Effect of pH on humic acid removal performance in coagulation–ultrafiltration process and the subsequent effects on chlorine decay

The effect of pH on coagulation performance and floc characteristics with respect to the treatment of humic acid (HA) solution using polyferric chloride (PFC) as coagulant was investigated in this study. Ultrafiltration was used after coagulation to further remove organics under three pH conditions. The subsequent effects on chlorine decay were studied using a first decay model and AQUASIM modeling software. The results indicated that PFC had different coagulation mechanisms in different pH regions. Charge neutralization was the dominant mechanism in acidic region, while sweep was the predominant mechanism at pH >6.0. HA was effectively removed in acidic region and turbidity removal efficiency was higher in neutral and alkaline regions. The median equivalent volumetric diameter (d50) of flocs formed in coagulation process in acidic region was larger than that of flocs formed at pH 9.0. The flocs formed at pH 5.0 were more compact because larger floc fractal dimension (Df) was observed. Floc breakage behavior showed that flocs formed at different pH values gave different floc strength and recovery ability, and the strength and recovery factors of the flocs varied in the following order: pH 5.0 >4.0 >9.0. The treated water at pH 5.0 possibly contained the minimal amount of disinfection by-products (DBPs) precursors due to the minimal reacting compounds present in the treated water. DBPs precursors were probably prone to be removed by coagulation process during which flocs with larger strength factor, recovery factor and Df were formed.

Humic acid solution was treated by coagulation–ultrafiltration–desinfection combined process. Effect of pH on coagulation performance and floc characteristics was investigated. Relationship between floc characteristics and removal of disinfection by-products was analyzed.Figure optionsDownload as PowerPoint slideHighlights
► Humic acid solution was treated by coagulation–ultrafiltration combined process.
► Waters treated were disinfected using NaClO as disinfectant.
► Floc characteristics were monitored using Mastersizer 2000.
► Effect of pH on coagulation performance and floc characteristics was investigated.
► Effect of floc characteristics on removal of disinfection by-products was analyzed.