The Effects of Potassium Permanganate and Ozone Preoxidation for Algae Removal by Coagulation
| Autor: | Jen-Jeng Chen, 陳振正 |
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| Rok vydání: | 2005 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 93 Reservoirs are the main source of drinking water in Taiwan. However, most reservoirs are eutrophic because of pollution by domestic sewage, farming and industrial wastewater. A continuing problem for Taiwan’s water treatment plant is the presence of algae in source water. Algae in drinking water supply can cause significant problems, such as tastes and odours, production of disinfection by-products (DBPs), obstruction to coagulation, and clogging of filter. Algae removal by conventional treatment is more difficult than removal of inorganic particle, due to their low specific density, motility, morphological characteristics, and negative surface charge. Pretreatment with oxidants are commonly used in drinking water treatment process to enhance algae removal. Numerous studies have showed that preoxidants such as ozone, chlorine dioxide, chlorine, or permanganate can improve algae removal by coagulation and filtration processes. However, prechlorination may increase the alum dosage required for algae coagulation, and there are also problems of trihalomethanes (THMs) and other chlorinated organics. In this study, pilot and laboratory scale experiments were conducted to understand the effects of ozone and permanganate preoxidation on enhancing the coagulation of algae. Initial efforts were focused on comparing the effects on algae coagulation in pilot scale test. The observations in the pilot plant were further verified in the laboratory, and special attention was then paid to understand (1) the change and damage of algal cell surface by preoxidation, (2) the interactions between the algal cell and the preoxidants, and (3) the effect of preoxidation and calcium on algae coagulation. Initial results from pilot scale tests showed that both ozone and permanganate were capable of enhancing the coagulation removal of algae. Both preoxidants had an optimal dosage for algae coagulation. Although ozone preoxidation is beneficial to THMFP and A254 removal, the NPDOC in water was increased with increasing ozone dosages. This released NPDOC could inhibit the coagulation removal of algae and particulate. In contract, no significant NPDOC release was observed for permanganate preoxidation. Permanganate preoxidation was found to enhance the coagulation removal of turbidity and THMFP. On the parallel comparison in pilot test, permanganate preoxidation had a better performance than ozone in algae coagulation removal. Results from laboratory experiments showed that ozone was prone to cause the release of extracellular organic matter (EOM) into the bulk water and alter the cell wall structure. These may have detrimental effect on algae flocculation. While permanganate stressed cells to secrete EOM, which mainly remained on cell surface, and could facilitate the inclusion of MnO2 into algae floc, and improve its removal. In the presence of calcium, algae flocculation after permanganate preoxidation was more significant than the absence of calcium. However, algae flocculation was not enhanced in the presence of calcium after ozone preoxidation. This may be explained by the function of Ca2+ as a bridge between MnO2 and EOM. Generally speaking, permanganate preoxidation had a better performance than ozone on algae removal, similar to the finding of pilot scale tests. Based on the overall observations from this study, it is proposed that the effect of ozone preoxidation on algae coagulation is through cell surface charge neutralization, while permanagante is via EOM adsorption and bridging mechanism. |
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