混沉程序中聚氯化鋁混凝劑鋁物種分佈對濁度顆粒移除及水中殘餘鋁型態之影響

Abstract

聚氯化鋁(PACl)作為淨水場混沉程序中普遍使用之混凝劑，其鋁物種分佈對於混沉除濁效能及上澄液殘餘鋁型態會產生影響。本研究之目的為探討PACl 混凝劑鋁物種分佈對濁度顆粒去穩定及水中殘餘鋁組成和型態分佈之影響。研究中選取三種單體鋁Ala、聚合鋁Alb、膠體鋁Alc 比例之PACl混凝劑，分別命名為PACl-1 (Ala/Alb/Alc=14/64/22)，PACl-2(Ala/Alb/Alc=47/31/22)，PACl-3 (Ala/Alb/Alc =56/22/22)。通過對以上三種混凝劑選擇性搭配FeCl3混凝劑進行高嶺土水樣瓶杯實驗，探討PACl混凝劑水解物種與濁度顆粒之間的相互作用。 研究結果顯示，高聚合鋁/單體鋁比例之聚氯化鋁混凝劑電性中和作用明顯，在最適加藥量(2.0 mg/L as Al)下具有最佳除濁及降低殘餘鋁效果，搭配FeCl3混凝劑之雙重加藥下亦然，且生成膠羽粒徑更大。此時PACl-1混凝劑生成之膠羽粒徑與碎形維度比PACl-2及PACl-3生成膠羽小。混沉上澄液殘餘溶解鋁量隨混凝劑聚合鋁/單體鋁比例的降低而升高。隨著加藥量的增加，PACl-2及PACl-3混凝劑混沉上澄液殘餘鋁中，單體鋁比例逐漸降低，膠體鋁比例逐漸升高；PACl混凝劑中聚合鋁/單體鋁(Alb/Ala)比例越高，混沉上澄液殘餘聚合鋁(Alb)之比例越高。

Polyaluminum chloride (PACl) is one of most frequently used coagulant for particle destabilization in water treatment. The speciation of PACl coagulant strongly affects the coagulation performance and residual Al species in the supernatant. The aim of this study was to investigate the effects of hydrolyzed Al species on the removal of particles and speciation of residual Al after sedimentation. Three kinds of PACl coagulants with different fraction of monomeric Al (Ala), polymeric Al (Alb) and colloidal Al (Alc), namely PACl-1 (Ala/Alb/Alc=14/64/22), PACl-2 (Ala/Alb/Alc=47/31/22) and PACl-3 (Ala/Alb/Alc=56/22/22), were used in jar testing to evaluate the performance of coagulation with and without FeCl3 dosing. For coagulation with dual dosing, the ratio between PACl and FeCl3 dosing is 2:1. The composition and speciation of residual Al in the supernatant after sedimentation for coagulation with various PACl coagulants were determined with and without dual dosing. The result has shown that PACl coagulant with high ratio of Alb/Ala possesses strong charge neutralization ability to effectively destabilize particles at the optimum dosage of 2 mg/L as Al, which lowers total residual Al as well as dissolved Al in the supernatant, even though dual coagulation (PACl dosing mixed with FeCl3 coagulant). At such conditions, the average size and fractal dimension of flocs formed by mixing with PACl coagulants with high ratio of Alb/Ala are much smaller compared to PACl coagulants with lower ratio. The dissolved residual Al in the supernatant increases with Ala/Alb ratio of PACl coagulants regardless of dosage. With increasing PACl dosage, the quantity of Ala decreases and Alc increases in the supernatant after PACl-2 and PACl-3 coagulation, while Ala increases and Alc decreases after PACl-1 coagulation. The quantity of Alb in the supernatant increases with Alb/Ala of PACl coagulant.