聚矽酸鐵混凝劑之鐵物種型態對藻類混凝去穩定之影響

Abstract

低濁含藻水常出現於以水庫為主要水源之淨水場原水中，由於藻類比重低而無法有效的藉由混沉程序移除，致加重淨水場過濾單元操作負荷甚至阻塞無法出水。雖然淨水場應用聚矽酸鐵混凝劑(poly-silicate iron, PSI)可提昇低濁含藻原水混沉除藻及除濁成效，但PSI混凝劑中鐵型態物種分佈及其表面電荷密度會因製備條件不同而產生差異，進而影響PSI之混凝成效。因此，PSI混凝劑製備條件將是PSI提昇混沉除濁及除藻效能之關鍵。 本研究首先調查不同季節時寶山淨水場及寶山水庫含藻原水之水質，接著以寶山淨水場原水配製特定藻類濃度水樣進行前氧化搭配混凝程序實驗，再以兩種合成方法(包括複合合成法及共同聚合法)製備不同矽鐵比及熟化時間之PSI混凝劑，並與另一種商用PSI比較各自之混沉除濁及除藻效能。研究結果顯示，寶山淨水場之藻數及葉綠素 a在冬季時最低，但兩者均在夏季時急遽升高，優勢藻種也隨季節變化，寶山淨水場原水各季節均存在之優勢藻種以小環藻為主，春夏兩季曾有大量舟形藻及卵囊藻出現於淨水場原水中。 在以相同合成方法下製備PSI混凝劑時，矽鐵比會嚴重影響PSI之物種分佈及電荷密度，其中高矽鐵比(Si/Fe=1)之PSI中聚鐵含量均高於低矽鐵比(Si/Fe=0.25)之PSI，而低矽鐵比之PSI之鐵物種表面電荷密度大於高矽鐵比之PSI。此外，以複合合成法製備PSI混凝劑時，熟化程序亦會對PSI之鐵型態物種分佈及其表面電荷密度造成影響，熟化24小時會使PSI混凝劑表面電荷密度減少。在無前加氯條件下混凝時，商用PSI之混沉除藻效能優於自製之PSI，其中以複合合成法自製之PSI因電性中和能力高於以共同聚合法自製之PSI，故混凝形成較小且結構緊密之膠羽。另一方面，前加氯程序能有效提升未熟化之自製PSI的混沉除藻效能，而已熟化之自製PSI混凝劑因熟化使分子增大，導致混凝時形成結構較鬆散之膠羽。相較之下，未熟化之PSI與商用PSI因混凝所形成之膠羽結構差異不大，因此兩者之混沉效能相當。

Low turbidity along with abundant algae is frequently found in raw water taken from reservoirs by water treatment plants (WTPs). Because the density of the algae is too low to be effectively removed from the water by coagulation-sedimentation, the burden on the operation of the filtration unit in the WTP is significantly increased. In addition, the algae may clog the filter and slow down or stop the filtration. Although the performance of turbidity and algae removal can be effectively improved by polysilicate iron (PSI) coagulation together with sedimentation, the Fe species distribution and charge density of PSI coagulant varies with synthesis method, which strongly affect the coagulation performance of PSI. Therefore, the synthesis method of PSI is a principal factor to improve the removal performance in turbidity and algae by coagulation-sedimentation. In this study, the quality of water and algae species of algae-containing water with low turbidity from Bao-San WTP in various seasons were investigated first. Then the preparation of PSI coagulants with various Si/Fe ratio and aged time were conducted by using two synthesis methods (i.e., composite algorithm and copolymerization). The removal performance in turbidity and algae through coagulation/sedimentation with two kinds of lab-synthesis PSI coagulants was evaluated to compare with that of one commercial PSI coagulant. The result has showed that the lowest algae count and chlorophyll a was found in the algae-containing water in winter, but they sharply increase in summer. The dominant algae species in the raw water is Cyclotella in four seasons, but large amounts of Oocystis and Navicula present in the raw water during spring and summer, respectively. In PSI preparation, the Si/Fe ratio is a significant factor affecting the Fe speciation, and its charge density in the preparation of PSI. It has been found that the quantity of polymeric Fe in PSI coagulant with high Si/Fe ratio (Si/Fe=1) was higher than that in PSI coagulant with low Si/Fe ratio (Si/Fe=0.25), while the charge density of Fe speciation in low Si/Fe ratio PSI coagulant was larger than that in high Si/Fe ratio PSI coagulant. Moreover, the aged time of PSI coagulant significantly affect the distribution of Fe species and their charge density. The charge density of Fe species decreases after PSI coagulant aged 24 hours. Without prechlorination, the removal performance in algae through coagulation-sedimentation by commercial PSI coagulant is superior to that by lab-synthesis PSI coagulant. Furthermore, the charge neutralization ability of PSI synthesized by composite algorithm is stronger than that by copolymerization, resulting in the occurrence of smaller flocs with dense structure during coagulation. On the other hand, with pre-chlorination, the performance of coagulation/sedmentation with non-aged PSI synthesized by composite algorithm can be effectively improved. Because the molecular size of PSI coagulant becomes larger after aging, the aged PSI with interparicle bridging ability can cause the formation of larger flocs with loose structure during coagulation. By contrast, non-aged PSI has a similar removal performance in algae to commercial PSI in coagulation/sedimentation because their floc structure is similar during coagulation.