标题: 聚矽酸铁混凝剂之铁物种型态对藻类混凝去稳定之影响
Effect of Fe speciation of poly-silicate iron coagulant on algae destabilization during coagulation
作者: 陈念妤
Chen, Nian-Yu
黄志彬
Huang, Chih-Pin
环境工程系所
关键字: 混凝;聚矽酸铁;聚铁;藻类;电荷密度;coagulation;poly-silicate iron;charge density;algae;Fe speciation
公开日期: 2011
摘要: 低浊含藻水常出现于以水库为主要水源之净水场原水中,由于藻类比重低而无法有效的藉由混沉程序移除,致加重净水场过滤单元操作负荷甚至阻塞无法出水。虽然净水场应用聚矽酸铁混凝剂(poly-silicate iron, PSI)可提升低浊含藻原水混沉除藻及除浊成效,但PSI混凝剂中铁型态物种分布及其表面电荷密度会因制备条件不同而产生差异,进而影响PSI之混凝成效。因此,PSI混凝剂制备条件将是PSI提升混沉除浊及除藻效能之关键。
本研究首先调查不同季节时宝山净水场及宝山水库含藻原水之水质,接着以宝山净水场原水配制特定藻类浓度水样进行前氧化搭配混凝程序实验,再以两种合成方法(包括复合合成法及共同聚合法)制备不同矽铁比及熟化时间之PSI混凝剂,并与另一种商用PSI比较各自之混沉除浊及除藻效能。研究结果显示,宝山净水场之藻数及叶绿素 a在冬季时最低,但两者均在夏季时急遽升高,优势藻种也随季节变化,宝山净水场原水各季节均存在之优势藻种以小环藻为主,春夏两季曾有大量舟形藻及卵囊藻出现于净水场原水中。
在以相同合成方法下制备PSI混凝剂时,矽铁比会严重影响PSI之物种分布及电荷密度,其中高矽铁比(Si/Fe=1)之PSI中聚铁含量均高于低矽铁比(Si/Fe=0.25)之PSI,而低矽铁比之PSI之铁物种表面电荷密度大于高矽铁比之PSI。此外,以复合合成法制备PSI混凝剂时,熟化程序亦会对PSI之铁型态物种分布及其表面电荷密度造成影响,熟化24小时会使PSI混凝剂表面电荷密度减少。在无前加氯条件下混凝时,商用PSI之混沉除藻效能优于自制之PSI,其中以复合合成法自制之PSI因电性中和能力高于以共同聚合法自制之PSI,故混凝形成较小且结构紧密之胶羽。另一方面,前加氯程序能有效提升未熟化之自制PSI的混沉除藻效能,而已熟化之自制PSI混凝剂因熟化使分子增大,导致混凝时形成结构较松散之胶羽。相较之下,未熟化之PSI与商用PSI因混凝所形成之胶羽结构差异不大,因此两者之混沉效能相当。
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.
URI: http://140.113.39.130/cdrfb3/record/nctu/#GT079919520
http://hdl.handle.net/11536/49686
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