低濁原水處理策略評估：實驗室及現場診斷

Abstract

低濁原水因顆粒濃度較低，混凝過程中顆粒不易碰撞而聚集，常導致混沉效能不佳。此外，當低濁原水中含有藻類或溶解性有機物時，會增加混沉處理之難度，造成後續固液分離單元之負荷。因此，本研究主要評估國內各淨水場遭遇低濁原水時之最適處理策略。 本研究收集了龍潭淨水場、板新給水廠、平鎮給水廠及寶山給水廠之低濁原水。先以實驗室批次試驗評估不同混凝劑種類對各低濁原水之混沉效能及混沉後上澄液之過濾性，並比較低濁原水與較高濁度原水之處理策略。在完成實驗室評估試驗後，分別對出現低濁低有機物原水及低濁含藻原水之淨水場(龍潭淨水場及寶山給水廠)現場診斷，以評估淨水場遭遇低濁原水之現場處理策略。 低濁原水水質及顆粒物化分析結果顯示，淨水場遭遇之低濁原水可分為低濁低有機物、低濁含藻及低濁高有機物原水(即白濁水)。實驗室混沉及過濾性試驗結果顯示，使用聚矽酸鐵混沉處理低濁低有機物、低濁高有機物及低濁含藻原水之效能皆優於硫酸鋁及聚氯化鋁，以致混沉後上澄液之過濾性較佳。以聚氯化鋁混沉處理低濁高有機物原水時，高純度聚氯化鋁混凝劑之電性中和能力較強，因此所需之最適加藥量較低，其混沉後上澄液之過濾性亦較佳。商用低純度聚氯化鋁混凝之最適pH值為中性，而高純度聚氯化鋁混凝之最適pH值在偏鹼性(pH 9)。在處理較高濁度原水時，高純度聚氯化鋁之混沉效能及混沉後上澄液之過濾性皆優於低純度聚氯化鋁及硫酸鋁。另一方面，低純度聚氯化鋁混凝劑搭配少量(0.1 mg/L以下)高分子凝集劑使用有助於各低濁原水混沉後上澄液過濾性之提升。 由龍潭淨水場駐廠診斷結果顯示，現場及實驗室混沉操作後上澄液STI值間之相關性(R2)達0.93。因此，STI可作為遭遇低濁低有機物原水之淨水場現場混凝加藥指標；由寶山給水廠駐廠診斷結果顯示，寶山第一水庫中表層水含有較多藻類，不利於混沉及過濾，且需添加較多之混凝劑方可有效改善沉澱池上澄液之過濾性。

Low turbidity water is hard to treat by traditional coagulation/flocculation process. The concentration of particles in water is too low to cause effective particle collision and aggregation. The situation is worsened when the raw water contains algae and soluble organics, resulting in the overloading of the subsequent treatment and unstable water supply. The aims of this work are to characterize the low turbidity raw water, to on-site diagnose the treatment problem, and to optimize the treatment process.

Five water purification plants, namely, Long-Tan water treatment plant, Ban-Shin water treatment plant, Liyuitan water treatment plant, Ping-Jhen water treatment plant, and Bao-San treatment plant, were the object of study including the characterization of the chemical and physical properties of raw water, coagulation efficiencies of various coagulants, and filtration of the coagulation effluent. Long-Tan water treatment plant and Bao-San treatment plant were selected for on-site diagnoses and evaluation. The low turbidity water collected from Long-Tan water treatment plant was found containing either high organics or low organics. Low turbidity water from Ping-Jhen water treatment plant contained only low organics while those from Liyuitan water treatment plant and Bao-San treatment plant contained algae. Among them, the low turbidity water with high organics is the so-called white turbidity water. The turbidity of the raw water for Ban-Shin water treatment plant was relatively higher than the rest.

Coagulation with various coagulants, namely, polysilicato-iron (PSI), alum and polyaluminum chloride, and the filtration of the supernatant after sedimentation were performed on low turbidity water with low organics, low turbidity water with high organics and low turbidity water with algae. Pre-coagulation with PSI was super to the other two coagulants in improving the filterability of the water. Up to 50% improvement has been observed. In treating low turbidity water with high organics content, lower dosage of PACl was required to achieve charge neutralization and satisfactory filterability. Optimum particle removal was observed at pH 9 for PACl and pH 7 for Alum. Poor filterability was found with Alum treatment. In treating the relatively higher turbidity water from Ban-Shin water treatment plant, PACl performed the best. Although the polymer improved the filterability of the pre-settled water, the dosage must be kept under 0.1mg/L. The on-site evaluation in the Long-Tan water treatment plant showed increased residual turbidity and poor filtration. It has been attributed to the broken flocs due to the strong shear force on flocs during the transportation from the sedimentation tank to filtration basin. The efficiency of the flash mixer was enhanced after improving the coagulant dosing system. The correlation coefficient between the result from pilot plant study and that from laboratory experiment was 0.93, indicating that the STI (suction time index) could be used as the index of operation. The on-site study in Bao-San water treatment plane discovered that the raw water supply from the Bao-San first reservoir contains great amount of algae, which not only interferes with the operation efficiency of Bao-San water treatment plane but also demands more coagulant.