| In this study, humic acid (HA) and fulvic acid (FA) were selected as organic matters to investigate the effect of floc characteristics on the removal of disinfection by-products (DBPs) precursors. A series of coagulants were used to treat the test waters containing organics and kaolin, and the treated waters were disinfected using NaCIO as disinfectant. According to the results of coagulation and disinfection, polyferric chloride (PFC) and composite coagulant polyferric chloride (PFC)-polydimethyldiallyammonium chloride (PDMDAAC) were chosen to treat the test waters in the coagulation-ultrafiltration-disinfection process. The mass ratio of Fe2O3 and PDMDAAC in PFC-PDMDAAC was 10:1 in coagulation process of humic acid and 2:1 in coagulation process of fulvic acid. The effects of dosage and pH on the efficiency of coagulation and coagulation-ultrafiltration were investigated and the floc size and fractal dimension were obtained by on-line monitor of coagulation process using Mastersizer2000. The data of chlorine decay were estimated using AQUASIM modeling software to get the concentration of DBPs precursors and the effects of floc size and fractal dimension on the removal of DBPs precursors were studied. The main conclusions are as follows:(1) The dominant mechanism and efficiency of composite coagulants with different mass ratios of Fe2O3 and PDMDAAC were different. In HA test water, the coagulation efficiency of PFC was not improved by adding PDMDAAC and the mechanism of PFC-PDMDAAC was also sweep. In FA test water, PFC-PDMDAAC was more efficient than PFC with smaller dosage, and the charge neutralization of PFC was strengthened by PDMDAAC.(2) In HA teat water, floc characteristics changed in the same potential as dosage or pH increased for PFC and PFC-PDMDAAC. More compact and larger flocs can be obtained by dosing more PFC or PFC-PDMDAAC. The effect of pH on floc fractal dimension was negligible, while floc size changed evidently as pH increased. Larger flocs were formed at acid ragion.(3) In FA test water, floc characteristics changed in the different potentials as dosage or pH increased for PFC and PFC-PDMDAAC. Floc size increased as PFC dosage increase, while the fractal dimension of flocs changed slightly. Fractal dimension and d0.5 both reached the peak values at dosage 5mg/L of PFC-PDMDAAC. Floc size increased as pH increased for both two coagulants and larger flocs were formed by dosing PFC at the same pH condition. More compact flocs were formed at acid region by PFC, while PFC-PDMDAAC formed flocs with large fractal dimension at alkaline region.(4) In HA test water, floc characteristics of PFC and PFC-PDMDAAC had the same effect on the removal of DBPs precursors. Removal of DBPs precursors was not enhanced by the increase of floc size in coagulation process. Removal efficiency of DBPs precursors increased with do.5 of the supernatant flocs increased in ultrafiltration process. DBPs precursors were prone to be removed by coagulation and coagulation-ultrafiltration process during which flocs with larger fractal dimension formed.(5) In FA test water, floc characteristics of PFC and PFC-PDMDAAC had the different effects on the removal of DBPs precursors. Removal of DBPs precursors was slightly affected by the increase floc size in PFC coagulation process, while the increase of floc size in PFC-PDMDAAC coagulation process could enhance the removal of DBPs precursors. The removal efficiency DBPs precursors decreased with d0.5 of the supernatant flocs increased in PFC coagulation-ultrafiltration process and increased with d0.5 of the supernatant flocs increased in PFC-PDMDAAC coagulation-ultrafiltration process.(6) The effect of ultrafiltration on the formation potential of DBPs was different in HA test water and FA test water. DBPs precursors could be effectively removed by ultrafiltration membrane in HA coagulation-ultrafiltration process with PFC as coagulant. When PFC-PDMDAAC was used to treat HA test water, DBPs precursors could be effectively removed by ultrafiltration membrane with dosage 21-30mg/L. Ultrafiltration process enhanced the formation potential of DBPs in pH 5.0-7.0 with PFC-PDMDAAC dosage 12mg/L. For FA test water, the formation potential of DBPs was increased by ultrafiltration for both PFC and PFC-PDMDAAC. |
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