Research On Flocculation Kinetics Based On The Coagulationand Precipitation Integrated Process

Coagulation and precipitation are two of the most significant processes for drinking water treatments, which determine the running conditions of the follow-up processes, quality of effluent and operation costs of water treatment eventually. Therefore coagulation and precipitation processes are two increasing popular concerns in water treatment discipline. Aiming at the frequent water quantity changes, big hourly variation cofficient, and large clean-water pool adjustable volume of traditional small-scale water supply process which can not guarantee outflow water quality with intermittent operation mode,"the coagulation and precipitation integrated process"was put forward based on the static precipitation. In this study, coagulation and precipitation experiments were conducted to optimize the experimental parameters and confirm the optimal operating conditions. Two flocculation effect control indexes as"energy input (GT)"and"shear strength (Fr)"were proposed on the basis of the establishment of kinetics model, and the optimal range of the indexes was confirmed by experiments.Coagulation and precipitation experiments on treating the raw water with a turbidity of about 85NTU using Polyalumin Chloude(PAC) as the flocculant indicated that, the best experimental conditions were: temperature T was about 22℃, pH was around 6.5⁸.0, the best dosage of PAC was 16mg/L; the optimal stirring parameters were: n₀=300r/min, t₀=1min; n₁=140r/min, t₁=4min, n₂=90r/min, t₂=16min; The optimum volume of floc mud was the same as 6% of the water volume of next processing period, and the corresponding dosage of PAC was low to 8 mg/L. Because of the repeated utilization of floc mud which had residual activity, this process could enhance the flocculation effect, and 50% dosage of the coagulant could be saved; the solid-liquid separation effect was improved based on the static precipitation, and the turbidity of effluent was less than 1.0NTU, which could reach the water quality standards through simple filtration and disinfection procedures.Morphology and dynamics researches on the flocculation process showed that, several physical parameters of the floc as floc densityρ, void ratioε, precipitation rate u were all the function of the fractal dimension Df; The shearing force Fr in the turbulent vortex flow was the main force in the flocculation process; the expression of floc intensity was k_σ(?)G^-2d_ρ^2Dt/3-4 as the function of the energy input G and the fractal dimension Df; the expression of the floc fractal dimension was D_f=K·f₄(GT , Fr) as the function of the energy consumption GT and the shear strength Fr on the basis of fixed equipment forms and fixed raw water quality. So that, in an efficient flocculation process kinetics indexes of G, GT, Fr and the fractal dimension D_f should be controled as the floc growing.In this study, the the optimal range of the flocculation energy input, shear strength and the floc fractal dimension was confirmed by experiments. The best energy distribution of each flocculation stages was: G₁=108.30s^-1, T₁=240s; G₂=54.59s^-1, T₂=480s; G3=20.84s^-1, T₃=540s. The optimal control range of the shear strength in each stirring stages was: Fr₁=10.3193¹3.6438, Fr₂=4.3451⁶.9332, Fr₃=0.8630¹.1465, and the ratio range of the shear strength in each stir stages should be controlled in Fr₁:Fr₂:Fr₃=(10.98¹2.61):(4.59⁶.10):1. The optimal control range of the floc fractal dimension in each stir stages was: stage I Df₁=1.340¹.367, stage II Df₂=1.513¹.562, stage III Df₃=1.785¹.873.The implementation of the coagulation and precipitation integrated process could provide high quality drinking water for residents from large or small-scale water treatment engineering, popularization and application of which could bring great economic and social benefits. In this study, comprehensive indexes which controlled the flocculation effect were established by establishing the kinetics model,and the optimal range of the flocculation energy input, shear strength and floc fractal dimension was confirmed by experiments,which could contribute scientific basis for the design and operation of coagulantion process and reasonable guidance for practical engineering application.