Principles of Depth Filtration

Filtration is the simplest of water treatment processes, but in some ways the most difficult to accomplish. If all suspended particles in water were the same size, they would be easy to filter out. A media with porosity slightly smaller than the particles could be selected, the water run through the media, and particulate matter removal would approach optimum. For large particles, a coarse filtering media would be chosen: for fine particles a fine porosity media would do the job. For higher delivered volumes, and to extend the service cycle of the filter between backwashes, large diameter vessels could be used to delay clogging of the media. Of course, the size of particles in water varies considerably, especially in surface water where debris as well as fine sand and silt are found. And large pressure vessels are uneconomical, they cost more to build and occupy too much valuable floor space.

Single-Media Filters: often-single media filters are selected as a compromise. If the media is too coarse, fine turbidity particles pass through and into the filtered water stream. If the media is too fine, large turbidity particles will soon clog it, and the filter must be backwashed more frequently. Frequent backwashing reduces the time the filter is in service and back-washing is costly.

Design engineers usually choose a fine media in a single media filter, since inefficient filtration cannot be tolerated and may do more harm than good. They then place the media in a small diameter vessel, to keep equipment costs down. In operation, the media soon clogs and must be backwashed. More frequent backwashing is tolerated as a concession to higher filtering efficiencies.

Clogging occurs in the top portion of the bed, rendering the bottom part useless. In other words, the filter does not function throughout the entire depth of the filter bed, and thus single media filters almost invariably operate inefficiently.

Multi-Media Depth Filters: the multiple media filter, or depth filter, solves all these problems. It combines more than one media in a single vessel, with coarse media on top, intermediate porosity media below it, and fine media below that. As water flows downward through the filter vessel, it encounters media beds of decreasing size, and thus higher filtering efficiency. The coarse media traps and hold large particles, preventing them from migrating downward through the bed. The intermediate portion traps medium size particles, and the fine media traps the smallest particles. This arrangement of media makes it virtually impossible for large particles to clog the finer media, since they are captured and held in the coarser media above. As a result, each layer of media functions near its optimum efficiency. It does not matter that the fine particles pass through the top layer; they will be trapped later. The fine media layer can do its job because it is not encumbered with large particles that would clog it.

Because of this arrangement, each media layer filters throughout its bed depth, as does the entire filter, hence the term depth filter. The depth filter also traps and holds more impurities than a single media filter because impurities are trapped throughout the entire bed. As a result of efficient media utilization and less frequent clogging, relatively small diameter vessels that require less floor space can be used.

Other Filtration Technologies

  • Bag and cartridge filter systems

  • Sediment: sand, depth and Next filters

  • Iron filters: Birm, Greensand, Pyrolox, Aeration, etc.

  • Arsenic filters

  • Micro filters and Ultrafiltration

  • Dissolved Air Flotation systems