混凝劑Al型態對高濁水混凝行為之影響

Abstract

台灣河川短小且湍急的地形條件與時常遭逢颱風、豪雨的氣候條件等天然因素，導致大雨夾帶大量泥沙形成高濁水，衝擊著下游的淨水場；此外，現況之淨水場使用的聚氯化鋁(PACl)品質參差不齊，其具有電性中和能力的Al13含量少，造成混凝效果不佳。目前淨水場採以預沉澱處理或加入助凝劑(Polymer)提升去除濁度效果，但有其限制。此外，高Al13含量的聚氯化鋁(EPACl，以電解法製備)已被證實能比一般混凝劑更有效使顆粒電性中和聚集，但是卻沒有深入探討不同濁度原水與混凝劑鋁形態分佈對混凝機制及效果的影響。本研究分析一般聚氯化鋁(PACl)及高純度聚氯化鋁(EPACl，以電解法製備)其鋁型態分布，並討論其在不同濁度原水下之基本混凝特性。接著試探討快混攪拌強度G值對混凝劑之分散程度及對後續混沉效果影響。最後對於後續沉澱後之污泥量測基本性質及脫水性指標，以判斷脫水性的優劣。 無論以天然(450~2,580 NTU)及人工濁水(50~5,300 NTU)進行實驗，由基本混凝特性結果發現EPACl的操作pH值較廣，在鹼性時因異相混凝之作用不會有濁度上揚趨勢。EPACl混凝劑具電性中和能力Alb含量較多，處理高濁水時加藥劑量較少即有良好效果，且較不會消耗鹼度。快混攪拌強度G值對於混凝劑分散程度有正面作用，但混沉效果濁度越高效果越不明顯，PACl因混凝機制關係快混攪拌強度G值對混沉效果更不明顯。兩者所產生之污泥皆是達電中性時脫水性最佳，此外，因為混凝機制形成的膠羽略為不同，其沉降性及脫水性皆以EPACl污泥較好。經由模廠實驗(10,000 NTU)印證，EPACl較低劑量時具有較好的處理效果，其產生之污泥沉降性及脫水性也較佳。

All of the rivers in Taiwan are short with small drainage basins and steep with rapid flows which causes severe erosion. The situation is even worsened by concentrated intensive rainfall during typhoons and storms in summer seasons. The excessive rainfall causes fluctuation in downstream turbidity and occurences of extremely high turbidity of the raw water for the water works. Most water works adopt strategies such as pre-settling and polymer addition for high turbidity removal in addition to the conventional coagulation process with polyaluminum chloride (PACl). Very limited result has been observed. Poor coagulation has also been attributed to the inconsistency in the Al13 content of the PACl used in most water treatment plants. A variety of dissolved aluminum species form when the PACl is added in water, in which the Al13 is the key to charge neutralization and particle-aggregation. However, very limited researches have looked into the individual coagulation behavior of different aluminum species. In this research, the coagulation behavior of the conventional PACl on high turbidity water is compared with that of the electrode-polyaluminum chloride (EPACl). The effects of rapid-mixing intensity on particles aggregation and the subsequent sedimentation as well as the sludge dewatering ability are also examined. The result indicates that the EPACl has a wider range of operational pH in both natural (450~2,580 NTU) and artificial (50~5,300 NTU) raw waters due to the heter-coagulation (charge-neutralization and enmeshment) in the basic range. Because of the higher content of Al13, less dosage of EPACl is required to treat the high turbidity water. Although the G value is directly related to the degree of coagulants dispersion, it does not affect the efficiency of sedimentation on high turbidity water especially for PACl. Optimal dewatering of the sludge occurs at charge neutralization for both coagulants. In addition, because of the different coagulation mechanisms, the EPACl sludge has better settling character and dewatering ability. Pilot study shows that the EPACl has higher turbidity removal at lower coagulant dosage.