general filtration principles

filtration mechanisms

Three mechanisms are involved in succession: capture, attachment and separation. Their importance depends on the properties of the particles to be screened out and on the filtering material used.

capture mechanisms

There are mainly two types :

• mechanical straining: this involves screening out particles that are larger than the filter’s mesh or than elements that have already been deposited that act to form the filtering matter. The occurrence of this phenomenon increases as the filtering material mesh becomes finer: it is of little significance in the case of a filter bed made up of relatively coarse material; however, it becomes preponderant in filtration over a thin support: screen, filter bag…

• retention in intergranular spaces: this particule’s size compared to that of the pores, could enable it to pass through the filtering matter without being held back and yet as it follows its convoluted path through the bed, through zones with even the smallest current and with particle/matter contact will make its retention possible. This constitutes an extremely important mechanism in in-depth filtration

attachment mechanisms

A low flow velocity will encourage particles to attach onto the surface of the filtering matter. This phenomenon is caused by forces that are physical in origin (jamming, cohesion…) and by adsorption forces, mainly the Van der Waals forces.

separation mechanisms

As the result of the action exercised by the previous mechanisms, the space between the walls of the matter covered with particle deposits will diminish. We then see an increased intergranular flow velocity. Deposits that have already attached themselves can then be partially separated from the matter and drawn further into the filtering matter (advancing “filtration front”) or even into the filtrate (“breakthrough”).

The solid particles found in liquid and colloidal particles that are more or less flocculated, do not all share the same properties and will not react in the same proportion to the various mechanisms described above. Direct filtration of a liquid where the suspended solids retain their state and, where applicable, their electric charge, will, consequently, differ considerably from the filtration of a coagulated liquid.

filtering matter fouling and washing

Fouling signifies the gradually obstruction of the interstices in the filtering matter. As already discussed, fouling causes increased head loss and, if we operate at a constant supply pressure, the filtrate output will decline (“declining rate” filter).

Therefore, if we wish to operate at a constant flow rate, we have to:

• either increase the pressure applied to the filter bed as fouling increases (e.g. “constant rate variable head” filter),
• or maintain this pressure constant and install a regulation system on the filter outlet to apply an additional head loss and the latter will decrease as the bed fouls up: these “constant rate clogging compensation” filters are the most widely used filters in water treatment applications (see chapter filters).

The fouling rate will depend on :

• the matter to be screened out: the fouling rate will rise when the liquid becomes more laden with suspended solids, when this matter is more cohesive and when the matter itself is susceptible to proliferation (algae, bacteria);
• filtration velocity;
• filtering element properties: pore dimensions, homogeneity, unevenness, shape of the material.

The filter is fouled when it reaches the maximum head loss set by construction. It must be washed efficiently so that it can be reinstated to its original condition. The wash mode will depend on the type of filter and on the nature of the matter screened out. The operating time that elapses between two successive washes is called a filtration cycle.

selecting the filtration mode

The selection to be made between the various types of filtration through a support or through a granular bed will depend on a range of criteria:

• properties of the liquid to be filtered, of its impurities and of the changes the latter undergo in time;
• quality of the target filtrate and permissible margins;
• installation conditions;
• options and facilities available for washing.

When selecting a filter, the ability to carry out a wash easily, efficiently and cost-effectively is as important as the need to achieve the best possible filtration quality; filtration quality will only persist in time if washing enables a clean filter to be reinstated at the start of each cycle.