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

Volume 8, Issue 2

D. Ward, S. Cupples, R. Poole and I. Owen (pages 114-119)

The findings reported in this paper demonstrate that ‘filter cake’, and its associated contribution to head loss in a media pressure filter system, can be suppressed by the employment of a swirl-inducing tangential inlet without any loss in separation efficiency. The likely mechanism for this performance enhancement is the creation of a vortex-induced crossflow scouring action which prevents cake formation. Instead of being retained as a thin, dense layer at the surface of the filter, particles are believed to be either held in motion above the bed or given the opportunity to penetrate the bed and collect over a greater bed depth, thus producing a lower contribution to overall system head loss by utilising media voidage more effectively. This further improves filter performance by reducing backwash cycle frequency and therefore the overall amount of contaminated backwash water requiring disposal.

T. Peters (pages 119-123)

Even though important innovations have been created in the last two decades that contributed to the realisation of a great variety of membrane-based solutions and industrial processes, there can still be further optimisation and important improvement in different areas of membrane technology. One of the areas is water treatment or purification with ultrafiltration, including drinking water, process water and waste water. Due to the different spectra of components or degrees of contamination in these liquids, a wide range of possibilities regarding the design, manufacturing and operation of related plants has had to be developed.

At one extreme are waters with a low level of contamination, that can be processed with the deadend operation mode. At the other are waste waters or process waters with a high burden of components or contaminants, that can only be treated with the crossflow operation mode. One example for an innovation in low contaminated applications is a module with capillary membranes with a much higher membrane area than usual and improved internal hydraulics based on flow dynamic calculation. At the other extreme a new system has been developed where the crossflow velocity of the liquid over the membrane is realized by pumping this liquid through nozzles into the open channel formed between two disc membrane cushions fixed as a disc cartridge.

P.R. Johnston (pages 124-128)

In rating a filter medium for a possible filtration job, the outline approach is to determine (1) materials of construction, (2) thickness, (3) porosity, (4) viscous permeability, then (5) calculate the viscous flow averaged pore diameter from (3) and (4). For some uses a strength test must be performed. Media composed of random arrays of materials, such as mats of short fibres, sintered granules, and solvent-cast membranes, all have the same kind of pore diameter distribution – the same ratio of the standard deviation to the mean – making it possible to understand fluid extrusion measurements that seek pore size distributions.

The method of determining the bubble-point – starting with a liquid-soaked medium, forcing air against the bottom face to blow open the largest pores on the top face – is an eyeball determination and not standardized. Extending that test to blow open successively smaller pores, gives the ‘mean flow pore’ – half the pores have been blown open. Yet gas flows are then in the inertia, and sometimes Knudsen, range rather than in viscous flow. The ‘mean flow pore diameter’ is 3/4 of the viscous flow averaged pore diameter. The liquid drainage test – air on the top face forces liquid out the bottom face, but stops short of blowing gas out of the bottom face – reflects the viscous flow pore diameter distribution on the bottom face. The mercury incursion test is not useful.

Abdel-Salam (pages 129-134)

Impacts of aerosol particles on human health have led to the design and development of a wide range of aerosol samplers; primarily for measuring airborne aerosol concentrations and human exposure. Many such instruments, based on a wide range of different physical principles, have emerged over the years, and some have been commercially successful. However, conventional pumped aerosol samplers still have limitations which make the personal sampling process difficult, including: high cost; high power demand; limited sampling time; and invasive to wearers due to high noise levels, heavy weight, large size, and loose tubing. The new aerosol sampler design described in this paper reduces or eliminates many of these limitations.

The new sampler design utilizes a low resistance electrostatic filter media (electret) to collect aerosol in a fan-driven system, eliminating the greater power requirement of traditional air sampling pumps. Use of electret media offers advantages in the design of an aerosol sampler. Lower filter resistance requires less power which allows use of smaller batteries and fans, and this has led to the design of an integrated electret-fan personal aerosol sampler. Besides size and weight reduction, the new sampler design also has the added benefits of quiet operation, long sampling duration, and low cost. The new personal aerosol sampler has less invasive design that allows more convenient use in home and office environments, as well as industrial and ambient environment settings.

T. Bahners and E. Schollmeyer (pages 135-139)

The scope of the presented work was to study the separation of micron and sub-micron model particles from fluid systems by a deep-bed filter concept consisting of a multilayer stack of combined nanofibre/nonwoven layers. The nanofibres were intended to form an irregular small pore system with both high porosity and high flow properties. Nanofibres of an average diameter of 12515 nm were produced from polycaprolactone (PCL) by electro-spinning, a commercial nonwoven serving as the base of the individual layers. Polystyrene (PS) spheres suspended in water served as the model system. The model filters were characterized with regard to separation efficiency, flow rate and separation morphology.

The measurements showed a significant improvement in the separation of 2 µm particles by adding the nanofibre web. A two-layer stack of the combined filter system exhibited a separation efficiency significantly higher than 90%, which compares to 45% for a similar stack of the conventional system. While the nanofibre web reduces the flow rate, the superior performance results in a positive net effect. High separation efficiency was also found in the sub-micrometre regime. SEM analyses indicate an increased importance of impact separation in the filtration process.

J.O. Babatola, T.O. Saiki and M.O. Ogedengbe (pages 140-143)

The role of aeration prior to coagulation in conventional surface water treatment was investigated. Samples of stream water, some aerated for varying periods by rigorous mixing and some unaerated, were treated with varying dosages of 1% solution of commercial alum. The samples were rapid-mixed, slow-mixed and settled. Turbidity readings and pH values were taken. The results show that pre-aeration is beneficial in terms of up to 30% saving in alum consumption and improved settling.

A. Häkkinen, K. Pöllänen, S.-P. Reinikainen, M. Louhi-Kultanen and L. Nyström (pages 144-153)

The behaviour of solid/liquid suspensions during filtration processes is strongly influenced by the properties of the particles and the liquid phase. Although prediction of common filtration characteristics, such as cake resistance, cake porosity and compressibility, has been studied extensively, general theoretical models that could be applied to complex real-life suspensions do not exist. Prediction of filtration characteristics has proved to be difficult even in those cases where an extensive set of experimentally obtained material data are available. This is due to the complexity of the cake formation process, which means that the number of influential variables that should be simultaneously considered in the models is large. Traditionally applied calculation and modelling techniques have been incapable of processing such large sets of input variables, which has consequently restricted the complexity of the models.

This paper introduces an alternative procedure for predicting the filtration characteristics of solid/liquid suspensions from measured material properties. Empirical models were created using multilinear partial least squares regression (N-PLS) for the experimentally determined pressure filtration parameters and the particle size and shape data obtained by an automated image analyzer. The density and dynamic viscosity of the liquid phase were also included in the models as input variables. The filtration characteristics of the test suspensions were described by four different parameters and separate models were derived for each parameter. All four models were tested with an independent set of samples in order to validate the created models. The results presented in this paper show that the procedure can be applied to create models that enable filtration characteristics to be reliably correlated with particle size and shape distributions.

W.W.-F. Leung, J. Chung, C. Tsang and C.-H. Hung (pages 154-164)

Submicron particles can be pollutants from engine emissions (0.02-1 μm), or airborne contagious virus (0.05-0.2 μm). The N95 face mask has been employed to remove the finest particulates in the submicron range. In our filtration experiments, exhaust gas from a diesel engine laden with submicron particles was used to challenge the test N95 face mask with a gas velocity of 0.05 m/s. Measurements were taken over an extended 6 h period under heavy solids loading.

Among the three layers only the middle ‘filtration’ layer of the N95 face mask was used in the test. Over the 6 h period when solids loading generated from the engine emission increased linearly from zero to 2.83 g/m2, the pressure drop across the filter increased nonlinearly from 19.6 Pa at no loading condition to 36.0 Pa. The ratio of downstream particles (penetrating the filter) to those upstream stayed relatively constant for 0.3 μm particles, whereas for 0.05 μm particles this penetration ratio decreased rapidly from 30 to 0.2 %. The reduction in penetration of the fine particles (less than 0.1 μm) with solids loading is quite interesting.

The quality factor qf which measures benefit versus costs showed that during extended use of the N95 face mask, the qf reduced for particles greater than 0.1 μm due to constant capture efficiency yet increasing pressure drop, whereas for particles less than 0.1 μm, qf increases due to lower penetration (i.e. much higher capture efficiency) but only at modest increase in pressure drop. Unless there is excessive sweating causing the face mask to be fully wetted with liquid for which the electrical charges of the fibres are neutralized, the capture efficiency of the mask may only be reduced slightly due to the loss of electrostatic charges in attracting particles with opposite charges.

A. Al-Mudhaf, A.J. Chamkha and J.M. Al-Humoud (pages 164-172)

A one-dimensional continuum deep bed filtration model with scale-dependent diffusion or dispersion coefficient and filter superficial velocity is considered. The diffusion coefficient and superficial velocity are assumed to increase according to a polynomial function with the filter longitudinal distance. A general mass transfer that characterizes particle attachment and detachment effects is considered. Since the general problem is nonlinear and possesses no analytical solutions, a numerical solution based on an efficient implicit finite-difference method is obtained. Comparisons with exact and approximate analytical solutions for special cases of the filtration model are performed and found to be in excellent agreement.

A parametric study of some physical parameters is conducted and the results are presented graphically to illustrate interesting features of the solutions. It is found that the presence of a particle detachment mechanism has significant effects on the solute concentration and specific deposit profiles at all filtration times. In addition, the scale-dependent polynomial-type diffusion coefficient and superficial velocity are predicted to yield significant changes in the solute concentration, specific deposit and filter efficiency at all filtration time stages compared with their corresponding constant cases.

D. Ward, S. Cupples, R. Poole and I. Owen