(a) Very small particles do not tend to sink. For a given type of particle in a given liquid, sedimentation rate is proportional to the particle’s diameter squared [7]. This is because the particle’s mass (and thus its weight, pulling it “down” and making it sink) depends on its diameter cubed. On the other hand, the frictional force working against the particle’s movement (Stoke’s Law) increases with actual diameter [8]. Thus for small particles, the weight is too small to overcome the drag force, yielding very low sedimentation rates.
(b) The weight of the particle also depends on its density. The density difference between the particle and the fluid affects the sedimentation rate [9]. In organic effluents most suspended solids are organic matter, with a density close to that of water, avoiding their separation from the liquid.
(c) In most cases colloidal particles are electrostatically charged. In mineral colloids those charges occur due to imperfections in the lattice of the minerals, whereas on organic matter such charges might derive from functional charged groups (amines, carboxyl, phosphate, etc.) [8]. Electrostatic repulsion between the particles avoids its aggregations, stabilizing the colloidal dispersion.
(a) Very small particles do not tend to sink. For a given type of particle in a given liquid, sedimentation rate is proportional to the particle’s diameter squared [7]. This is because the particle’s mass (and thus its weight, pulling it “down” and making it sink) depends on its diameter cubed. On the other hand, the frictional force working against the particle’s movement (Stoke’s Law) increases with actual diameter [8]. Thus for small particles, the weight is too small to overcome the drag force, yielding very low sedimentation rates.
(b) The weight of the particle also depends on its density. The density difference between the particle and the fluid affects the sedimentation rate [9]. In organic effluents most suspended solids are organic matter, with a density close to that of water, avoiding their separation from the liquid.
(c) In most cases colloidal particles are electrostatically charged. In mineral colloids those charges occur due to imperfections in the lattice of the minerals, whereas on organic matter such charges might derive from functional charged groups (amines, carboxyl, phosphate, etc.) [8]. Electrostatic repulsion between the particles avoids its aggregations, stabilizing the colloidal dispersion.
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