| Summary: | 碩士 === 國立成功大學 === 環境工程學系碩博士班 === 94 === In Taiwan, over 65% of source water for public water supply is from reservoir. Owing to eutrophication, the growth of algae in the reservoir is quite common. Algae can cause water quality problems such as taste and odor and/or toxicity. During water treatment, if algae are not removed by coagulation and sedimentation, some species may clog filter. While other species can penetrate filter media, and cause water quality deterioration in distribution system.
In this research, pure-culture of green algae (Chodatella sp.) and diatom (Navicula sp.) were used to prepare algal suspensions. Both alum and polyelectrolyte were used as coagulant. Polyelectrolyte was also used as coagulant-aid, while alum was the primary coagulant. The effect of preoxidation with ozone and potassium permanganate on coagulation removal of algae was also studied. The algal floc size distribution after various mixing conditions were measured with particle counter, and the data used to look into floc strength.
The results show that preoxidation with ozone or potassium permanganate improved coagulation removal of green algae with either alum or polyelectrolyte as primary coagulant. However, ozone worked better when alum was used, and potassium permanganate was the choice of preoxidant when polyelectrolyte was the primary coagulant.
For diatom, ozone preoxidation had detrimental effect on alum coagulation. It is speculated that this may be due to the high oxidation power of ozone, which could oxidize those extracellular polymeric substances(EPS) of algal cells. EPS was said to be helpful to cell aggregation. Potassium permanganate preoxidation was found to improve the diatom coagulation by alum. The results also indicate that if alum doses were high enough, diatom removal rate could be higher than 90%, no matter if preoxidation was practiced or not. When polyelectrolyte was used as primary coagulant, potassium permanganate preoxidation still was beneficial to diatom coagulation removal, and preozonation also had bad effect.
When polyelectrolyte was used as coagulation-aid to alum, the cell removal rates of both green algae and diatom were improved. However, it was more significant for green algae than that of diatom. It could significantly reduce the required alum dosage.
As far as floc characteristics is concerned, the floc formed with preoxidation were more loosely packed, with lower density and settling velocity. Preoxidation also reduced floc strength, especially with potassium permanganate. When polyelectrolyte was used as coagulant-aid to alum, it generally formed larger floc with lower density and lower settling velocity. However, its effect on floc strength was mixed.
Based on the results from this study, it can be said that the strategy for algae removal from source water should be based on the dominant algae species. For example: when green algae dominate, either ozone used as preoxidant, or potassium permanganate couples with cationic polyelectrolyte can be adopted to enhance alum coagulatioin. However, preozonation should be avoid if diatom is the dominant species.
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