表面水處理之問題藻種分析及加藥對其去除之影響

Abstract

低濁高藻原水常出現於以水庫為主要水源之水場原水中，由於藻類比重太低而無法有效的藉由混沉程序去除，因而加重淨水場過濾單元操作負荷，甚至阻塞無法出水。前氧化結合混沉程序常被採用來去穩定且移除水中藻類，但原水中之針狀或條狀藻在淨水場砂濾程序前很難有效被去除，導致這些藻類阻塞於濾池中，阻礙濾池的操作而致反洗頻率增加，因此前處理加藥程序(前氧化及混凝)之最適化為提昇藻類去除效率之關鍵。 本研究調查寶一水庫不同季節之優勢藻種及水質特性與寶山給水場沉澱及過濾單元之問題藻種。此外，以寶一水庫原水配製特定藻類數量水樣進行前氧化搭配混凝程序實驗，探討加氯前氧化(次氯酸鈉及二氧化氯)對藻類細胞特性(藻體結構、葉綠素a與藻類破損程度)之影響，並瞭解前氧化搭配不同混凝劑(聚矽酸鐵及硫酸鋁)對含藻原水之混沉效能及上澄液過濾性之影響。 研究結果顯示，水庫優勢藻種受氣候影響而變化，氣溫較低時，優勢藻種主要為小環藻、舟形藻、沼澤顫藻、針桿藻及盤星藻；氣溫較高時，水中藻類多以小環藻、四角藻及裸藻為主。水場淨水處理單元之問題藻種多為不易被混沉移除且容易堵塞於過濾池之小環藻、盤星藻、舟形藻、沼澤顫藻、針桿藻及直鏈藻等藻類。 在氧化程序中，次氯酸鈉與二氧化氯氧化均能破壞藻類細胞，致使藻類活性降低，提昇藻類之沉降性，因二氧化氯氧化對藻類之破壞性較次氯酸鈉氧化強，故二氧化氯提昇藻類沉降性之成效優於次氯酸鈉。當藻類活性降低時，水中葉綠素a濃度會減少，且藻體細胞完整性變低且受損數量增多。此外，加氯氧化時，因二氧化氯氧化力強而導致加氯後藻類破裂致使芳香族蛋白質大量溶出，且水中天然有機物(類腐植酸及黃酸)之濃度亦降低。在加氯前氧化結合混凝程序上，加氯前氧化可提昇低濁含藻原水之混沉及過濾效能，在硫酸鋁混凝條件下，二氧化氯前氧化對原水混沉效能及上澄液過濾性之提昇成效優於次氯酸鈉前氧化，而搭配二氧化氯或次氯酸鈉前氧化對聚矽酸鐵混凝效能之提昇差異不大。

Low turbidity along with abundant algae is frequently found in raw water taken from reservoirs by water treatment plants (WTPs). Because the density of the algae is too low to be effectively removed from the water by coagulation-sedimentation, the burden on the operation of the filtration unit in the WTP is significantly increased. In addition, the algae may clog the filter and slow down or stop the filtration. While a hybrid pre-oxidation/coagulation-sedimentation process is commonly adopted to destabilize and remove the algae in raw water, it is difficult to effectively remove strip or needle algae before the sand filtration in the WTP because the setting rate of these algae is rather poor. As a result, the filter is easily clogged with these algae, which significantly impedes its operation and causes an increased frequency of backwashing. Therefore, the optimum pretreatment, such as oxidation and coagulation, is the key to improve the removal performance in algae by water treatment plants. In this study, the characteristics of dominant algae and water quality in raw water from Bao-San first reservoir were investigated in different seasons. The species of problematic algae in the sedimentation and filtration process of Bao-San WTP was also investigated. In addition, the raw water taken from Bao-San first reservoir was concentrated to the desirable algal concentration in order to carry out the experiments of pre-oxidation combined with coagulation. The effects of pre-chlorination by NaOCl and ClO2 on the characteristics of algal cell (i.e., algal structure, chlorohyll a and cell integrity) before and after pre-chlorination was studied. It is simultaneously estimated that the effect of pre-chlorination coupled with coagulation by different coagulants (i.e., Alum and Polysilicate iron (PSI) on the performance of coagulation/sedimentation and sand filtration for algae-laden water treatment. The results have shown that the dominant algae of reservoir vary with the seasons. During fall and winter seasons, Cyclotella, Navicula, Oscillatoria, Synedra sp., and Melosira are in full bloom, while Cyclotella, Tetraedron, and Euglenophyta bloom in the spring and summer seasons. The problematic algae species existing in the sand filter include Cyclotella, Pediastum, Navicula, Oscillatoria, Synedra sp., and Euglenophyta. In oxidation, algal cells can be destroyed by NaOCl and ClO2 oxidation as a result of the reduced activity of algae accompanied by the increased settleability. Algal cells have been undergone more strong damage by ClO2 than by NaOCl treatment thereby improving the settleability of algae better. When the activity of algae reduces, the concentration of chlorohyll a was decreased along with the reduced quantity in integrated algal cell and the increased quantity in impaired algal cell. Furthermore, the ratio of various dissolved organic matter (DOC) in raw water was not changed obviously after chlorination with NaOCl and ClO2, while only the increased aromatic protein along with reduced natural organic matter (i.e., humic substance-like and fulvic acid-like) was found after ClO2 oxidation of raw water due to the stronger oxidation ability of ClO2. In pre-chlorination/coagulation of algae-laden water with low turbidity, the performance of coagulation/sedmentation and filtration can be effectively improved by pre-chlorination. For alum coagulation, the improvement in the performance of coagulation/sedimentation and the filterability of supernatant by ClO2 pre-treatment is superior to that by NaOCl pre-treatment. However, for PSI coagulation, NaOCl has the similar performance to ClO2.