Handbook of Filter Media

D. Purchas, K Sutherland

• Publisher: Elsevier Science
• Year: 2002
• Print ISBN: 9781856173759
• eBook ISBN: 9780080507774
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Chemical & Biochemical Engineering
• Index: Technology & Engineering

This comprehensive handbook provides a complete and updated overview of filter media. From classification to performance date to practical selection tables.

CHAPTER 1

An Introduction to Filter Media

The process of filtration is widely used throughout industry, commerce and domestic life. It covers the production of potable water by reverse osmosis, through the protection of delicate components from the impact of large solid particles, and the clarification of beer, to the separation of waste sewage sludges. Filtration involves the physical separation of one or more components from a suspension or solution in a fluid, by passage of that fluid through or across a barrier that is permeable only to some of these components.

The key element in this description is the barrier, permeable only to part of the feed suspension or solution. This barrier is the filter medium, and a filter is then any mechanical structure that holds the medium in the most efficient way.

1.1 Definition of Filter Medium

It has rightly been said that the heart of any filter is the filter medium. Unless it is fitted with an adequate medium, even the most ingenious filter is useless. So what exactly is a filter medium?

The, now ageing, Filtration Dictionary and Glossary⁽¹⁾ defined a filter medium as ‘any permeable material used in filtration and upon which, or within which, the solids are deposited’. This definition was not broad enough for the first edition of this Handbook, because it surprisingly assumed that only solid particles are relevant, and therefore excluded the many instances where particles comprise droplets of liquid. The following improved definition was then suggested as sufficiently all embracing: ‘A filter medium is any permeable material upon which, or within which, particles are deposited by the process of filtration’.

However, even that breadth of coverage is no longer enough, because a large proportion of filtration operations now concern molecular and ionic species in solution. Accordingly, a revised definition is suggested in this new edition of the Handbook, to take note of this additional need:

A filter medium is any material that, under the operating conditions of the filter, is permeable to one or more components of a mixture, solution or suspension, and is impermeable to the remaining components.

The retained components may be particles of solid, droplets of liquid, colloidal material, or molecular or ionic species in solution, while the permeate (or filtrate) will normally be the suspending fluid or solvent, possibly together with some of the other components.

The purpose of this Handbook is to describe the materials used to make filter media, to highlight their main characteristics, and to advise upon their selection and use.

1.2 Filters and Their Media

In the definition of filter medium given above, the nature of the filter itself is not defined. A filter is any device in which a separation is achieved among other components of a suspension or solution, in a fluid – which may be a liquid or a gas – where the separation is caused by mechanical means, without the involvement of a change in phase (such as the melting of a solid, or the evaporation of a liquid). Filtration is almost entirely a characteristic of the size of the particle, droplet or molecule being separated, whereas relative particle density is a more important feature in sedimentation.

As is often the case in attempting a perfect definition or classification, the boundaries involved in defining filtration in this way are less than absolutely precise. Thus, some membrane separations do involve a change in phase; some filtration processes involve electrical as well as mechanical forces: and some processes involve chemical forces as well as physical ones. Most significantly, the process of dry screening (sieving or sifting) involves the passage of part of a mixture of granular solids through a screen – the filter medium – with little or no passage of a fluid at all, yet this is clearly a filtration process.

That said, the bulk of filtration processes involve the removal of particles, droplets or molecules from a fluid, by means of a physical barrier, the filter medium, through which they will not pass by virtue of their size. A particular form of filter may be able to use a wide variety of filter media, to achieve the same, or different, separations.

1.2.1 Purposes of filtration

There are two main purposes in filtration:

• to remove impurities from a fluid, and

• to recover valuable materials from suspension in a fluid (usually a liquid).

The first of these (‘clarification’) normally employs finely porous filter media, and aims to remove as much of the impurity as possible, and preferably all of it. The second (‘harvesting’) also aims for as complete a recovery as possible of the wanted material, but uses coarser media, mainly because the cake of recovered solid does most of the filtering, and also is less concerned with the clarity of the filtrate.

Here again the division is not exhaustively precise: some harvesting processes remove waste solids for subsequent treatment, while some clarification removes only some of the suspended solids (ahead, for example, of a finer filtration process). Filtration may also be used to classify a suspended solid into two separate size fractions.

The choice of filtration equipment involves selection both of the right medium, and of the...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• Foreword to the First Edition
• Chapter 1: An Introduction to Filter Media
• Chapter 2: Woven Fabric Media
• Chapter 3: Non-woven Fabric Media
• Chapter 4: Wet-laid Fibrous Media
• Chapter 5: Air and Gas Filter Media
• Chapter 6: Screens and Meshes
• Chapter 7: Coarse Porous Sheets and Tubes
• Chapter 8: Membranes
• Chapter 9: Replaceable Filter Elements
• Chapter 10: Packed Beds
• Chapter 11: Testing Filter Media
• Chapter 12: Filter Media Standards
• Glossary
• Index of Advertisers
• Editorial Index