預先混凝處理之截留式微過濾薄膜回收淨水場砂濾反洗水之研究:膠羽特性影響

Abstract

在台灣地區淨水場砂濾反沖洗水大多回流至淨水處理單元，常造成一些問題，如加藥量控制不易和產生原蟲及化學物質長期累積，且每當颱風暴雨期間，反沖洗水回流造成水場處理單元負荷過大，為了解決這些問題，本研究將預先混凝反沖洗水再以截流式(Dead-end)微過濾薄膜去除反沖洗水中之懸浮物質，以利過濾水經消毒後可直接作為公共給水；同時探討預先混凝所成的膠羽，其特性對於微過濾薄膜產水率之影響。本研究主要使用新竹第一淨水場反沖洗水，利用0.5 □m之微過濾薄膜，以固定壓力(0.40及0.67 bar)抽真空方式，混凝機制選擇電性補丁、快混電中性點及沉澱掃除三種機制，以及選擇90%、60%及10%Alb含量之混凝劑(PACl-3、PACl-2及PACl-1)進行預先混凝處理。 研究結果顯示，預先混凝反沖洗水後經由MF過濾，可提升薄膜通量，減低薄膜阻塞的程度以及減低濾餅阻抗。至於出水水質，預先混凝過薄膜濁度及SS去除率可達到99%以上，DOC及UV254去除率分別為50∼60%及65∼70%。此外，過膜通量隨著膠羽特性不同而異，當膠羽碎型維度大及膠羽強度強，所形成濾餅阻抗較低且過膜通量較大。不同混凝機制對於薄膜通量提升順序為電性補丁(EPE) ＞電性中和之電中性點(CN) ＞沉澱掃除(SW)，而不同聚合鋁(Alb)含量混凝劑對薄膜通量提升順序為PACl-3＞PACl-2＞PACl-1。對於薄膜阻塞機制，過濾阻抗以濾餅阻抗為主，其佔總阻抗之75∼95%，隨混凝劑及混凝機制不同而異。由結果可知，混凝形成之膠羽特性將會影響薄膜上濾餅的特性，且進一步影響過膜通量及薄膜阻塞行為。

Spent filter backwash water (SFBW) is recycled into previous water treatment units for most water treatment plants (WTP) in Taiwan. The water treatment plants often encounter problems such as difficult control of optimal coagulant dosage and accumulated amounts of protozoa and chemical substances existed in SFBW. In order to solve these problems, the recovery of SFBW was facilitated by MF after the pre-coagulation process. The goal of this study is to investigate the effects of floc properties on membrane flux and the water quality of the filtrate. The SFBW was collected from a local WTP in Taiwan. A bench-scale MF (0.5μm) was set up to simulate a full-scale filtration of the SFBW. The trans-membrane pressure was controlled at 0.4 and 0.67 bar by a suction pump. The pre-cogulation conditions produce different floc properties. The different pre-coagulation mechanisms of electrostatic patch effect (EPE), charge neutralization (CN), and sweep flocculation (SW) were investigated to realize their effects on the formed cake sand PACl coagulants with three different contents of polyaluminum species (Alb) were used to produce different floc properties in this study. The results have showed that the pre-coagulation process can enhance membrane flux and reduce specific cake resistance for multiple filtrations. Ordering membrane flux performance from most effective to least effective was EPE, CN, SW. Regarding floc properties made of coagulants with different content of Alb (PACl-1, PACl-2 and PACl-3), Ordering membrane flux performance from most effective to least effective was PACl-3, PACl-2, PACl-1 The higher membrane flux was cause of the smaller sizes and greater strength flocs which produced more porous cake and lowered specific cake resistance. Regarding the water quality of filtrate, the removals of turbidity, DOC and UV254 were up to 99%, 50% and 65% respectively. As for filtration resistances, the cake resistance (Rc) was the mean portion (＞75%) of total resistance (Rt). The floc properties make influence on cake properties, and then impact performances of membrane flux and fouling.