FACTORS AFFECTING PARTICLE GROWTH AND RELATED ORGANIC MATTER REMOVAL DURING ALUM COAGULATION (SIZE DISTRIBUTION, TRIHALOMETHANES, HUMIC).

Abstract

Effects of several important source-related and operation-related factors on particle formation and growth as well as potential particle and dissolved organic matter removal by alum coagulation are described. Two representative natural water sources, with low turbidities and high concentrations of dissolved organic matter, and one commercially available crystalline silica, with defined characteristics, were employed to establish initial aquatic particle and dissolved organic matter conditions. Six experimental variables utilized for evaluation include initial pH, initial turbidity, applied pre-ozonation dose, alum dose, flocculation time and sedimentation time. A bench-scale experimental apparatus with capabilities of ozonation, coagulation, sedimentation and membrane filtration was employed to conduct a series of selected experiments. Each factor investigated in this research proves to be able to inpart, individually or collectively, statistically significant effects on particle formation and growth during alum coagulation. While the addition of model particles shows significant enhancement in particle growth, it fails to demonstrate significant improvement in the removal of dissolved organic matter. On the contrary, effects of pH and alum dose on particle formation and growth are accompanied by corresponding effects on the removal of dissolved organic matter. Pre-ozonation of dissolved organic matter renders the dissolved organic matter more hydrophilic by increasing the number of carboxylic acid functional groups. This phenomenon can significantly improve or impede particle growth as well as dissolved organic matter removal during alum coagulation, depending on raw water chemistry and other operational factors. Alum coagulation under all of the conditions investigated in this research is demonstrably more effective in removing aquatic humic susbtances with higher apparent molecular weights and fewer carboxylic acid functional groups, as opposed to those with lower apparent molecular weight and more carboxylic acid functional groups. The predominant removal mechanisms were found to occur at the beginning stage of the coagulation process; that is, the rapid mixing period. The remaining dissolved organic matter and humic substances can form significant amounts of trihalomethanes upon reaction with chlorine.