In order to discuss the effect of humic acid on improving coagulation performance at low temperature, an optical method was employed to investigate the aggregation kinetics of kaolin with and without humic acid at low temperature using a photometric dispersion analyzer (PDA). Moreover, the fractal dimension (D-2) of the aggregates was obtained by photographic technique coupled with image analysis. The presence of humic acid in the water significantly increased the rate of aggregation at low temperature and caused a decrease in the number of small flocs. It was found that the optimal dosage of coagulant decreased in the presence of humic acid. In the model water with only kaolin, the fractal dimension of flocs with different dosage of alum was nearly the same when coagulation time was longer than 20 min. However, in the presence of humic acid, the fractal dimension of the flocs initially decreased, and then increased with dosage of coagulant, which suggested that median dosage of alum caused looser flocs. Comparing the flocs with and without humic acid, D-2 was always lower in the presence of humic acid, which showed that the coagulation mechanisms for these two model waters were dramatically different. The residual turbidity in the model water with humic acid was significantly lower than that without humic acid, and the optimal dosage of alum was two times lower than that without humic acid. Al(OH)(3)-humic acid precipitates worked as polymers, which interact with charged particles by bridging flocculation, charge neutralization or other interactions. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.