| Summary: | 碩士 === 國立臺灣科技大學 === 化學工程系 === 91 === The objective of this study was to study the two-stage coagulation-flocculation process in water treatment. Raw water of various turbidities were used in this experiment. Jar tests were carried out to assess optimum conditions for turbidity removal. Results from single-stage and two-stage coagulation-flocculation processes were compared utilizing polyaluminum chloride (PACl) and alum as coagulants. The two-stage coagulation-flocculation process involved three different dosing schemes and different ratio of coagulant dosed in two separate stages. They are scheme I: rapid mixing → settling → rapid mixing → slow mixing; scheme II: rapid mixing → slow mixing → rapid mixing → slow mixing; and scheme III: rapid mixing → rapid mixing → slow mixing. Floc size and structure were investigated to delineate reaction mechanisms.
In sigle-stage, raw water with initial turbidity of 384 NTU needed more than 8mg/L of PACl to attain residual turbidity of 0.12 NTU. In two-stage coagulation-flocculation, 4 mg/L of PACl resulted in residual turbidity of lower than 0.16 NTU. Improved treatment efficiency was also observed when initial turbidity was 26.8 NTU and 1.63 NTU, respectively. The results indicated that two-stage coagulation-flocculation process was more efficient than single-stage.
Flocs formed by PACl in two-stage coagulation-flocculation were larger than those in single-stage. However, flocs formed by alum showed the opposite trend. In scheme III, the result indicated that the 1:4 dosage ratio was better in terms of turbidity removal and fractal dimension of floc. In two-stage coagulation-flocculation process, the first stage’s coagulants will act as nuclei and the second stage’s coagulants will provide more active sites for particle aggregation.
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