Wine Filtration
Depth Filters for Wineries
Pressure-leaf, lenticular disc, and plate and frame filters, are the most common forms of depth filters used in winemaking.
Pressure Leaf Filters
Pressure leaf filters consist of 10 to 60 leaves, stacked inside a vertical or horizontal vessel (Figure 11.1). The leaves are drainage systems that support a wire or textile cloth cover. The screen provides the support of the diatomaceous earth cake during the filtration. The leaves may be circular or rectangular and are assembled on a frame with ports for removal of the filtrate. The leaves are completely immersed and surrounded by the juice or wine that is being filtered. Pressure leaf filtration can be used from the rough filtration of tank settlings through to the pre-filtration of wines for membrane filtration.
Diatomaceous Earth
Pressure leaf filters use diatomaceous earth (DE, kieselguhr) as the filtration medium. There are various grades of DE depending on the fineness or particle size, which ranges from about 2.5 to 38 ?m. The finer particle size produces a more polished filtration.
Filter Operation
Normally, use of DE in filtration is a two-step operation. First, a thin layer of clean filter aid is coated on the filter element (cloth or wire screen) by recirculating a dilute slurry of filter aid through the filter. This is referred to as precoating the filter. The precoat serves two purposes; it protects the filter element and also produces almost instant clarity when the filter goes on line. Following precoating operation, a small amount of filter aid (body feed) is continuously added to the liquid being filtered. The first 20 to 40 liters of filtered juice will have an earthy taste (from the DE) and is typically discarded.
Lenticular Disc Filters
A lenticular filter, also referred to as a disc filter, is simply a circular housing or canister that is secured to a base plate with a clamp closure (Figure 11.2). Built to housings hold circular stacks of filter discs, sometimes called frisbees, either 12 inches or 16 inches in diameter. Several will be contained as a single package, hence the term stack.
Plate and Frame Filters
Plate and frame filters, sometimes called pad or sheet filters, contain a series of plates that hold the pads between them (one plate, one pad) (Figure 11.3). Although plate and frame filters are more commonly used with filter pads (commonly referred to as sheets) as the filtration medium, they can also can be setup using DE as the filtering medium. These plate and frame filters are equipped plastic or stainless-steel filtering plates for DE. The units are set up with multiple plates within the frame, and each plate is fed by a common manifold which provides access ports to all other plates.
Plates
The plates are generally square in shape and hang from two supporting bars at each side of the filter stack. The number of plates that are used determines the filter area and the number may be adjusted for the filtration run. Plate and frame filters are generally sold as holding 20, 40, 60, 80, or 100 plates.
Pads
Most pad filters are made of a blend of cellulose fibers, sometimes with the addition of granular components, such as DE, perlite, polyethylene fibers, and resins. The resins provide strength and also impart a zeta potential, a positive charge that increases the collection of small, negatively charged particles and microorganisms. The common dimensions of the commercial pads are squares of 20 x 20 or 40 x 40 centimeters (Figure 11.4).
Filter Operation
Before closing and compressing the plates for new filtering, wet the cloth pads with water. Check the edges of the pads before closing the plates, to assure that no bent pad is caught between the plates. If this happens, the filter will leak when the pressure rises.
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