| Three composite coagulants polyaluminum chloride (PAC), polyaluminum ferric chloride-polydimethyldiallylammo-nium chloride (PFAC-PDMDAAC) and polysilic aluminum ferric chloride (PFASiC) were used to remove disinfection byproduct precursors (DBP) in a seasonal variation reservoir water. The influence of basicity and composition ratio on turbidity and organic matter removal was investigated to optimize the preparation conditions of composite coagulants. The optimized coagulation conditions were determined by the investigation of influence of pH and hydraulic conditions on coagulation effects. The characteristics of the flocs formed by composite coagulants were investigated, such as floc size, floc growth rate, floc strength and floc recovery capability. Chlorination experiment was also conducted to investigate the formation of DBPs in reservoir water samples, which were treated by different composite flocculants. The effect of coagulant types and enhanced coagulation process on DBPs formation potential was analyzed to prove the control ability of composite coagulant against DBPs formation potential. The results were shown as follows:1. In the treatment of spring reservoir water, the optimum conditions for PFAC were B=0.5, dosage12mg/L, pH=6.00-6.25; and composite ratio4:1, dosage8mg/L, pH=6.75-7.00for PFAC-PDMDAAC. In the treatment of low-temperature and high-algae (summer reservoir) water, the optimum conditions for PFAC were B=1.0or1.5, pH=6.0, dosage8～10mg/L; and dosage6mg/L and pH6.75for PFAC-PDMDAAC. In the treatment of low-temperture low-turbidity water, PFASiC and PFAC-PDMDAAC both obtained best coagulation effect at6mg/L dosage, and PFASiC have highest removal efficiencies of turbidity and organic matters.2. Under the same coagulation conditions, PFAC-PDMDAAC obtained lower residual turbidity and better organic matter removal effect. In comparison to PFAC, the application of PFAC-PDMDAAC could decrease coagulant dosage.3. The primary hydraulic factor of turbidity removal was slow stiring time, while the influence of hydraulic conditions on organics removal was small. The optimum hydraulic conditions of PFAC-PDMDAAC in high algae-laden water treatment process were:rapid mixture60s at200rpm, and slow stirring for25min at40rpm.4. Flocs formed by PFAC-PDMDAAC were larger, and have higher growth rate and better recovery ability than those formed by PFAC. In PFAC-PDMDAAC coagulation system, floc size and growth rate increased as coagulation pH increasing. Highest growth rate was obtained when composite ratio (PFAC/PDMDAAC) was4:1, while floc sizes increased with the composite ratio. 5. The main reactions between natural organic matter (NOM) and disinfectants will finish in24h. The proportion of CCl4and CHBr3in THMs were low in high algae-laden water after disinfection. The primary DBPs were CHCl3, CHBrCl2and CHBr2Cl. Great amount of bromide by-products were generated, and increased with the disinfection period. Coagulation could significantly decrease the formation of DBPs, and PFAC-PDMDAAAC had better effect on DBPs control under same coagulant dosage. Enhanced coagulation combined with adsorption technology was better than individual enhanced coagulation treatment, and activated carbon had better effect on removal of DBSs precursors.6. In the treatment of low-temperature low-turbidity water, almost1/2DBPs were formed, in comparison to summer water, with more chlorination DBPs and less bromide DBPs in proportion. The order of DBPs amount in effluent was:PFASiC> PFAC-PDMDAAC> PFAC. Enhanced coagulation combined with adsorption technology could significant decrease the formation of DBPs in winter water treatment, but the removal efficiency of DBPs precursors was slightly lower than those in summer reservoir water treatment. |
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