Strategy evaluation of low turbidity water treatment: laboratory and field diagnosis

• Main Authors: Liou, Yi-Fu, 劉奕甫
• Other Authors: Huang, Chih-Pin
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/26488760898503401379

碩士 === 國立交通大學 === 環境工程系所 === 96 === Low turbidity water is hard to treat by traditional coagulation/flocculation process. The concentration of particles in water is too low to cause effective particle collision and aggregation. The situation is worsened when the raw water contains algae and soluble organics, resulting in the overloading of the subsequent treatment and unstable water supply. The aims of this work are to characterize the low turbidity raw water, to on-site diagnose the treatment problem, and to optimize the treatment process. Five water purification plants, namely, Long-Tan water treatment plant, Ban-Shin water treatment plant, Liyuitan water treatment plant, Ping-Jhen water treatment plant, and Bao-San treatment plant, were the object of study including the characterization of the chemical and physical properties of raw water, coagulation efficiencies of various coagulants, and filtration of the coagulation effluent. Long-Tan water treatment plant and Bao-San treatment plant were selected for on-site diagnoses and evaluation. The low turbidity water collected from Long-Tan water treatment plant was found containing either high organics or low organics. Low turbidity water from Ping-Jhen water treatment plant contained only low organics while those from Liyuitan water treatment plant and Bao-San treatment plant contained algae. Among them, the low turbidity water with high organics is the so-called white turbidity water. The turbidity of the raw water for Ban-Shin water treatment plant was relatively higher than the rest. Coagulation with various coagulants, namely, polysilicato-iron (PSI), alum and polyaluminum chloride, and the filtration of the supernatant after sedimentation were performed on low turbidity water with low organics, low turbidity water with high organics and low turbidity water with algae. Pre-coagulation with PSI was super to the other two coagulants in improving the filterability of the water. Up to 50% improvement has been observed. In treating low turbidity water with high organics content, lower dosage of PACl was required to achieve charge neutralization and satisfactory filterability. Optimum particle removal was observed at pH 9 for PACl and pH 7 for Alum. Poor filterability was found with Alum treatment. In treating the relatively higher turbidity water from Ban-Shin water treatment plant, PACl performed the best. Although the polymer improved the filterability of the pre-settled water, the dosage must be kept under 0.1mg/L. The on-site evaluation in the Long-Tan water treatment plant showed increased residual turbidity and poor filtration. It has been attributed to the broken flocs due to the strong shear force on flocs during the transportation from the sedimentation tank to filtration basin. The efficiency of the flash mixer was enhanced after improving the coagulant dosing system. The correlation coefficient between the result from pilot plant study and that from laboratory experiment was 0.93, indicating that the STI (suction time index) could be used as the index of operation. The on-site study in Bao-San water treatment plane discovered that the raw water supply from the Bao-San first reservoir contains great amount of algae, which not only interferes with the operation efficiency of Bao-San water treatment plane but also demands more coagulant.