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Understanding Filtration

Water from the tap always contains a certain amount of contaminants, such as dirt, organic material and rust, along with chemicals and dissolved minerals. These contaminants can be broken into three basic groups:

  • Microbiological and suspended solids
  • Chemicals
  • Dissolved minerals and gases

Filters can remove some microbiological contaminants, most suspended solids and some chemicals using either mechanical filtration, chemical filtration or both. Dissolved minerals (TDS) and gases require treatments such as softening or reverse osmosis.

Filters often feature a radial flow design that allows water to flow from the outside of the filter through to the core providing. Many carbon block filters use this design. Another type of filter is Evepure’s patented Precoat, which utilizes a pleated membrane coated with powdered carbon.

Suspended solids and cysts

Microbiological contaminants can include harmless bacteria, as well as pathogens. Pathogens are families of microbiological organisms that are known to cause disease in humans, and can be a virus, a bacteria or a cyst. Cysts are living particles, and can be as small as 1 micron (the width of a human hair is 40 microns). These can be removed by filtration. A group of pathogens, known as Oocyst Cysts, are larger microorganisms, typically 4 to 7 microns, and are very resistant to chemicals used in municipal disinfection. An example of an Oocyst is Cryptosporidium. The presence of Cysts has been verified in almost every major surface water supply within the US.

Suspended solids can be both inorganic materials such as silt, clay and ferric iron, and organic materials such as decaying plants, that are carried along by water as it runs off the land and enters the surface water supply. Suspended solids create a variety of problems for the foodservice customer. They can cause poor appearance, or water that is hazy, cloudy or dirty looking. It can cause issues with equipment maintenance, such as clogging and abrasion of small valves, orifices, and smooth surfaces.

Mechanical Filtration

Mechanical Filtration is the process of removing or separating suspended solids from the water. In simple terms, a mechanical filter is a barrier with a large number of tiny holes. The mixture of water and solids is pushed through the barrier by water pressure and any solid particles larger than the holes are trapped. The size of these “holes” determines the micron rating. A micron is the measurement used to describe the physical size of solid particles in any water supply. As a point of reference, one micron equals approximately 1/25,000 of an inch, and solid particles that measure less than one micron in size are occasionally referred to a “colloidal,” or sub-micron, particles.

The majority of mechanical filter products use simple, flow-through designs with Nominal Micron Ratings. Nominal means that approximately 85% of particles the size of the micron rating will be blocked by the filter. A few product designs are available with extremely high levels of particle reduction at small micron ratings. Most of these products were developed for consumer drinking water and foodservice applications, and usually carry one of the NSF / ANSI Standard 53 certifications for the reduction of cysts, turbidity, or asbestos fibers. Three features of mechanical filters need to be considered: the micron rating, the flow rate requirements for a mechanical filter product, and the total capacity of the mechanical filter.

Sediment prefilters are an example of a mechanical filter. Most foodservice prefilters have a micron rating ranging from 5 microns up to 50 microns, and are designed to remove larger particles. These are placed before other filtration systems to extend their life.

Chemical Filtration

Filtration of chemicals such as chlorine and chloramine is typically achieved using carbon. Food grade carbon can be manufactured from various natural materials such as wood and coconut shell. Carbon is extremely porous, which gives it tremendous surface area. It is typically either carbon block, or granular activated. With carbon block filtration, the filter cartridge is manufactured with powdered carbon and polymer resins that are either baked in molds or extruded into a spool shape. The carbon spool is often wrapped with a filter membrane that provides the mechanical filtration. Granular activated carbon, or GAC, is carbon that is ground into powder and then fused together into granules of consistent size.

Carbon can remove chemicals by either absorption or by adsorption, also called catalytic reduction. Absorption is where the molecules of the chemicals are attracted to the carbon and adhere to it. Adsorption or catalytic reduction is a process by which the carbon breaks the chemical down. In the case of chlorine, carbon reduces it to a chloride ion.

Organic chemicals are removed by absorption and disinfectants such as chlorine and chloramine are removed by adsorption. The amount of chemical reduction by carbon will depend on the specific chemical, the amount and type of carbon, and the flow rate or contact time through the carbon bed. Chloramine removal requires a much longer contact time with the carbon than chlorine.

Source : Everpure, Inc.