Evaluation of coagulants for turbidity removal from raw water

• Main Authors: Jun-hong Li, 李俊泓
• Other Authors: Jie-Chung Lou
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/2ysct6

碩士 === 國立中山大學 === 環境工程研究所 === 101 === The turbidity of raw water is caused of fine particles which are including sediments, clay, microorganism, and organic matters. In high turbidity levels the operational loading of water treatment plants was found. Particularly during the season of heavy raining and storm, the turbidity level in raw water can be further increased up to more than thousands of NTU. With an extended period under this circumstance, the source water with high turbidity becomes difficultly to treat. This result forced water treatment plants reducing or temporarily stopping their water supplies, and caused inconvenience to people’s living and economic losses. The objective of this research is to study the characteristics of sediments and their settling performance in Kao-Ping river. Also, the correlation between the turbidity level and concentration of suspended solid in raw water as well as the effect on removal efficiency of high turbidity from raw water using three different coagulant are discussed. Using analysis of the particle size distribution from the sediment of river showed the results of particle sizes smaller than 0.044 mm was account for 45% of the sediments in Kao-Ping River. In preparing the raw water with high turbidity contents, we were to understand the effect of various particle size on the level of turbidity in artificial raw water. The 45% of sediments were further divided into three distinct groups(0.074-0.044 mm, 0.044-0.034 mm, and smaller than 0.034 mm), and were used to prepare three kinds of artificial raw water containing 1000 mg/L of suspended solid stirring for two hours for measuring the turbidity level. The results showed that the turbidities were 201, 1210 and1330 NTU(noted the larger, middle-range, and fine particles) respectively. The preparing turbidity of the large particles (0.074-0.044 mm)was higher than another two group particles up to six times difference. From the results using the analysis of particle size distributions and the prepare of turbidity tests, the sediments that consist of particles smaller than 0.044 mm is selected by us in following tests in this work. The result of correlation between the turbidity and suspended solid concentrations showed the standard deviations increased with increasing the concentration of suspended solid. More importantly, the standard deviations corresponding to different suspended solids concentrations were consistently below 10%. The function of correlation is obstained as a linear equation of y=0.7x-228.53 with a R² value of 0.99. Where y is turbidity level and x is concentration of suspended solid. This equation shows a high correlation between the turbidity and suspended solid concentration and was used to prepare artificial raw water with high turbidities. By using this equation that the turbidity is about 474 NTU when 1000 mg/L of suspended solids is added in water sample in all test. As to compare the three different coagulants and dosages in removal of turbidity in artificial raw water, the optimal dosage of PAC is ranging from 2 to 6 mg/L, and the high turbidity in raw water were treated under 30 NTU for a settling time of 20 minutes. The optimal dosage of Al2(SO4)3 was ranging from 2 to 6 mg/L, and the residual turbidity of all artificial raw water as found lowered than 20 NTU when settling time was at 30 minutes. We found the alkalinity depletion at the optimal dosage of coagulants, using Al2(SO4)3 consumed slightly more alkalinity as well as lower pH than PAC did. The optimal dosage of FeCl3 was ranging from 6 to 10 mg/L, results showed all higher than dosage of Al2(SO4)3 and PAC.