Coagulation, in the physical context, is looked upon here first from the fundamental perspective of collision and coalescence of individual particles. A Monte Carlo technique is used to investigate the particle size distribution in a suspension of coagulating particles when one or more collision mechanisms operate. The effect of interparticle forces - hydrodynamic, van der Waals' and electrostatic - on the collision probability of the particles is examined. The results obtained are used to evaluate the well-known dynamic equilibrium hypothesis according to which an equilibrium particle size distribution is assumed to exist under the action of a given collision mechanism. It is shown that dimensional analysis cannot, in general, be used to predict steady state particle size distributions, mainly because of the strong dependence of the interparticle forces on the sizes of the interacting particles.
The insight into particle kinetics thus gained from the Monte Carlo simulation of collision processes is used to develop a numerical simulation of a rectangular settling basin. The computer model follows the spatial and temporal development of the influent particle size distribution towards the outlet of the tank, accounting for all of the basic kinetics of particle collision and coalescence processes and including transport processes such as particle settling, advection, resuspension and turbulent mixing. The influence of the particle size-density relationship and floc deaggregation by turbulent shearing are also modeled. Of necessity, modeling of some of these processes has been somewhat empirical since the physical and biochemical nature of the flocs are unique to a particular suspension and their determination requires experimental work. The results of the simulations performed indicate that the particle size-density relationship, the collision efficiencies between flocs and the influent particle size distribution are of major importance to the performance of the sedimentation basin. Clearly, further modifications, improvements and trials are needed in order to use the model for the design of new facilities. Nevertheless, the computer model may serve as a guide for selection of several design and operation variables for the successful treatment of a particular waste or the selective removal of pollutants whose concentration depends the shape of the effluent particle size distribution.
