| Summary: | 碩士 === 國立交通大學 === 環境工程系所 === 100 === 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.
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