Question

Base your answers to the following questions on the analogies drawn for the collision mechanisms between suspended particles in coagulation/flocculation and between a suspended particle and a static collector grain in filtration.
a) List four types of physical transport (collision) mechanisms that are important to filtration and their analog among flocculation transport mechanisms (if any).
b) Which mechanism(s) would be expected to govern the collection efficiency of 0.4 micrometer particles passing into a filter?
c) If the media grain size were increased, which mechanism(s) would decrease in effectiveness of collection?
d) If the density of the influent particles increased which mechanism(s) would increase in collection effectiveness?
e) If the approach (superficial) velocity increases, which mechanism(s) would decrease in effectiveness of collection?

Answer 1

a) The four types of filtration mechanism are as follows:

Straining: This is a very common mechanism of filtration. This mechanism occurs when the particles are larger than the designed diameter of the filtration media. The particles are removed when they are restricted from crossing over the filter media. This mechanism solely depends on the filter media spacing, the media density and the particle diameter.

Interception: In this mechanism the particle follows the streamline of the flow before it hits the filter media. The particle sizes are small and once the particles encounter the filter media they get attached to them and lose interia to cross over them.

Inertial Impaction: In this mechanism the inertia of the particles are changed suddenly and thus the particles are removed from the flow. This is applied when there is a high concentration of coarse particles in the flow.

Diffusion: This is otherwise called the Brownian mechanism. Here the particles flow in brownian motion and once they reach the filter media they get attached. this inturn creates a vaccuum in the flow which acts as a driving force for the next particle to move towards the filter media and get attracted. This mecahnism usaully occurs at low velocities and small particle sizes. To enhance this mechanism the area of filter media should be large and the velocity of the flow should be low.

Whereas in Coagulation and flocculation process the suspended particles normally carry a negative charge and this keeps them stable in the solution. The process involves three stages

1. Destabilisation

2. Formation of micro flocs

3. Formation of Macro flocs

         When an inorganic salt ( electrolyte ) is added to the solution the the hydrate layer on the surface of the particles gets removed and the double elctric layer on these particles get compressed. The particles become neutral and thus are destabilised.

        This is followed by adding a polymer whcih would bind these particles to form micro-flocs which grow and later form macro-flocs. These macro-flocs then settle down enabling them to be separated from the solution.

b) For a particle size of < 0.4 micrometer the filtration mechanism would be dominated by diffusion. This is the predominant mechanism for the particle sizes < 1 micrometer.

c) If the media grain sizes would increase mechanisms like interception and diffusion would decrease the effectiveness of filtration. This is so because as the media particle sizes are large which mean that the designed filter particle size would be small. If the particles get attached to the media and keep piling up this would eventually block the entire filter media and thus reduce the effectiveness.

d) If the density of influent particles increase the mechanisms like straining and inertial impaction would improve the effectiveness. These mechanisms are especially designed for the coarse particles with high densities.

e) If the appraoch velocity of the particles would increase the mechanism of diffusion would decrease the effectiveness of filtration as this mechanism requires more time to be spent in the filter or the supeficial velocity needs to be very low for Brownian motion to happen.