To: Regional Wine Producers

From: Bruce Zoecklein, Head, Enology-Grape Chemistry Group, Virginia Tech

Subject: Filtration, continued; Precoating and Body Feeding

The depth filters in common use in the wine industry include the pad filter-like plate and frame filters, as well as pressure leaf or cake filters.

Cake filtration. Cake filtration is perhaps the most widely used filtration system in the wine industry.

In cake filtration, the solid material separated from the liquid accumulates on the surface of the medium so that, after a short initial period, the deposited solids form a cake through which the liquid must pass. The cake, in the form of diatomaceous earth, is deposited initially, on a coarse screen in the case of pressure leaf filters, or pads in the case of plate and frame filters. In the wine industry, filter cloths made of cotton, or other suitable synthetic fiber, such as nylon, are used. The process may continue, increasing in depth of the cake, until the space available is filled or until the pressure differential becomes so great that the flow is reduced to an uneconomical level.

In order to improve the filtration characteristics of this system, the wine industry uses diatomaceous earth (D.E.) for precoating of the screen or filter pads, as well as for a continuous proportioned body feed throughout the filtration cycle. By selecting the particle size of the D.E. used, different fineness of filtration can be achieved, from rough filtration to polish filtration.

D.E. is composed of the fossil remains of microscopic marine plants called diatoms. These plants extract silica from the water and form exoskeletons or shells. The skeleton remains after the plant dies and settles to the bottom and accumulates.

Diatomaceous earths are processed at 1500-2000EF to burn off all organic matter. This leaves a residue which is almost pure silica.

There are various grades of D.E. depending on the fineness or particle size which range from about 2.5 to 38 microns. The finer particle size produces a more polished filtration.

The amount of D.E. needed to deposit an effective precoat depends on the flow characteristics of the filter, the type of screens and filter pads used and the pump characteristics. The most effective amount can only be determined by actual experimentation.

Precoating and Body Feeding. Plate and frame filters consist of a number of plates and frames, corresponding in size and shape, which are arranged alternately and which are supported on a pair of rails. The plates have a ribbed or waffle surface to facilitate the flow of filtrate. They may be constructed of stainless steel or plastic.

The feed channel in this filter is formed by corresponding holes in each plate and frame that register together when the filter is tightened, so that they form a continuous flow path. Each frame has an opening that leads from this channel into the inside space of the frame. There is another opening in the bottom of each plate, that connects the down flow side of the filter cloth to an outflow channel formed in a manner similar to the feed channel, and which leads to the filtrate outlet port.

When the filter is in operation, liquid flows into the filter through the inlet port, frame ports, cake, filter cloths, plate ports, and out through the filtrate outlet port.

During the initial stage of the operation, the liquid is filtered through the filter pad only. Therefore a circulation phase is needed in order to deposit a cake on the surface of the filter cloth. This is known as "PRECOATING". Once this is accomplished, the flow is diverted into a tank and the filtration continues while filteraid is added to the wines as a "BODY FEED". The body feed prevents rapid plugging of the filter by providing a continuous supply of new porous filter elements.

As a general rule, 10 to 15 lbs. of D.E. per 100 sq. feet of surface will be ample for a 1/16 inch precoat, if the cake is evenly distributed. The next issue will include additional information on filtration. Such topics as integrity testing, the new membrane pads, etc., will be discussed.

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Dr. Bruce Zoecklein
Associate Professor and Enology Specialist
Head Enology-Grape Chemistry Group
Department of Food Science and Technology
Virginia Tech
Blacksburg VA 24061
Enology-Grape Chemistry Group Web address: www.vtwines.info or www.fst.vt.edu/zoecklein/index.html
Phone: (540) 231-5325
Fax: (540) 231-9293
Email: bzoeckle@vt.edu