Every pool owner wants clear and sparkling water, and one of the most important aspects of achieving this is the filter system. Filtration science can become very complex, but there are some basic mechanisms at work and a straightforward set of metrics that are used to compare filtration media. Let’s explore how these relate to cartridge systems.
Cartridge is popular because it delivers a balance of high water clarity and ease of maintenance. To understand how cartridge meets these needs, we first need to understand filtration mechanisms. There are three major mechanisms that occur: surface filtration, depth filtration and cake filtration.
Surface filtration is the retention of captured particles on the surface primarily by a sieving process. In other words, particles larger than the media’s pore size will be captured.
Depth filtration occurs as smaller particles pass into the channels of the media and become trapped. Depth filtration is not a principal mechanism but over time does affect its overall useful life.
Filtration is primarily based on both surface filtration and cake filtration. As soon as the first layer of particles has accumulated on the surface of the filter media, this “cake” begins to act as the filter. As the cake builds, a gradual reduction in the effective pore size occurs as some of the pores of the media become blocked, and the filtration becomes more efficient at removing smaller particles.
The cake’s ability to capture small particles relies on the stability of the cake, which is controlled by flow rate and particle size. Generally, lower flow rate allows a more stable cake. However, flow rate can be too low, and the captured particles will not form a firm cake. Conversely, a high flow rate will physically drive particles through the media support.
To determine how a product will perform in surface, depth or cake filtration, competing yet interrelated characteristics must be evaluated. By looking at key attributes of pool filter media, it is possible to quantitatively evaluate multiple media options.
Uniformity of the filter media — This refers to consistency of thickness, weight, and resistance to flow. Uniformity is the single most important property. Thick and thin spots in media adversely affect other media properties. A thin spot allows pool water to pass, which limits the filter’s ability to maintain a consistent filter cake.
Filtration efficiency — Efficiency is dependent primarily on the size and uniformity of pores. Mean pore size is measured in microns by instruments called porometers. (1 micron = 0.0000394 inches.) As an example, a grain of sand is 90 to 110 microns. Particles less than 10 microns are usually those that cause pool water to be cloudy. However, the pore size and the resultant size of particles removed by a filter media is a complex function of several interrelated structural variables including fiber size and shape.
Pleat rigidity — This is the ability of the pleats in a cartridge filter to resist deflection. Retaining pleat shape and spacing under the force of water flow is important to maintain the surface area of the cartridge. This property is measured with specific lab tests, but easily noticeable to the touch.
Mass — The overall mass (unit weight and thickness) of the filter media affects surface area, higher dirt loading capacity, and overall greater strength. Performance generally increases as the weight of the media increases. The heavier weight and thickness provides more mass for greater strength that is related to longer life, more surface area for small particle capture, higher dirt-holding capacity and stronger pleats.
Fiber size — Denier per filament (dpf) is the standardized measure of fiber weight at a length of 9,000 meters. A decrease in denier will increase the number of fibers in a given area, resulting in smaller fiber diameter and pore size. As the fiber diameter decreases, smaller particles will be captured.
Fiber shape — The surface area of the fiber also has an effect on filter performance, improving the development of all three filtration mechanisms. A trilobal fiber cross section has 42.5 percent greater surface area than a round fiber at the same dpf and polymer. The greater surface area of a trilobal fiber can capture more small particles by adsorption because of van der Waals forces (See sidebar). A trilobal fiber, with its higher surface area, greatly improves filtration efficiency.
Neely is global marketing manager at Fiberweb.
Baker is a senior research fellow at Fiberweb.
Van der Waals force is a weak force of intermolecular interaction between electrically neutral particles that collide with or pass very close to each other. The van der Waals force is caused by the attraction between electron-rich regions of the colloid particle and electron-poor regions of another. The attraction is much weaker than a covalent, ionic or hydrogen bond. Van der Waals forces are the intermolecular forces that cause molecules to cohere in liquid and solid states of matter and are responsible for surface tension and capillary action.