Optimal Use of Chemicals and Filter Media for the Treatment of Low Turbidity Water with Problem Algae
響，並與Alum 作比較。此外，將評估前氧化結合PSI 及Alum 混沉程序對問題藻類去除之效
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. While a hybrid pre-oxidation/coagulation-sedimentation process is commonly adopted to destabilize and remove the algae in raw water, it is difficult to effectively remove strip or needle algae before the sand filtration process in the WTP because the setting rate of these algae is rather poor. As a result, the filter is easily clogged with these algae, which significantly impedes its operation and causes an increased frequency of backwashing. Meanwhile, microbial hyperplasia may occur in the filter because microorganisms within the filter can feed on the algae cells, resulting in an increase in the quantity of bacteria and biofilms. To prevent the algae from flowing into the filter, the traditional coagulation and filtration technology needs to be improved. Therefore, this study aims to improve the coagulation of algae through preparing the polysilicate iron (PSI) coagulant, a new coagulant approved by the EPA. In addition, the effects of different pre-oxidants of various dosages on algae removal using the coagulation-sedimentation process will be investigated. Furthermore, the differences in the operation of filters equipped with anti-microbial and traditional filter media in the presence of algae will be evaluated. In this study, various problem algae in raw water during different seasons will be collected on-site and identified using a high-resolution optical microscope. The PSI coagulants will be prepared in the lab, which will be used to evaluate the effects of polyferric species and alum speciation on the coagulation of algae-rich raw water, and determine the filtration performance of the supernatants after coagulation-sedimentation. The removal efficiency of the algae by PSI and alum coagulation coupled with a pre-oxidation process will be determined by counting residual algae after coagulation. Then pilot-scale filtration columns equipped with anti-microbial filter media (i.e., manganese sand) and traditional filter media (i.e., quartz sand) will be simultaneously operated to filtrate the effluents of sedimentation unit of WTP, followed by filtration and backwashing experiments of both media. The results of this study will help water utilities to establish optimization strategies for coagulation and filtration operation to treat low turbidity water with abundant algae.
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