Have you ever wondered why it is that some filters cost more than others? Or perhaps how two filters that physically look identical can be vastly different in price? Or even why it is that filters that seem to be very similar in porosity (e.g. 0.5μm vs. 0.45μm) can vary enormously in performance and price? In this short article we will explore some of the reasons for these apparent anomalies in the world of filtration products.
Filter Types and Construction
There is a wide variety of filters available on the market, and previous publications have discussed many of these differences in terms of wine filtration products (Bowyer, Edwards and Eyre, 2012; Bowyer, Edwards and Eyre, 2013; Bowyer and Edwards, 2014). Differences between filters are not always apparent, although with some understanding of certain factors these differences can be ascertained, and for this a technical data sheet is typically required. Filter construction varies according to the design function of the filter. While it is possible to filter pretty much any liquid with 1 filtration stage, this is not a cost effective enterprise. (For example, sterile wine filtration can be achieved by simply pushing the wine through a 0.45μm integrity-tested membrane, which is fine if you don’t mind installing a new wine membrane every 5 minutes) For this reason filters are constructed according to purpose.
Nominal vs. Absolute
This terminology refers to the manufacturing tolerances employed during construction of the filter. A nominal filter will have broader manufacturing tolerances, and so the variations in effective porosity are greater than for an absolute filter. These differences can be accurately defined, and for absolute filters this is referred to as an efficiency rating or a β-ratio. The β-ratio of a filter is the ratio of particles captured to that passed for a given grade. For example, if a 1μm filter retains 1000 particles of a size greater than 1μm but allows 1 through for every 1000 retained, the filter is said to have a β-ratio of 1000. This can be converted to a % efficiency by using the formula below:
Efficiency = 100(β – 1)/β
|1000||99.9||For some manufacturers|
Table 1: Conversion data for β-ratio and efficiency.
An efficiency of 99% sounds pretty good, right? This value is perhaps acceptable for a nominal filter, but not for an absolute filter, and this is where the technical data sheet becomes invaluable. Some manufacturers do not make it clear whether their filters are nominal or absolute. Others do not present this information in a format that is easy to understand. Yet others consider an absolute filter to have a β-ratio of 1000 (99.9 % efficiency), but for most this figure is 5000 (99.98 %).
As an example, consider the filters in Figure 1. The upper filter is an Amazon SupaGard, a nominal depth filter often used for water filtration. The lower filter is an Amazon SupaSpun, an absolute depth filter, also used for water filtration. Both filters are made of spun-bonded polypropylene of the same porosity rating and have a polypropylene core. Both filters physically look the same and have similar void volumes, yet their efficiencies and performance vary due to the different ways in which they are fabricated, and this is reflected in the price difference between them. The correct filter for the job depends on the application, desired outcomes for the process and the budget of the customer, and it is BHF’s job to determine which is fit for purpose.
Figure 1: A comparison of two visually indistinguishable depth filters, one nominal (upper) and one absolute (lower).
To complicate matters further, no standard method exists for determining filter performance. Moreover, β-ratio will change throughout the life of a filter, since as it loads up during usage the effective porosity decreases, meaning the filter becomes more effective as it blocks up. The end result is that any nominal filter will be less “efficient”, in the sense of particulate retention capability, when it is new.
In terms of wine depth filters, these are typically in sheet or lenticular format. In almost all cases these filters are nominal, which is why performance across brands is not as simple as comparing sheet materials of the same stated porosity. Porosity typically spans a range (e.g. 1 – 2μm) for any given grade of filter sheet, to encompass the spectrum of retention capacity of that nominal grade (Figure 2). Further, some grades of sheet are described as “sterile”, but this again is a nominal term, since only an integrity-tested membrane can truly be considered to be a sterile filter.
Figure 2: Excerpt from a Becopad porosity chart, indicating both nominal porosity (μm, on the ordinate axis) and nominal flow rates (Lm-2min-1, on the abscissa). Porosity and flow rate are usually proportional.
Construction Material & Filter Type
Filter construction varies according to application. Situations involving harsh chemical treatments require highly resistant production materials, such as polypropylene. If no harsh chemicals are being used (caustic soda being a problem in some applications in this regard), a glass microfiber medium can be extremely effective. End caps on filters can be reinforced with glass fibres to provide increased strength is repeated sanitisation is required, such as for wine membranes. We have discussed in detail the differences between nylon and polyethersulfone (PES) wine membranes in a previous publication (Bowyer, Edwards and Eyre, 2013). Aside from chemical differences between these two polymers, there are significant physical differences that impact on the way the filters function, in terms of colour stripping, flow rates and even integrity testing.
If the application requires a high dirt-holding capacity, typically a depth filter will be used, such as the lenticular filters commonly used for cellar filtration or as wine membrane pre-filters. These have some thickness to the filtration medium, designed to entrap and retain particulates. The very nature of this physical entrapment mechanism makes the regeneration of depth filters quite difficult, unless the retained particulates can be dissolved and pushed through the medium, in which case a forward flush is equally valid.
If the application requires a higher flow rate, or of the filtration stream is not overly burdened with particulates or microorganisms, a pleated filter is more suitable, as it has less depth capacity but presents a much higher surface area to the flow stream, which allows a higher flow rate. Wine membranes represent the extreme of this philosophy, in that they have very high surface areas, facilitated by a thin layer and excessive pleating, but very little depth.
When visualising a membrane cross section it helps to think of a very thin sponge. In the case of nylon membranes, they are typically cast in a symmetrical manner in terms of cross section, so when they encounter load and block, they block on the surface and minimal “depth” in the membrane is used. PES membranes are typically cast asymmetrically, with coarser outer regions and a progressively tighter core, which allows them to not only flow faster but also to exhibit some “depth” capacity, a characteristic not usually attributed to membranes (Figure 3). This in turn can result in a longer service life.
Figure 3: A sectional comparison of nylon (left) and PES (right) membranes, illustrating the typical differences in symmetry between membrane types. Note the tighter, inner section of the PES membrane.
Filtration porosity is typically expressed in terms of microns (μm). Unless two particulate filters being compared are absolute-rated with the same β-ratio, porosity comparisons are relatively meaningless. Filters come in a range of porosities, and so can be optimised for performance according to the task at hand. In terms of wine this pertains to the specific loading that the wine will present to the filter. A question that is often asked is “what grade of filter should I use to filter my wine at X NTU to get it down to Y NTU?”. This is impossible to answer accurately without an understanding of the particle size distribution and colloidal status of the wine. Usually a filtration grade estimate is made based on a combination of experience and historical data.
Considering only the stated porosity of an absolute-rated filter, there should be little difference between a 0.5μm particulate filter and a 0.45μm membrane, since there is only a difference of 0.05μm, yes? Not so! Particulate filters and membranes are evaluated for efficiency in two different ways, and they are not transposable. Particulate filters use β-ratios, but membranes use log reduction values (LRV)
Log Reduction Value (LRV)
A log reduction value is another way of expressing filter efficiency. It is typically used to describe the efficiencies of membranes at removing micro-organisms, as the numbers become too high for β-ratios to be conveniently used. For example, the Parker Domnick Hunter Bevpor PH 0.45μm wine membrane is fully retentive of Saccharomyces cerevisiae, and also has a stated LRV of 9.1 for the organism Pseudomonas aeruginosa, meaning that the β-ratio for the latter organism is 109.1, or 1,300,000,000:1, or 99.9999999% (Figure 3). LRV’s are often expressed for several micro-organisms on a technical data sheet, but the organisms tested are not common to all filter manufacturers, and a direct comparison requires commonality of test organisms and conditions. LRV’s are also more meaningful for organism retention since organisms are able to deform, whereas particulates typically are not, and so a measured organism challenge yielding an LRV will provide more meaningful data than a simple particulate retention test.
Figure 4: Excerpt from a technical data sheet for the Parker Domnick Hunter Bevpor PH indicating LRV’s for several organisms across 3 levels of porosity.
Integrity testing is a method whereby a membrane can be confirmed as being integral, with no holes or leaks in or around the filter. The filter is wet out completely, then the upstream side of the membrane is sealed off and pressurised, and the pressure drop over time measured, often with the use of a specialised pressure measuring device called an integrity tester. The gas (usually N2), will slowly diffuse through the wet membrane to the open downstream side of the filter at a defined rate. All test parameters are specific to the housing and upstream pipework volume, filter type, size and porosity, and the test is strongly affected by temperature. Provided the pressure loss over the test period is below the calculated allowable value, the filter is declared as integral and fit for purpose. Since this process cannot be applied to cross-flow filters, they should be considered as non-sterile filters only.
Comparisons of different filters is not a straightforward process, and several factors should be taken into consideration. Ultimately, testing and/or process trials must be undertaken to evaluate true cost-effectiveness of any filter. A filter that is cheaper to buy initially may ultimately lead to higher ongoing costs in terms of filtration performance and associated staff time allocation for change-outs.
Paul K. Bowyer