Filtration involves the separation of a turbid fluid (unfiltered beer) to produce a clean filtrate and retentate, or residue, via a filter medium. Separation mechanisms include dead end filtration (one-way) and tangential flow filtration (cross-flow). High solids applications like yeast removal from green or fermented beer requires methods that can efficiently handle high loads, such as DE or cross-flow systems. Filtration downstream of DE generally includes particle and microorganism reducing depth and membrane filters to achieve beer brightness and stability. These methods utilize depth or surface filtration, and in some cases, a combination of both.
Diatomaceous earth (DE) is the fossilized remains of single-celled diatoms, consisting mostly of silicon dioxide (85-90%) and up to 4% aluminum oxide. Deposits are mined from a few regions, including the western USA, Canada, Australia, Italy, France (Auvergne) and Germany (Lüneburger Heide). DE is an excellent filter aid for the primary clarification of beer, due to its high surface area. These powdery substances are coated onto a filter surface (reinforcement or sheet) to form a cake. The cake of filter aid and retained solids is responsible for the filtration effect.
Calcined grades of DE are termed kieselguhr; fine, medium and coarse grades may be blended to accomplish the level of clarity required by the brewery. Fine kieselguhr provides a higher clarification, but does not provide as high a surface area (thus dirt holding capacity) as medium and coarse grades. DE consumption for beer filtration ranges between 70 and 200g per hL of beer; normal usage is about 100 g/hL. Other filter aids, including perlite and cellulose, are commonly combined with DE, though these aids are better suited for pre-coat applications to enhance cake formation.
Pre-coat (DE) Filtration Systems use filter aids by first pre-coating onto a filter surface, and then dosing into beer flow in order to build a filter cake. The most common types of pre-coat filters for breweries include:
” Plate and Frame filters
” Horizontal pressure leaf filters
” Candle filters
Plate and Frame Plate and Frame Filters consist of a frame (or chassis) in which quadratic plates and frames are alternately mounted. The plates are equipped with filter sheets on both sides to seal plates and frames against each other. Pre-coating and the following dosage (body-feed) are pumped into the frames and the filter cake forms on the sheet. After filtration DE is spray-washed and the sheet can be used again.
Horizontal Leaf Filter
Horizontal Pressure Leaf Filters consist of an upright cylinder with numerous round filter elements arranged to a hollow shaft. Horizontal plates have fabric made from chrome nickel steel with a pore size of 50 to 80 µm and provide the base for building a filter cake. The design of horizontal filters provides more flexibility and ease maintaining the filter cake compared to plate and frame or candle filters. Horizontal filters are produced in a range of sizes, and are also commonly applied to PVPP stabilization processes.
Candle FilterCandle Filters are cylindrical upright, conical pressure tanks, holding as many as 700 candles fixed to a top plate and suspended in the housing. DE is coated around the candle cartridges, which are radial wrapped profile wire with spacing of 50 to 80 µm. Candles may be 2 meters ormore in length and spaced compactly, providing a high filtration capacity for beer. Candle filters are well suited to long filtration batches of single brands, therefore tend to be applied in high production breweries.
Filter Sheets continue to be widely employed in beer filtration, typically for particulate filtration (trap filtration following DE), fine filtration (polishing and bio- reducing) and final filtration for microbiological removal. Sheet filtration provides excellent beer quality and flexibility, but lacks the security of a cartridge filter with a fixed pore structure. Therefore, constant attention to operating conditions is necessary to prevent system pressure shocks and potential microorganism unloading from sheet filters.
Filter sheets used in breweries consist primarily of cellulose, DE, and perlite bound by a food quality resin. Sheets exhibit both surface and depth filtration mechanisms, and may have a zeta potential for adsorptive effects. Large solids are retained on the sheet surface, whereas smaller particles are caught in the tortuous path of depth media.
Sheets are available in various grades (nominal micron ratings) for the required level of clarity or bio-reduction, and may have different formats depending on equipment used. Common sheet sizes for breweries are 40 cm x 40 cm, 60 cm x 60 cm, 100 x 100 cm and 120 x 120 cm.
Sheet Filters, also known as plate sheet filters, or filter presses, consist of a chassis holding plates and sheets together. Unlike pre-coat filters of similar format, a sheet filter does not use frames. Single flat sheets are inserted between filter plates, or fold-over types may be inserted into two filter positions to reduce the number of sheets that operators handle. In either case, a plate separates the inlet and outlet side of each sheet, with incoming flow to the rough sheet side and beer exiting the smooth side. Once sheets are loaded to the equipment, the frames are tightened against each other by means of a hand crank or hydraulic ram. Special attention must be given to inlet/outlet frame o-rings to obtain a good seal and reduce leakage.
Sheet filters are very often used for fine and polishing beer filtration. In smaller breweries pre-coat frame filters and sheet filters may be combined to one filter via a conversion plate. The disadvantages to sheet filtration include:
- open system, requiring attention to beer quality and loss
- high operating cost, mostly due to labor for manual operation and beer loss
- nominal rated filter media (not optimum security for fine/final filtration)
Lenticular Sheet Filters
Lenticular filter Lenticular Sheet Filters were developed from flat sheet filters, to an enclosed, modular design. Early types had a disc-in-housing format which has developed into modules, and more recently to a cartridge style filter. All formats of disc filters are based on depth filter sheets in lenticular cell form. Therefore, these modules offer similar filtration characteristics to sheet filters, with the advantages of a closed system.
Cross-section Oxygen pick up in the beer and product loss are reduced, and labor to operate and change out filters is lower than a sheet filter. Use of lenticular cartridges continues to increase among microbreweries due to relative low capital investment and ease of use. Lenticular modules have a higher media cost than filter sheets, though overall operating cost tends to be lower than with a filter press. As with traditional sheet filters, modular sheet filters offer a flexible range of sheet grades and certain types may be regenerated by backflushing.
Depth Filter Cartridges
Depth filter cartridges are used in breweries for particle removal such as kieselguhr and PVPP and fine filtration for removal of yeast. Cartridge style filters are more modern formats than flat or lenticular sheets. Most large brewery operations use cartridges, as this filter type can handle much greater flow rates and volume throughput in a compact design. Depth filter media in a cartridge format is most often a graded density polypropylene, which can have a stable media matrix and absolute rating. Depth filter cartridges have a high solids holding capacity and are ideal for particle and yeast reducing (membrane pre-filter) applications; however, unlike membranes, depth filters (including sheet formats) are not integrity testable so users must be aware that while the filter may meet a manufacturer’s standard of removal rating, they cannot be reasonably verified for microbial removal once in service. For this reason, membrane cartridges are preferred over all types of depth filters for microbiological stabilization.
The preferred beer cartridge is a 30 inch (762 mm) single open-ended filter with double 226 o-rings and locating fin. Various filter manufactures label their end cap codes differently, but the configuration and dimensions are industry standard. Double open ended filters are available, but are not as secure and therefore, not recommended for final filter applications. The best design for depth filters combine a traditional depth format with pleating for increased surface area. Grades applied to particle and fine removal applications should be backflushable to attain the longest life.
Cartridge housingsCartridge Housings for beer filtration should be food and beverage grade, holding filters with a 226 style, double o-ring, bayonet lock. Housings may have conventional upright mounting of the cartridges, but an inverted configuration offers the following advantages:
- Rapid and complete venting of the system resulting in minimum oxygen pick-up during the start-up phase
- Even flow distribution enhances the regeneration effect during back flushing.
Membrane filter cartridges are used in final beer filtration applications to remove yeast and beer spoiling micro organisms (yeast or bacteria) that can lead to secondary fermentation, turbidity and off-flavor after bottling. Polymeric membranes have become one of the most commonly used materials for final filtration of beer. These membranes are characterized by a very stable and consistent pore structure, excellent cleaning and sterilization characteristics, and most importantly their verifiable ability to remove beer spoiling microorganisms. Membranes most commonly applied for beer filtration are Nylon and Polyethersulfone.
Membrane cartridgeMembrane cartridge filter systems are best installed directly prior to the packaging equipment and should be sized according to the feed pattern to packaging. This particularly applies to keg plants where the intermittent feed rate can be two or three times higher than the effective hourly output of the plant. Up to 30% higher flow rates should be allowed if the beer is directly routed from the filter station to the bottle or can filler.
Membrane Cartridge Housing with Clusters are fully automated final filter systems designed for medium-sized to large breweries. With the exception of manual mounting and dismounting of the cartridges, all processing steps, including integrity tests, are performed via a PLC. Simple operation and supervision, process visualization and individual programming requirements are standard.
Cross-flow filters are applied in breweries for primary (bulk) yeast removal, recovery of fermentation bottoms, water filtration and caustic recovery. Hollow fiber membranes are used for primary beer clarification water and caustic treatment, and ceramic membranes for beer recovery. In each case, the fluid to be filtered is tangentially passed across a fine-pored membrane. A part of the liquid (filtrate or permeate) is passing through the membrane as the residual liquid (retentate, unfiltrate) is concentrated by recirculation, eventually to be flushed by cleaning or discharged. In the case of recovering fermentation tank bottoms, the filtrate is clarified beer to be returned to the process and retentate is concentrated yeast for transfer.