to simultaneously deform algae and oxidize Mn ions and complexed Mn. For alum coagulation, the optimum algae destabilization can be achieved when KMnO4 preoxidation is performed. However, ClO2 preoxidation has a better ability to improve the removal in algae and manganese by coagulation/sedimentation.