Abstract: Since the particle size distribution of sludge is difficult to observe, and the fractal structure and the efficiency of flocculation are rarely considered in previous simulations, we established an improved aggregation model to simulate the mechanism of sludge particle aggregation and finally carried out three-dimensional simulation of flocculation-settling of sludge based on population balance model and two-phase flow mixture model. Results revealed that the established mathematical model and simulation method could well reflect the regularity of flocculation-settling of sludge. Furthermore, the particle size distribution range of floccules is widened with the decrease of fractal dimension, the number of large diameter floccules grows rapidly, and the number of small diameter floccules reduces gradually; the decay rate of initial particle number increases with the rising of fractal dimension, and the number of floccules generally increases and then decreases with time, but the maximum floccules number increases with the rising of fractal dimension, the occurence time of the maximum floccule number is shortened with the rising of fractal dimension.

Key words: improved aggregation model, population balance model, sludge flocculation, settling, particle size distribution, fractal dimension, numerical simulation