Modelling the settling behaviour of fractal aggregates–a review

Sedimentation is one of the simplest and cheapest methods used for the removal of particulate matter from wastewater. However, the method is only viable for large particles and hence pollutants need to be flocculated to form large aggregates, normally behaving as fractals. Knowledge of the structure of fractal aggregates and their settling behaviour are crucial for optimisation of the sedimentation process. The structure of fractal aggregates can be readily measured using methods such as light scattering. However, the characteristics of the settling behaviour of fractal aggregates are not well understood since these aggregates do not behave as spheres with constant density. The settling behaviour of fractal aggregates depends on various aspects, such as the porosity, size, and buoyant density. The models used in this paper to predict settling velocity of fractal aggregates show that aggregates with particles packed closely together settle more easily than open flocs of particles when the size of aggregates exceeds the transition radius, RT. RT exists due to porosity of the flocs. When the sizes of the flocs are smaller than RT, the passage of fluid through the interior of the aggregates allows the reduction of the drag coefficient, which further results in faster settling of loose aggregates compared to compact aggregates. However, when flocs are bigger than the RT, gravity plays a more important role, which allows compact aggregates to settle faster than loose aggregates. This result suggests that by controlling aggregation processes to ensure aggregates have compact structure and are bigger than RT, sedimentation could be further utilised in pollution control processes. Additionally, the usage of coagulant can be greatly reduced since less coagulant is needed to produce compact aggregates.