How the natural organic matter to coagulant ratio impacts on floc structural properties

Periods of elevated natural organic matter (NOM) loadings at water treatment works (WTW) can lead to operational problems as a result of deterioration in floc structural quality. This study used a range of diagnostic tools to evaluate floc structure with increasing organic fraction in the floc. It was observed that when the organic fraction in the floc went significantly over a mass ratio of 1 mg of DOC to 1 mg of Fe (coagulant), the floc size, settling velocity, fractal dimension, and strength were seen to decrease even when the NOM removed during coagulation remained high. A model was proposed to explain these changes that was dependent upon the adsorption of NOM on primary particle surfaces. The operational significance of these results suggests that for the coagulant under investigation the correct coagulant dose must be applied to give good floc structure.