- 28 June 2006-

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Can you measure it?

A vital factor in any discussion of purity as an indicator of quality is the ability of the process operator to measure impurity levels at low concentrations to a sufficient degree of accuracy. It is as true in determining a product's quality as it is in running the national economy that: "if you can't measure it, then you can't manage it".

With some modern filters and filter media, the lowest levels of impurities passing through the filter may be below the detection levels even of modern-day testing equipment. It is thus pointless to stipulate a maximum level for an impurity in, say, drinking water, if detection equipment or methods are not able to determine its presence at this low level.

Some filter media, especially membranes, are capable of this almost complete removal, certainly of particulate contamination, and if it were not for the problems of cost and high operating pressures, membranes could probably solve all product quality problems.

The medium is the key

The comments just made about membranes suggest that better filter media are the key to better product quality, wherever filters are part of the production process. This is true whether the objective is total removal of solids or a classification of the solids in suspension, to remove the oversize. The world of filter media is continually expanding, with the advance driven partly by the continual development of new polymers, which quickly find themselves available as woven filament or extruded non-woven media.

The greatest advances are, of course, in membrane media, because the wish for higher quality, leading to demands for better filtration performance, frequently points to the microfiltration and ultrafiltration membranes as the right material for the job. The extra cost of membrane processes, itself reducing as demand increases, is easily offset by the benefits of the more precise separation.

Applications

This article set out to describe how filters are helping to achieve better product quality, and the reader will expect this to refer to actual manufacture of useful or saleable products, rather than the increasingly wide, and increasingly important, uses in waste treatment and protective air systems. There are, of course, hundreds of applications for filters in industrial processes, and in almost every case, better product quality can be achieved by use of a better filter - as can be seen from every issue of this journal.

Probably the most important application is the treatment of raw water, to make it drinkable, or suitable for use as a coolant in industrial processes (including power generation) or as a process feed material. For much of its long history, fresh water treatment has seen slow development of methods, with major reliance upon the deep bed filter. Only recently has the need become apparent for the more efficient filtration capable of dealing with, first, bacteria, and latterly viruses and other pathogens. On top of this, there is growing demand for ground water to be decoloured, and, to a lesser extent, hard water to be softened. This has led to extensive use for membrane systems as polishing units after the main filtration is complete, and even to the use of micro- and ultrafiltration by membranes as the main treatment process. The Water Framework Directive is having pronounced effects on water company investment, and filter manufacturers should be seeing good business for the resultant planned extensions.

From fresh water it is a short mental step to beverage production - the home for a large number of filters. After the hiccough over asbestos use in filter sheets, the filtration industry has developed media able to give the required clarity to beers, ciders, wines and bottled waters. A wide range of media is employed in such filters, with beverage quality an important feature in the constant jostling for market share.

The consumer is increasingly looking for clarity in extracted juices and oils, and whereas a sedimenting centrifuge was once able to produce an acceptable product, polishing filters are now used more and more to give this clarity.

The pharmaceutical industry has a deep-rooted interest in clean processing, and filters can find wide application for this purpose, although the final production step is quite likely to use non-filtration separation processes such as chromatography.

Tolerances in most manufactured things are getting tighter, so that fuels and lubricants must be less contaminated by suspended dirt. The filters used on engines, mobile or stationary, must improve accordingly, and paper media are giving way to non-woven media to this end.

The recovery of waste oils suffers, as do all such recycle systems, from the problems of collection of small quantities into volumes that are worth processing - but, once this problem is solved (by legislative or financial means), there is a large scope for filtration in the recovery process. Waste oils will probably have to be treated to even higher standards than fresh oil supplies, in order that people will buy them.

These applications have all featured liquid products. The mineral dressing and mineral product processing sectors, however, have a major interest in the solid fraction from a filtration process. Now, although new media are having an important part in product quality improvement, it is as much in filter design that better quality can be achieved. An extra percent or two of mother liquor removal or of final dryness of the cake can work wonders on the overall process economics, and give better products at the same time. More gentle handling in the cake discharge zone of the filter can ensure that the final dried product has the as-produced crystal or particle shape, with minimal damage.

Whether it be a clearer product or a safer one, the continual improvements in product quality, expected by the final consumer, owe a great deal to the better filters and filter media that are being developed to meet these demands.

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