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Key Principles of Filtration Process Control

To achieve these six filtration quality control measures, a brewer must use sound, robust process control. So how should a filter plant be set-up and operated to achieve bright, clean, “brewery fresh” beer?

Table: Filtration process parameters

Principle

Process Control Measure
Minimise high solids entering the filter, which otherwise would “blind” the filter and shorten run length Systematic and effective removal of solids in the maturation tank before the vessel is transferred to filtration will minimise this risk. As mentioned previously, a filter pre-buffer tank can help manage any excessive solid material at the beginning, during and/or at the end of filtration. A turbidity meter will continuously monitor the beer clarity during transfer to the filter and will divert any turbid beer into the pre-buffer tank to avoid blinding the filter.
Minimise/eliminate oxygen ingress into the transfer line Air must be evacuated from all transfer lines, hoses, filters, ancillary equipment, and vessels before product is introduced. This is best achieved by filling lines with deaerated water and then displacing the water to drain with the beer. Maturation tanks must not be drawn 100% empty as this risks pulling air into the line. A low level probe should be positioned in the vessel outlet pipe which when uncovered, will cause the beer transfer from that vessel to cease.   
Prevent beer with a low temperature from entering  the filter Beer with an excessively low temperature (for example -4ºC) may contain ice crystals or create ice formation on the filter bed. It may consequently damage the filter bed or membrane integrity. Conversely, beer with too high a temperature can cause the chill haze to re-dissolve, causing haze instability in the final pack. We can prevent this using in-line temperature probes to monitor with interlocks or alarms. 
Minimise pressure shocks in the system To prevent displacement of DE from the filter screen, damaging the filter bed or causing spent DE to break through on the filter (subsequent high hazes in bright beer tank), a brewer will install a filter pre-buffer tank and/or variable speed control on the pumping to the filter to allow ramping of the flow. This principle of pressure shock protection also applies to ceramic cross-flow filters, which can crack due to pressure shocks.
Minimise fobbing during production Avoid producing fob during vessel filling; this will minimise the loss of foam positive proteins or collapsed foam, which may lead to hazy beer (due to the dissolution of proteins from the foam into the beer). Slow fill speeds at the start of transfer and maintaining consistent CO2 top pressures on vessels will aid this. 
For DE filters, effective pre-coat and filter material / body feed should be correctly managed Uneven or inadequate pre-coat could cause DE and/or yeast to break through the filter. Dosing these filter aids at the correct flow rate velocity and sizing the pumps correctly will ensure an effective filter bed is created. Maintaining a consistent target differential pressure or ∆P (difference between the filter inlet pressure and the outlet filtrate pressure) will ensure effective beer clarification. This value will vary depending on filter technology and manufacturer. It will normally be around 1.5-2.0 bar. ∆P should not exceed 6 bar. When this value is reached, it indicates that the filter has become blocked and the process should stop, washed down, and a new bed applied before re-starting. 
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