Feasibility of humic substances removal by enhanced coagulation process in surface water

Aims: This study evaluates the effectiveness of polyaluminium chloride (PACl)as a coagulant for enhanced coagulation (EC) of humic substances (HS) in surface water. Materials and Methods: Dissolved organic carbon (DOC), ultraviolet absorbance (UV ₂₅₄ ), specific ultraviolet absorbance (SUVA), and humic acid (HA) were determined as the HS indices. HS was extracted by Amberlit XAD-7HP from water and HA was analyzed by high performance liquid chromatography (HPLC). The HS index was determined in raw water and after clarification and filtration in all seasons. The efficiency of PACl for removal of HS, both in the jar test and pilot scale were determined. Results: The average extraction efficiency for XAD-7HP was 77%, and the HA concentration was determined in all seasons ranging from 0.15 to 0.29 mg/ L in raw water. The DOC in raw water was in the range of 1.5 to 4mg/L, in all seasons. The average removal efficiency of PACl for EC was 52% for HA in the pilot scale. Also the average removal efficiencies of PACl EC for DOC and UV ₂₅₄ were 43 and 57% on the pilot scale and 38 and 41% in the jar test, respectively. The optimum pH for removal of HS was 7.5 ± 0.2 and the optimum PACl dose was 12 ± 0.5 mg/L. Conclusions: After EC, the aluminum concentration was lower than the maximum concentration level (MCL) of aluminum in drinking water (< 0.05 mg/L). The efficiency of PACl EC was more effective in the high molecular weight fraction of HS and UV ₂₅₄ . In addition, PACl EC was more affected by the increasing coagulant dose than the pH decrease. The removal efficiency of PACl EC at high pH > 8.5 was very poor for the HS index and turbidity.