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
