Water Filters: The Basics **

Water filters can be traced to ancient times, evolving out of necessity, first to remove materials that affect appearance, then to improve bad tastes and finally to remove contaminants that can cause disease and illness. In present times, filters are used to clean water for manufacturing processes and for individual water treatment requirements, mainly to remove chlorine from home municipal water systems and other impurities that we may not want in our drinking water like fluoride, excess sediment and by-products of chlorine. Well water often requires filtration for sediment, iron, hydrogen sulfide, heavy metals, bacteria and other fluid contaminants.

Filter Definitions

Filtration is defined as the removal of a suspended particle from fluid, liquid or gas by passing the fluid through a porous or semi-permeable medium. Separation is the removal of dissolved substance from a carrier fluid stream. Cartridge filtration is usually pressure driven and the filter system is subject to pressure drop through the housing and cartridge which is affected by the fluid flow rate and density, the filtration material density and the inlet and outlet port sizes. Depending upon the application, filters may be piped in parallel or series.

Particle Capturing Mechanisms

Seven mechanisms are involved in filtering particles from fluid. 1. Direct Interception works when a gets captured after running into a physical barrier. 2. Bridging is when a two single particles together are stopped by the filter medium, but would pass through one at a time. 3. Sieving occurs when a particle is too small to pass through the opening or pore of the medium and stays on the surface or gets trapped within the medium. 4. Inertial Impaction happens when a particle starts through the medium and gets trapped within. 5. Diffusion Interception works on a molecular level. A particle is more likely to be trapped by the filter media because the molecules are in constant random motion. 6. Electrokinetic Effects mechanism works because electrical charges of the particles and filter medium can cause the particles to be deposited on the media. 7. Gravitational Settling works because particles have a mass and settle in the filter medium.

Retention

The contaminant is held in the filter by several methods. Mechanical Retention occurs when a particle is restricted from passing through a filter medium. Adsorptive Retention is when the particle stays in the filter media due to interactions between the particle and the surface of the medium. Surface and Depth Filtration relate to the particle size and pore size of the filter media. A surface filter is seen as a screen that is covered with particles too large to pass through. Particles form a filter cake on the surface and the retention will be absolute since no particles may pass, known as sieving. Depth Filtration allows particles to penetrate the filter matrix and get captured throughout the depth of the medium.

Surface Filtration

A surface filter is a screen that is challenged with particles too large to pass through the openings. Particles will collect on the surface forming a filter cake. Retention will be absolute because no particles can penetrate through the surface. This mechanism of capture is called sieving.

Sieve Retention: Uniform Pore Size

Pleated filters are designed to enhance surface filtration when correctly used. The micro-fiber sheet media has a narrow pore size distribution, allowing absolute sieving and large surface area.  This increases its capacity to retain particles on the surface. The thin medium allows higher flow rates with lower pressure drops.  This promotes the formation of a filter cake, allowing the filter to hold a high amount of dirt.

Depth Filtration

A true depth filter allows particles to penetrate the filter matrix and get captured throughout the depth of the medium. This holds true when the particle size/pore relationship matches the filtration design requirements. The depth filter matrix has a broad pore size distribution, therefore depth cartridges rely on adsorptive retention for a portion of their dirt holding capacity. Some have a gradient pore structure with tighter pores near the center core to maximize mechanical retention.  Most are made from extruded melt blown fibers (polypropylene, nylon, polyester) or twisted yarn. And there cartridges should not be subject to high flows like the pleated filters. They can filter particles from 100 microns down to 1 micron.

Depth Versus Surface

Choosing between Depth or Surface filtration will depend on your application and the cost allowed.  Pleated cartridges are more expensive then the equivalent depth cartridges, but they offer lower micron ratings and more dirt holding capacity.

Fiber Filtration Principles

Pore size of the filter is an important consideration when choosing a cartridge, and is dependent on three factors: Fiber Diameter, Porosity and Thickness of  Filter Media.

Fiber Diameter

As fiber diameter increases, mean pore size decreases. So, you would use thinner fibers for a smaller pore size and more dirt retention.

Porosity

Porosity is the ratio of the void volume to the total volume of a filter medium. It can be decreased by winding a cartridge more tightly.  Decreasing the porosity will decrease if the mean pore size, thus making the filter finer.  But decreasing porosity will also increase the flow resistance of the cartridge.

Thickness of Filter Media

As the filter medium becomes thicker, the mean pore size decreases. Also, as layers are added, the pores become smaller; and adding layers will increase the flow resistance.

Filtration Variables

Filtration performance and cartridge life depends on several factors including the following operating conditions.

Flow Rate

High flow rates through cartridges or filter media degrade the adsorptive retention mechanisms and decrease efficiency.  And, a decrease in flow rate increase cartridge efficiency and performance by enhancing adsorptive retention and the ability to from a filter cake.  Optimum efficiency for pleated cartridges average from 0.5 to 0.75 gpm

Differential Pressure

To maintain a constant flow rate through a filter cartridge as it accumulates contaminants, more fluid must flow through the progressively smaller unplugged portions of the cartridge.  This increases differential pressure and decreases efficiency.

Viscosity

Increasing viscosity (thickness of the fluid passing through the filter) increases the hydrodynamic drag of the fluid and increases the differential pressure required to push the liquid through the filter medium. Increasing the viscous drag decreases the adsorptive retention, decreasing filter efficiency.

Contaminant

The relationship of particle size distribution to pore size determines the degree of surface versus depth filtration.

Flow Conditions

Cartridge water filters are designed for use with steady flow rates, and pulsating flow rates can disrupt a filter cake and dislodge particles that were adsorptively or mechanically retained. Pulsing can also cause structural damage to a cartridge.

Compatibility

Fluids not compatible with a filter media can reduce filter efficiency. They can cause filter media to swell, became brittle, dissolve, shrink and separate from end seals causing the cartridge to fail or even break apart.

Area

Increasing filter area while maintaining a constant flow rate reduces the flux or flow density (flow rate per unit area) and increases filter efficiency.

** Reference: Filtration Product Handbook, USFilter - Plymouth Products