Effect of Coagulant Type on the Coagulation of Low-Turbidity Source Water
| Autor: | Jia-Hong Liou, 劉嘉宏 |
|---|---|
| Rok vydání: | 2007 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 95 The treatment process of Wu San Tou Water Works is the conventional coagulation, sedimentation and filtration process. Its raw water is characterized by low turbidity (< 10 NTU) and high pH value (about 8). Alum was used as the coagulant, and its dosage was reduced with the decreasing of the turbidity of the raw water. However, under this operation strategy, field sampling found that sometimes the turbidity of effluent from sedimentation basin was higher that of the influent. Other problem encountered by the plant was the accumulation of sludge mat on the surface of the inclined plate in the sedimentation basin. When the thickness of sludge mat was excessive, the sludge may be carried by the water flow into the following sand filter, and causing premature turbidity breakthrough. As this happened, the operation needs to be interrupted, and the basin cleaned manually. Therefore, it reduced productivity and also was quite a burden to the operators. The research is aimed at solving the above-mentioned problems by both field investigation and lab-scale studies. The latter is mainly to compare the suitability of aluminum and iron salts as coagulants for Wu San Tou raw water. Based on the results from field investigation, it is realized that the problems encountered by the treatment plant was due to the lower than optimum alum dosage used by the plant, when the raw water turbidity was low. The insufficient alum dosage coupled with high pH value of the raw water prevented the formation of aluminum hydroxide floc. This also means that, under this condition, most of the alum added existed as soluble Al species in the water. Therefore, the coagulation performance were poor. In the lab-scale studies, the effect of aluminum sulfate and ferric chloride on the coagulation of both the raw water from Wu San Tou and artificial raw water, made up mainly from kaolin clay particles, were investigated. The results show that the floc generated from iron salt have higher density and higher fractal dimension than those from aluminum salt. This means iron floc is heavier and structurally more compact than aluminum floc. Floc from Wu San Tou raw water generally have lower density, fractal dimension and settling velocity than those from artificial raw water. This probably is due to the more complex constituents in the nature raw water, which contains algae cell and NOM, besides inorganic particles. Further, no matter which coagulants were used, those with sweep coagulation mechanism have higher turbidity removal than those with adsorption - destabilization mechanism. For Wu San Tou raw water, iron salt coagulant required lower dosage than that of aluminum salt. Under same coagulant dosage, the treated water from iron salt also have lower residue turbidity than those from aluminum salt. Further, the residue color problem from iron salt can be avoided, if the dosage and pH value are located in the region appropriate for solid ferric hydroxide formation. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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