Colloid and Particle Fouling

Colloids are defined as particulates in the size range of 1 nm to 1 pm (Potts et al. (1981)). Below that range, particulates are dissolved solids. While this size range includes organic matter, this section is mainly devoted to inorganic colloids. Fouling depends on colloid size and membrane pore size. A number of models exist for colloidal fouling and these were summarised by Bowen and Jenner (Bowen and Jenner (1995)). Here, major mechanisms and forces on colloids will be summarised. 

A particle in the feed stream will be exposed to a number of forces and the resulting force will determine the particle s destiny. The forces are dependent on the particle size (Altmann and Ripperger (1997)). 

Three mechanisms are important for the backtransport of particles from a membrane. For small solutes and submicron colloids. Brownian Diffusion (determined by the Stokes-Einstein equation (Sethi and Wiesner (1997)) dominates backtransport of the colloids from the membrane into the bulk solution. Inertial Lift, which is caused by the presence of a wall is important for large particle sizes and high shear rates. Shear Induced Diffusion is an orthokinetic mechanism also more important for larger colloids. 

Sethi and Wiesner (1997) predicted a most unfavourable size of 0.4 pm where the backtransport, considering all mechanisms, was at a minimum and thus the resulting flux lowest. When also considering cake permeability as a function of particle size, this minimum shifted to 0.01 - 0.1 pm. The dependence on membrane pore size and initial flux was not addressed in this study. Chellam and Wiesner (1998) pointed out the complicating effects of polydispersity on the use of such models. Most natural systems are polydisperse. 

In a further study, Chellam and Wiesner (1997) showed that the specific resistance of the deposit increased with shear rate and decreased with initial flux. This implied that the deposit structure is also important. Additionally, Veerapaneni and Wiesner (1994) simulated the deposition of particles on permeable surfaces. Small particles ( 1 pm) and low fluid velocities favoured the formation of loose deposits on the surface, while particles 1 pm formed dense deposits. These results show the impact of colloid size on particle packing and thus the permeability of the deposit. Particle-particle interactions, however, were neglected. 

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