- The Filtration Society - http://www.filtsoc.org -

Volume 15, Issue 3

A REVIEW OF SOME DEVELOPMENTS IN FILTER MEDIA
Richard Lydon (pages 146-152)

It is often said that at the heart of any filter is the filter medium. It determines whether a filter process will perform adequately to give the desired separation efficiency. One of the key roles of a filter medium is to create a clear separation of particulate from a liquid or gas with the minimum consumption of energy.

Technical textile filter fabrics remain one of the most popular groups of filter media and are an essential part of countless industrial processes, contributing to product purity, savings in energy/production costs and a cleaner environment. For many years the basic structures used have remained unchanged with minor incremental improvement in fabric construction. In the past few years the development of new and improved technology combined with new measuring techniques has resulted in new filter media constructions.
This paper describes some of the recent trends and improvements in filter media and how these factors are affecting application performance.

RESEARCH AND DEVELOPMENT ACTIVITY ON FILTRATION AND SEPARATION IN JAPAN – MILESTONES AND STATE OF THE ART
Eiji Iritani (pages 153-161)

Up to now Japanese researchers have been making important contributions to the developments of solid-liquid separation engineering. Modern cake filtration theory, which can elucidate the internal flow mechanism in compressible filter cakes, was developed by Shirato, together with Tiller. Several ingenious testing methods have been presented for easily obtaining compression-permeability characteristics. Properties of compressible filter cake are considered to be one of the critical factors influencing the fouling behaviour in membrane separation such as microfiltration and ultrafiltration. In clarifying filtration, Iwasaki laid the basis for deep bed filtration theory, and Shirato et al. established the generalised blocking filtration law that is also applicable to non-Newtonian, fluid-solid, mixtures. The Terzaghi-Voigt combined model which considers the creep effect of secondary consolidation, proposed by Shirato et al., served as a basis for evaluating consolidation behaviour during the expression of compressible materials. Also in industry, many important contributions have been made, including the production of various kinds of membranes, submerged membrane bioreactor, fully-automatic filter press, expression-type filter press, and so on.

FILTRATION AND DEWATERING IN WASTEWATER TREATMENT
Morten Christensen (pages 162-165)

Wastewater contains organic materials, nitrogen and phosphorus that are usually removed biologically, after which solids material is separated from the liquid in a clarifier or by a membrane. A by-product from biological treatment is excess sludge which has to be dewatered by using, for example, belt filters or sludge mineralization beds. Sludge is difficult to filter and the filtration rate does not increase with pressure due to high cake compressibility. However, a large variation is observed between different wastewater treatment plants.

In the work presented here the best dewaterability is observed for sludge that contains large strong compact flocs, without single cells and dissolved extracellular polymeric substances. Calcium ions improve floc strength and dewaterability, whereas sodium ions, for example from road salt, seawater intrusion and related industries, reduce dewaterability because the flocs tend to disintegrate at high conductivity. Storage lowers dewaterability, especially when storage takes place under anaerobic conditions. High shear has a tendency to destroy the flocs and reduce dewaterability. Thus, pumping and mixing should be done gently and in pipes without sharp bends.

THE IMPORTANCE OF PARTICLE SHAPE IN DETERMINING THE MEASUREMENT UNCERTAINTIES IN THE MAXIMUM PORE SIZES OF SAND SCREENS
Keith Brocklehurst, Andrew Hassall, Graham Rideal and Abi Stewart (pages 166-174)

Unlike plain square meshes such as test sieves that can be measured directly by microscopy, the complex 3-dimensional structures of sand screens means that they are opaque to image analysis. Challenge Testing with glass beads has been used for many years but there has been some uncertainty in the results, especially when measuring maximum pore size because the penetrating beads are not always spherical.

The work presented in this paper took a 180 µm (micron) test sieve as a baseline and measured the cut point by three different methods: direct measurement by microscopy, a gravimetric Sonic Challenge Test and measuring the penetration of precision glass microspheres. Provided that non-spherical particles were removed from the analysis, excellent agreement was seen between the methods with uncertainties of less than +/-5 µm. When challenge testing a 175 µm sand screen, a cut point (D97) uncertainty as low as +/-3 µm was found. However, when looking for the Maximum Penetrating Spherical particle (D100), an uncertainty of +/-50 µm was seen making this parameter statistically unsound. The uncertainty increased to +/-68 µm if particle length rather than the Geometric Pore Size was used in the calculation.

NON-STOCHASTIC LATTICE STRUCTURES FOR NOVEL FILTER APPLICATIONS FABRICATED VIA ADDITIVE MANUFACTURING
HazmanHasib, Allan Rennie, Neil Burns and Louise Geekie (pages 174-180)

Non-stochastic lattice structures are widely used in a variety of applications such as biomedical implants and heat exchangers. However, the utilisation of these structures for filtration applications is rather new. Additive manufacturing techniques such as selective laser melting allow lattice structures to be bespoke depending on the type of filter and its intended function. This study considers the flow characteristics and structural strength of a disc filter with a layer of repeated 1.8 mm lattice unit cell as the filter mesh. Computational fluid dynamics simulation is used to analyse the pressure and flow velocity across the filter, while finite element analysis (FEA) is used to analyse the structural characteristics of the lattice mesh under fluid load.

The results show a minimal decrease in pressure and small increases in velocity, with the mesh capable of withstanding higher loads. The ultimate failure load of the structure is also determined. These findings indicate that more layers of lattice structures could be used as filter mesh and the flexibility of AM allows the filter properties to be tailored as required for a given application.