Characterisation of Natural Organics and Colloid Systems

Particle analysis is often difficult in natural systems due to the presence of larger particles which necessitates fractionation of the samples. Also, the particles present are often unstable and the concentrations of submicron particles are low. Concentration variations were given by Filella and Buffle (1993) as between 0.1 mgL in sea- or groundwater to 10 gL in a river. Iron containing particles are, according to the authors, very common and occur in sizes of 2-450 nm. Colloids smaller than 50 

Of particular interest in water treatment is the permeability of aggregates. When a regates deposit on a membrane, flux is determined by the resistance of the formed cake. Furthermore, the drag force on the colloid is determined by the aggregate porosity. Veerapaneni and Wiesner (1997) predicted the resistance to fluid flow as a function of fractal dimension. They determined that fluid flow through the aggregates decreased with increasing fractal dimension. 

Grout et al. (1998) question the description of natural samples by simple power laws. According to these authors natural systems are often multifractal which means the fractal dimension varies for different size scales. Veerapaneni and Wiesner (1997) studied the effect of filtration velocity on the fractal dimension of deposits formed by 69 nm colloids. Fractal dimensions increased with increased velocities where a lower head loss was measured, this was attributed to columnar structures formed. 

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