Using microscopic and optical monitoring techniques, the process of coagulation of humic substances was studied paying attention to the structure of flocs and their settleability and filterability with information from image analysis, and HPLC, zeta potential, TOC and UV absorbance measurements. The results show that Al-humic flocs formed at pH 5.0 appear more compact with higher fractal dimension while those formed at pH 7.0 are looser with lower fractal dimension. During growth, Al-humic flocs also undergo a decrease in their fractal dimension. The attainable removal of TOC is about 40-50% at both pH values, but the optimum Al dose at pH 7.0 is about 5 times of that at pH 5.0. Charge neutralization is believed to be the main mechanism of coagulation at pH 5.0 and sweep coagulation plays an important role at pH 7.0 as well as at pH 5.0 under an overdosing condition.

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