Microporous depth filters provide a physical barrier to the passage of particles, and are characterised by nominal particle size ratings. Depth filters are matted fibre or material compressed to form a matrix that retains particles by random adsorption or entrapment. Most raw waters contain colloids, which have a slight negative charge (measured by the Zeta potential). Filter performance can be enhanced by using micro filters that incorporate a modified surface, which will attract and retain these naturally occurring colloids, which are generally much smaller than the pore sizes in the membrane.

Depth filters (typically 1-50 μm) are commonly used as an economical way to remove the bulk of suspended solids and to protect downstream purification technologies from fouling and clogging. They are replaced periodically.

Multi-media filters are generally used for higher flow rates. It is a depth filtration method where the media is held in a tank (vessel). Media filters can be single or multi layer with different kinds of media and porosity depending on the application. Sand is the most common media used, but anthracite, carbon or manganese green-sand are used as well. Accumulated particles within the tank are removed by a backwash cycle with water or air, lasting approximatly 30 minutes.