Water often contains particles that are incapable of being dissolved in solution. The particles may be comprised of fine sand, silt, clay or other materials. We often refer to these particles as particulate matter, suspended solids and/or turbidity. The size of the particulate matter varies and is usually measured in microns. Suspended solids that are greater than 30 micron size are generally visible to the naked eye. Filtration of particulate matter in water, in many ways, is the simplest of the treatment processes however thought must be given when deciding which type of equipment should be chosen.
Cartridge and bag filtration
Cartridge and/or bag filtration usually involves the use of a filter housing designed to hold a single or multiple number of filter elements. The filter housings, depending upon the application, may be constructed of carbon steel, stainless steel or a form of plastic such as PVC, polypropylene or polyvinylidene flouride (PVDF). The filter cartridges or bags that are used in the filter housings are rated for removal of a specified particle size identified in microns. Cartridge and/or bag filters may be used to remove essentially any particle size even down to submicron ranges. Cartridge and bag filters can be extremely effective for removal of particulate matter but can be costly if not applied properly. Cartridges and bags are generally disposable and ongoing replacement costs may be expensive.
Single media filters have been widely used for decades in municipal water treatment. The most used single media is sand. The size of the media particles determines the effectiveness of the filter to remove particulate matter. If the media is too coarse, fine particles will pass through the filter media and into the filtered water system. If the media is too fine the particulate matter will build up rapidly on the top of the filter bed thereby creating the need for frequent backwashing of the filter. Typically, single sand media filters will remove suspended matter down to around 25 micron size particles. Since single media filters only use one grade or size of media throughout the filter, the efficiency of the unit is limited to the top 3 – 4 inches of the bed. Design engineers usually choose a fine media in a single filter since inefficient filtration cannot be tolerated and does more harm than good. Increased frequency of backwashing is tolerated as a concession to higher filtering efficiencies. The inability of sand filters to utilize their entire media bed for filtration quite often renders them inefficient when compared with other forms of filtration.
Multi-media filters, also known as depth filters, combine more than one media in a single vessel. The top layer or level is generally a coarse anthracite media designed to remove particulate matter down to approximately 50-70 micron size particles. The second layer or intermediate level is generally comprised of sand with a rated removal of approximately 25-30 microns. The bottom layer is usually made of fine garnet designed to remove down to 10 micron size particles. As water flows downward through the filter vessel it encounters the different media layers thus increasing the filtering efficiency of the unit. Because of its multiple layers of media, a depth filter traps and holds more impurities than a single media filter because impurities are trapped throughout the entire bed. As a result of the increased efficiency of media utilization and less frequent clogging of the filter relatively small diameter vessels that require less floor space can be used for multi-media filters. The multi-media filter is like having three filters in one without the cost of acquiring and maintaining three separate filters.
Automatic screen filtration systems are quickly becoming the choice for architectural and engineering firms that are faced with the efficient design of new buildings. As the availability for mechanical space is diminished in new construction more efficient boilers, hot water generators and smaller footprints for equipment are being sourced. Screen filtration systems can be stacked and require much less floor space than large cumbersome sand and multimedia filter vessels. They also require a fraction of the backwash water required by media filters. The increased operating efficiency of screen filtration coupled with its minimal maintenance requirements makes it an economical choice.