Cellulosic depth filters, such as Millistak®+ filters commercially available from Millipore Corporation, are typically used in the production of biopharmaceuticals, as derived from mammalian cell culture for the purpose of clarifying various crude product fluids.  These composite filters include a layer of tightly structured cellulosic depth media, and can be optimized to a specific application, such as retaining colloidal particles and cell debris or retaining whole cells and larger debris.  They combine sequential grades of media in a single filter cartridge.  These filters are most commonly used in polishing or secondary clarification processes to remove small quantities of suspended matter from aqueous product (protein) streams.  The primary function of these filters is to protect or extend the service life of more expensive downstream separation processes, such as sterile filtration and affinity chromatography.  That is, a common application for these filters is as “prefilters”, protecting downstream equipment and media from colloidal contaminants and other cell debris.  In addition, such depth filters are also used for the protection of viral clearance filters by removing trace quantities of agglomerated proteins. 
It is also known in the industry that composite depth filters also can retain, to varying degrees, some soluble contaminants commonly found in mammalian cell cultures, such as nucleic acids, host cell proteins, lipids, surfactants, etc.  This retention capability for certain soluble contaminants is based on the adsorptive properties of the depth filter media. 
The filter media typically employed in these depth filters includes refined cellulose fibers (wood pulp and/or cotton derived), diatomaceous earth, and a water-soluble thermoset resin binder.  The diatomaceous earth (a natural form of silica containing trace amounts of various silicates) in these composites is typically 40-60% by weight, and is believed to be the essential component, adsorbing colloidal size biological matter such as cell fragments, organelles and agglomerated proteins, as well as various soluble biochemicals such as proteins, lipids and nucleic acids. 
However, one of the principal drawbacks of the use of these cellulosic depth filters for the production of parenteral drugs and other pharmaceuticals is the relatively high level of water-soluble contaminants they release into the system.  Indeed, extensive pre-flushing is required to reduce the level of these organic and inorganic contaminants to acceptable levels prior to use.  Furthermore, the maximum loading of diatomaceous earth adsorbent within the depth filter media is limited to about 60% by weight, and the minimum particle size for the adsorbent to be retained in the fiber matrix is about 10 microns. 
It is therefore an object of the present invention to reduce or eliminate the release of contaminants from adsorbent filters. 
It is another object of the present invention to increase the content or loading of adsorbent in filter media. 
It is a further object of the present invention to provide a filter with smaller adsorbent particles in order to maximize the available surface area for adsorption. 
Other objects and advantages of the invention will be apparent from the following detailed description and the accompanying drawings.