by Steve Parkes
Owner and lead instructor at American Brewer’s Guild
Diatomaceous Earth (DE) filters are a large class of filters that are used in many industries for a variety of liquid-to-solid separation tasks. They are used throughout the beverage industry to remove particulate matter and produce clear product. Many small breweries employ them to remove yeast and haze from beer before packaging or serving.
The two most common filter types employing DE used by brewpubs are sheet filters and pressure leaf filters.
Pressure Leaf Filters
Pressure leaf filter operation is fairly straightforward, but it is more complex than sheet filtration. At first the filter may look quite complex, with many valves, vessels, and pumps. But don’t be intimidated; all of these procedures follow some basic rules, and there are a lot of ways to correct problems that arise during filtration.
First, you should become familiar with the basic operation of the filter. The beer is filtered by passing it through a bed of diatomaceous earth, which traps any solids suspended in the beer. The result is a bright, clear beer, with no visible chill haze. The filter is not an absolute filter, and the median pore size in the filter bed is much larger than that of most beer spoilage bacteria. Hence, it cannot be considered a “sterile filter.” In fact DE filters are often a source of micro-organisms, especially if not properly sanitized.
One major difference between how filters are operated in brewpubs and the procedures used in the wine industry is that brewers filter the beer after it is carbonated. Because of this, the filter is pressurized to keep the CO2 in solution. Often the DE filters used in microbreweries are not actually certified to withstand CO2 pressure. Check with your manufacturer or supplier.
The filtration procedure is broken into several steps, the first being sanitizing (cleaning and disinfecting) the filter. Next, the filter is filled with water (or clear beer), and DE is added to pre-coat the screens. Adding 25 percent to 30 percent cellulose fiber to the pre-coat aids in bridging the screens and removing the filter cake at the end of the run.
After pre-coat is complete, the water is drained from the filter and filled with beer. Then the beer is filtered to the bright beer tank. Once the fermenter is empty, CO2 (or water) is used to force the remaining beer out of the filter bell and transfer lines. Finally, the filter is cleaned and stored.
There are several important warnings you should note before starting the filter. Stories are often told about pressure leaf filters exploding and knocking down walls or throwing stainless steel parts around the brewery. Although many of these stories are greatly exaggerated, there are some over-pressure concerns with the filter. There are several things you should always watch when operating the filter:
1. Check the pressure gauge on the top of the filter bell as often as possible. If it rises above 5 bar or starts to rise sharply at any time, turn off the filter.
2. Never leave the dosing pump running with no place for the pressure to be released. The filter is a closed system, and if the dosing pump — which is a positive displacement piston pump — is left running, it will over-pressure the system very quickly. There are specific times during the process when this is a risk, and they are noted below.
3. Never shut valves on the discharge side of the filter while the filter is pumping. Although the filter pump is a centrifugal pump, it is a two-stage, high-pressure pump with a double impeller, quite capable of blowing hoses that are shut off against it. If you need to shut off flow from the filter, either put it in recirculation or turn off the filter.
4. If you are filtering a beer with extremely high solids (such as a fruit beer, a very yeasty beer, or dry-hopped beer), it is possible that you will blind off the filter bed, and the pressure will increase sharply. It is important to be aware of this throughout the filtration. Watch the pressure gauge carefully if the beer has high solids.
Another warning often heard about pressure leaf filters is that if you turn them off during operation, the filter beds will be disturbed and you will have to pre-coat all over again. While there is some truth to this for vertical leaf filters, turning off a horizontal leaf filter during operation will not destroy the beds. The best way to stop flow on the filter is to put it into a recirculate mode, and if you do turn off the filter during transfer for some reason, you should recirculate for a few minutes before resuming the transfer. In an emergency it is always best to hit the “off” button on the filter and worry about the beds later.
Cleaning the Filter
Note that while cleaning the filter, you also may wish to clean the hoses, bright beer tank, or both as part of the same cleaning loop. When you are cleaning the filter you will need to make sure that the solutions run through the filter under a slight (1 bar or 15 psi) pressure. This ensures that all the crevices in the filter and hoses will be cleaned properly.
Remember that the filtration takes place under pressure, so the cleaning should be performed under pressure as well. Cleaning the filter is the most time-consuming task for most filtration runs. You must first clean and then disinfect the filter to assure that no micro-organisms that could spoil the beer are present.
Many small breweries operate filters only one or two days each week. How the filters are cleaned after use and stored affect how they must be cleaned before the next filtration. It is best to store the filter with some sort of mild sanitizing agent in the filter to keep the bacterial count down between filter runs. You also may store the filter dry with the valves open after cleaning.
To determine the best method for your brewery, take into account how often you will brew, other finishing processes (such as sterile filtration) you will use downstream of the DE filter, and the amount of time between filtrations. Do not store the filter with a caustic or chlorine solution, since this will promote corrosion of the stainless steel parts.
Even if you clean the filter with chemicals after filtration, you may wish to repeat the cleaning process before the next filtration. This is especially helpful if the filter is stored dry or for long periods.
Put on your gloves and goggles! Once the CIP loop is running and the dosing pump is primed, add a scoop of cleaning chemical directly into the dosing sump. Notice that the mixer in the dosing sump is running, since it is powered by the same motor as the dosing sump. Let the CIP loop run for 20 to 30 minutes.
Pressure Rises Dramatically
During a difficult filtration the pressure in the bowl might begin to rise dramatically. This is due to gaps in the filtration bed becoming clogged with the particulate matter you are trying to remove. In this situation it isn’t possible to make the filtration easier, but it is possible to stop it from getting worse.
The reason for the rise in pressure is that the ratio of DE being added to yeast in suspension is too low. Simply increase the speed of the dosing pump. If the dosing pump is already operating at full speed, then it’s possible to thicken the DE suspension or just slow down the beer flowing through the beer pump. Do this until the pressure stops rising.
Different Grades of DE
DE comes in several grades. Choose it based on the type of solid being removed from the beer. Don’t blend different grades to try to average the particle size. Because DE works by forming bridges, it relies on a consistent particle size to work effectively.
The DE brewers use is not the same as the DE used in gardens and pool filters. Some DEs contain iron, which is detrimental to beer quality. A simple test can be done on your DE. Fill two bottles with beer from your bright tank. Add 2.5 grams of DE to one, and keep both in a refrigerator for a day. Pour both beers, taking care not to disturb the DE sediment in one of them, and taste the beers. Taste any difference? You shouldn’t.
De-aerated Water for Dosing DE
A problem often cited with DE filtration is oxygen pickup by the beer across the filter. The biggest problem area is in the open dosing tank. Its agitator whips the DE and the beer or water it is suspended in full of air. One way to help reduce this is to bubble nitrogen or CO2 through the dosing tank. This creates a blanket of gas over the dosing tank to prevent air ingress.
Also de-gas the bell housing before operating the filter. Fill the filter with water, de-aerated if possible, then gently push it out through the discharge with CO2 or N2. Repeat. Very little air will remain in the housing. Be certain that the discharge is open to the atmosphere before applying the top pressure. Use caution!
It is important to flush the filter and the beer lines completely with CO2 prior to filling the filter with beer, and it’s fairly easy to create sterile, de-aerated water to operate the filter. Brewers often cite leaking pump seals as a source of oxygen up-take in the beer. Despite the dramatic pressure drop on the outlet of the filter pump, the system is being operated at a high enough pressure to prevent this ingress.
More brewpubs perhaps use sheet filtration as their primary clarification method. Again, DE is the primary medium in the compressed sheets that fit into a metal or plastic frame. Cloudy beer is pumped through the sheets in one direction, and yeast and haze particles are trapped in the shallow body of the sheet.
These filters are far less forgiving than horizontal pressure leaf filters. Because they cannot remove substantial volumes of particulate matter, they don’t have the same “dirt holding” capacity. So in general the beer must be quite clear to the naked eye before it is filtered or the filter needs to hold a lot of sheets.
One common problem occurs when the sheets are inserted into the filter the wrong way around. All the plates down one side of the filter are the inlet plates where beer flows into the filter chambers on either side, and all the plates down the opposite side are the outlet plates that collect the beer from the other side of the sheet and send it out of the filter. Thus the filter sheets should be aligned so that the rough side of the sheet is facing either side of the inlet plate, and the smooth side of each sheet faces the outlet plate.
A fact of life with sheet filters is that they drip beer to some degree. This can be reduced by applying the correct pressure in tightening the filter. Over-tightening will reduce the filter’s performance. Manufacturers recommend replacing the rubber seals and gaskets after 1.5 years of operation and the new silicon seals every 2 to 2.5 years. Worn seals force operators to over-tighten the filter and increase leaks and contamination risks.
All the seals should be replaced at the same time to ensure uniform tightening. A small piece of grit or dirt between the tightening piston and the end plate can result in non-uniform tightening and leaks.
Cleaning and Sterilizing the Filter
The standard method of sterilizing a sheet filter is with heat. However, most brewpubs use Noryl (a type of plastic) plates, and they cannot be exposed to extreme temperatures. If you are using stainless steel sheets, then simply pump 190° F water through the filter until all parts of the filter reach at least 180° F. The Noryl plates should be sterilized using chemical methods.
During their manufacture the sheets are exposed to temperatures of more than 145° C. So provided that the sheets are clean, well stored, and wrapped in their plastic covering, they should be relatively microbe free. Most brewery sanitizers are appropriate for sanitizing the filter sheets, although using an oxygen-donating sanitizer (peracetic acid) isn’t advisable unless you flush the filter dry with CO2 prior to adding beer.
One advantage sheet filters have over DE filters is that they can be regenerated during a filtration run by flushing backward with hot water. The filter can be set up with stainless steel T-sections and valves before the inlet and the outlet to rapidly switch from hot water to beer during a filter run.
It is necessary to keep a 0.5 psi back-pressure on the filter during back-flushing to prevent the sheets themselves from eroding. Back-flush initially with cold water for 5 minutes or so, then use water at 160° F for 10 minutes or so. Cool the filter with cold water before sending beer through it again.
Other Quick Hints
It is a good idea to have some spare plates in reserve in case a batch is extra cloudy.
Operate the filter slowly, allowing the filtration to take a couple of hours. This will better use the full depth of each sheet and ensure that the whole surface area is used. Use a variable speed pump and set it to pump slowly.
Use the sheets more than once, by all means. Just store them in the walk-in cooler between uses. Sterilize the sheets in place after use, and don’t store the sheets for more than three days. Always re-sterilize the sheets before the next use. If there is mold growing on the outside of the sheets, then it’s on the inside, too. Throw the sheets away.
Professional literature suggests that a caustic clean is possible with the assembled filter, and there’s really no reason why not, provided the caustic is not too strong (less than 2 percent). The problem of rinsing all the caustic out of the sheets is difficult, time consuming, and costly, however, and requires a neutralizing rinse with acid before the filter can be used.
It is possible to do a two-stage filtration — rough followed by polishing — within the same frame. A special diverter plate is needed that collects the output from the first section and diverts it through the second.
The sheets fresh out of the packet contain some loose DE. They must be forward rinsed with hot water to hydrate the sheets and remove the cardboard/DE taste from them.
Most brewpub brewers use DE or sheet filtration to clarify their beer, but brewers who distribute beer in bottles may wish to add the additional filtration step of cartridge filtration to their process to improve the shelf life of their bottled products.
Some small breweries use cartridges to filter water early in the brewing process, but most frequently cartridges are only used in distributing breweries to sterilize beer prior to packaging. This is because all the filtration in the cartridge method is done on the surface, relying on the fact that the holes in the membrane are smaller than the bacteria trying to get through. If a cartridge filter is presented with a beer that contains a lot of yeast, haze, or bacteria, the filter will quickly block, making membranes less preferable for liquid-to-solid separation. Only a microbiological analysis in the laboratory can detect the benefits of cartridge filtration.
Non-distributing brewpubs should be forewarned that this extra-fine filtration method can remove some flavor and color compounds from the beer.
In an article on membrane filtration (“Sterile Filtration for Brewpubs,” May ’98 BrewPub), Mark Witty points out that it is unwise to rely on a “catch-all” filter to correct any process errors after the fact. Instead, breweries should — and most often do — focus on quality throughout the entire process.
Ensure that the cartridge is not being asked to remove too much yeast, haze, or bacteria. If it is, using the filter will become expensive. The bubble test apparatus to test the integrity of the filter is a wise investment. To check the effectiveness of the membrane, develop a microbiological testing laboratory on site.
Published in the July-August 2000 issue of BrewPub Magazine.