Filtration is the act of passing a liquid through pores or ducts in a media with the aim of removing particles from it. The effectiveness of the filtering action of a filter is determined by the size of the pores, or the permeability, of the filtering medium. The flow rate of a filter is proportional to the surface area, pressure and permeability, and inversely proportional to the viscosity and the thickness of the filtering medium.

A filter is also defined by its efficiency; the volume of wine which can be filtered per cycle, until the filtering surface becomes blocked. The clogging power of a wine depends not only on the percentage of solids that it contains, but on the nature of the particles, as some clog more than others, in particular, ‘plastic’ molecules such as gums and mucilages. Other particles that block filters easily are colloidal proteins, bentonite and isinglass. Bacteria clog more than yeasts, as they tend to agglomerate in groups, and so wines having undergone malo-lactic fermentation are more difficult to filter.

Filtering surfaces are defined according to their rating:

• Nominally rated surfaces are rated according to their average pore size.
• Absolutely rated surfaces are rated according to their maximum pore size.

There are two major mechanisms involved in filtration:

Absorption (depth filtration) – this is when particles adhere to the filtering medium because of their opposing electrical charge.
For instance, cellulose fibres have a positive charge, and so attract yeasts which have a –ve charge. In this type of filtration, the pore size is not as important as the bulk, type and surface area of the fibres. The flow rate remains constant (though it may drop due to screening caused by a bridging effect of absorbed particles), but the wine comes out progressively cloudier until saturation point when the filter no longer has any effect.

Screening (surface filtration) – this is when particles are retained by pores which are smaller in diameter than the particle. The flow rate of a filter that uses screening exclusively decreases with the volume of liquid passing through.

The above diagram is by David Bird MW from “Understanding Wine Technology”

Only screening filters can be rated absolutely, adsorption filters will only remove a proportion of particles larger than their rating. Depth filters are much more sensitive to ‘break through’ due to increased pressure, pressure surges or pulses, than screening filters. On the other hand, depth filters have a much higher dirt holding capacity than membrane filters.