Lesson 8 - Clarification – Filtration

Filter aid preparation aims primarily to mix the filter powder slurries in a consistent manner so that it is consistent batch after batch.

The filter powder slurry must be purged with CO₂ to remove any oxygen trapped in the voids of the powder. Continuous agitation of the slurry helps to create homogeneity and displaces trapped oxygen. 

An important design factor of a filter handling system is effective dust extraction and ventilation. Filter powders can create fine dust and are thus considered a health and safety hazard. Therefore, we must install a dust extraction system to ensure that personnel do not inhale fine filter aid particles when emptying the bags into the mixing tank.

So, over and above these key requirements, other features of a filter aid mixing plant include:

• Deaerated water supply – this water should also be sterile to avoid any microbiological contamination. As mentioned above, it must have a negligible amount of dissolved oxygen to avoid oxidation and staling flavour development.

• Powder addition and make-up control – this can be manual or automated, but it must minimise dust creation. Volume measurement must also be accurate to achieve the target water to dry material ratio.

• Powder slurry outlet – this must be sized and positioned just above the base of the tank to ensure that it does not become blocked.

• Baffle plates – may be used in the tank to promote agitation and mixing, but they are not a necessity.

• Tank volume measurement – level transmitter or sight glass can be used in this instance. As a word of caution, sight glasses are liable to block due to poor drainage of the viscous filter aid powder slurry at the end of a production run.

• CIP installation – the vessel’s design must be hygienic with effective rinsing and cleaning capabilities to minimise microbiological contamination and avoid the formation of undesirable off-flavours.

• Sampling – the plant’s design must facilitate the taking of samples for microbiological monitoring.

Critical operating parameters

With all filter powders, we must build up a thickness of powder on a cloth or filtration surface that will permit the filtration of particles from the beer. As you may recall from earlier, this is called the pre-coat.

The pre-coat consists of a coarse variety of powder that is slurried with water or beer and pumped onto the filter This is then repeated with a finer layer of filter powder to form a second pre-coat. The total pre-coat layer is normally 1.5-3.0 mm thick.

It is essential that we create an even pre-coat coverage to mitigate thin regions where particles could burst through. Beer is then pumped onto the filter containing kieselguhr body feed to progressively build up the filter bed and achieve 0.5 EBC before filling the post-filter buffer tank or bright beer tank (BBT) / final product tank (FPT). Breweries use a typical target haze reading of 0.5 EBC as an acceptable reading on the outlet of a filter.

Pressure shocks must be avoided during filtration as they could destroy the integrity of the filter bed. If this was to happen, the whole process would have to stop so the filter can be washed down and a new pre-coat re-applied. We can avoid shocks by opening and closing valves slowly (manually or automatically). We should also only start beer pumps when the route to the filter is open; this minimises any pressure build-up in the line.

At this stage, kieselguhr dose rates are typically 100 g/hL. A continual increase in differential pressure from inlet to outlet of the filter (normally 0.2 bar/hr) ensures a satisfactory filtration. When this differential pressure approaches 4-6 bar, this indicates that the filter’s cavities are completely filled with kieselguhr and solid particles. The filtration run is then stopped. The filter is opened up and the spent kieselguhr is removed before either re-applying another pre-coat or doing a filter CIP.

From a production efficiency perspective, long filter runs are desirable. We can achieve this by using the correct grade of filter powder and correct pre-coat formation. If differential pressure rises rapidly, this indicates that something has gone wrong. This is normally caused by high solids coming on to the filter, perhaps greater than 0.2 million cells/mL. Again, we must ensure that we are in control of processes back up stream to minimise yeast and suspended solid material concentrations in the beer post-maturation.

Let us explore differential pressure in a bit more detail.