快混強度對天然濁水混凝效能之影響

Abstract

混凝為淨水場去除濁度顆粒之主要處理單元，其中尤以快混單元最為重要，快混效能之優劣將決定顆粒去穩程度及膠羽結構特性，進而影響後續沉澱及過濾單元之效率。本研究目的在於探討快混強度(G值)對天然濁水混凝效能之影響，首先，分別採取低濁度及高濁度原水，改變不同快混G值(150 ~ 1000 s^-1)進行瓶杯試驗，以評估快混G值對濁度及溶解性有機物(DOC)去除效率之影響，並且在實場架設一管中快混設備產生遠大於現場快混操作強度之G值(>5000 s^-1)，以探討高速G值對後續混沉過濾效能之成效，實場試驗中，採取管中快混後處理水及現場快混後處理水，同時利用瓶杯試驗機在一相同條件下慢混並在混沉結束後量測上澄液殘餘濁度及溶解性有機物。 研究結果顯示，低濁度及高濁度原水混凝在最適加藥量下，濁度去除有一最適Gt值範圍(9×10^4 ~ 1.2×10^5)，當快混時間短，混沉後濁度去除率及上澄液過濾性隨快混G值增加而提升，反之，當快混時間長則混沉後濁度去除率及上澄液過濾性隨快混G值增加而下降。在DOC混凝部分，混沉後上澄液殘餘DOC首先會隨快混G值增加而下降至最低，接著再隨快混G值增加而上升。另外，快混G值也會影響到混凝膠羽之結構特性，增加快混強度可增強混凝膠羽強度，且混凝膠羽形成之機制決定破碎膠羽回復率。電性中和機制下形成之膠羽強度遠弱於沉澱掃除下所形成之膠羽。隨著快混強度之增加，由電性中和作用形成之膠羽在破碎後之膠羽回復率亦會增加，但由沉澱掃除作用形成之膠羽在破碎後之回復率卻與快混強度無關。另一方面，淨水場原水經管柱高速G值快混處理後，其處理水混沉後相較於現場快混單元可明顯提升濁度去除及上澄液過濾性，但對DOC去除之助益並不明顯。

關鍵詞：水處理、混凝、快混、攪拌強度、管中快混

Coagulation process is the most widely used to remove particle for water treatment plant (WTP). The operation of rapid-mixing in coagulation dominates the performance of particles destabilization, and affects the properties of floc, which significantly influence subsequent solid-liquid separation such as sedimentation and filtration. This study aims to investigate the effect of rapid-mixing intensity (i.e., velocity gradient (G)) on the performance of coagulation for the treatment of natural turbidity water. First, coagulation was conducted by Jar test in laboratory at various G ranging from 150 to1000 s^-1 to determine the removal efficiency of turbidity and dissolved organic matter (DOC) from two types of raw water. Furthermore, a pilot-scale in-line mixer yielding much higher G (>5000 s^-1) than mechanical mixing of WTP were conducted to investigate the effect of high G mixing on the efficiency of coagulation-sedimentation and filtration. In pilot-plant test, the effluents after rapid-mixing induced by in-line mixer and mechanical mixing of WTP was simultaneously flocculated at a constant slow-mixing using Jar test to determine the residual turbidity and DOC of supernatants after sedimentation. The result showed that the optimal Gt value ranging from 9×10^4 to 1.2×10^5 for turbidity removal in the coagulation of low and high turbidity water at optimal dosage was obtained. At such condition, the turbidity removal and filterability of supernatant after coagulation-sedimentation increased with rapid-mixing intensity under a shorter rapid-mixing, but decreased with mixing-intensity under a longer rapid-mixing. For DOC coagulation, the residual DOC is first decreased to minimum and then increased with increasing rapid-mixing intensity. Moreover, the floc structure was affected strongly by rapid-mixing intensity during coagulation. The floc strength increased with mixing intensity for the coagulation of low and high turbidity water. Furthermore, the mechanism of floc formation dominates the recovery rate of the broken floc. Floc formed by charge neutralization (CN) is much weaker than that formed by sweep flocculation (SW). The recovery rate of the broken floc increased with the strength of CN floc; however, the recovery rate of the broken floc is insensitive to the strength of SW floc. On the other hand, the efficiency of turbidity removal through mixing with high G generated from in-line mixer is superior to that from mechanical mixing of WTP, in which the filterability of the supernatant after coagulation-sedimentation can be effectively improved using in-line mixer; however, the DOC removal was insensitive to mixing-intensity of coagulation at such condition.

Key words：Water treatment, Coagulation, Rapid-mixing, Mixing Intensity, In-line mixer.