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In the water treatment process, the coagulation process plays a very important role. Researchers have made a great deal of thorough study about coagulation for years, and have proposed computing models and theories. In the late 20th century, with the development of modern analytical tools, like electron microscopy and computer, coagulation in the modern field of nonlinear science has made a rapid development.This paper introduces fractal theory to the process of coagulation, uses fractal dimension for quantitative study of flocculation morphology, and thus comes to conclusions:1) This paper takes PAC, PAFC and PFS as coagulants, and draws a conclusion that the effluent turbidity and fractal dimension are well correlated after the coagulation in different dosage and raw water pH in single factor experiments. The effluent turbidity is lower; the corresponding flocculation fractal dimension is higher. Flocculation fractal dimension formed by on-line monitoring reflects the coagulation effect2) On the basis of best dosage and PH value scope determined by univariate coagulation, the paper makes thirteen factors and three levels coagulation condition tests, of the orthogonal experiments, and come to a conclusion that the main factors that affect coagulation are mixed in coagulant dosage C, a slow stirring speed mixing sectionⅡfile n2, coagulant type A, the slow stirring speed mixing sectionⅠn1 and other files. Optimal working conditions are as follows: raw water pH 7 to ferric chloride （PAFC） as a coagulant, coagulant dosage of 12mg/L, （G0, T0） = (455.38s-1, 0.5min), （G1, T1） = (137.46s-1, 0.5min), （G<sub>2, T2） = (84.37s-1, 3min), （G3, T3） = (40.73s-1, 4min) and （G4, T4） = (14.40s-1, 2min).3) Summarizing and analyzing the researches of fractal characteristics, we draw a conclusion that two-dimensional floc settling velocity u and its fractal dimension D2, characteristic length L （floc size） exists power-law relationship, have the （u, D2, L） data set by experiments. We write a running program which can be applied to this experiment, and obtain the u and L, D2 mathematical model with the help of matlab data analysis software: u =8.1477 L0.3608D2 （Unit: mm/s）In this formula, u means sedimentation rate in units of mm/s, L means the size in units of mm, and D2 means the two-dimensional fractal dimension. In actual engineering we can monitor the size and fractal dimension according to this formula, thereby managing to control of effluent turbidity requirements.
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