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| Multi-Media Filter, Highly-Activated Carbon Filter, Water Softener with Brine Tank & Reverse Osmosis Module (fully automatic backwash & regeneration) |
Benefits of Multimedia Filtration over
Conventional
Sand Filters
Multimedia filtration refers to a pressure filter vessel which utilizes three or more different media as opposed to a "sand filter" that typically uses one grade of sand alone as the filtration media.
In a single media filter,
during the "settling" cycle, the finest or smallest media
particles remain on top of the media bed while the larger, and heavier
particles, stratify proportional to their mass lower in the filter.
This results in very
limited use of the media depth since virtually all filterable particles are
trapped at the very top of the filter bed or within 1-2 inches of the top where
the filter media particles have the least space between them.
The filter run times
are thus very short before the filter "blinds" or develops so much
head pressure that it must be backwashed to avoid seriously impeding or
stopping the flow.
Multi media filters
typically utilize three layers of media for multimedia filtration: anthracite,
sand and garnet.
These media are
often chosen for use in multi-media filters due to the distinct differences in
their densities.
Anthracite is the
lightest filtration media per unit volume, followed by sand, and then garnet.
The idea behind using media with differing
masses is that during backwashing the lightest media with the largest particles
(anthracite) will naturally stratify at the top of the filter, while the
intermediate sized media (sand) will settle in the middle, and the heaviest
media with the smallest particles (garnet) will settle to the bottom.
This layering of the
filtration bed encourages the very largest contaminants to become trapped in
the first layer of the filter, with smaller particulates sifting farther down
into the lower layers.
Trapping contaminants
in this manner allows for more efficient turbidity removal and for longer run
times between backwash cycles.
A simple sand filter
can be expected to eliminate particles down to 25-50 microns in size, as
compared to a multi media filter that can remove particles down to 10-25
microns.
Operating at higher
pressure differential is liable to drive particles so deeply into the media bed
that backwash is not able to remove them all.
Over time the
build-up of dirt deep in the filter will cause shortened filter runs and high differential
pressures.
Filter backwash may
include air scour to help loosen packed dirt in the media bed.
When this step is
included, it is preceded in the backwash cycle by a "drain down" period
for water to be bled out of the filter vessel.
This process is
called "agglomeration" and, with proper chemical dosage, adequate
mixing and adequate contact time, it will enable the filter to remove particles
below 10 microns in average diameter.
Flocculants
/ coagulants may be used upstream of the filter to induce the tiny dirt
particles to join together to form particles large enough to be removed by the
filter.
The Benefits of Multimedia Filtration over
Conventional Sand Filters
Unlike traditional sand filters, multi-media
water filters are composed of three filtration media, ordered in
decreasing porosity.
Because
of their multi-layer design, multi-media water filters are able to trap and
retain a far larger number of particles than traditional sand filters before
backwashing becomes necessary.
Trapping sediment and particulates throughout
the entire depth of the filter bed, allows multi-media water filters to operate
for much longer periods of time than conventional sand filters.
The process of multimedia filtration produces
high quality, filtered water at much faster flow rates than traditional sand
filtration.
Pressure vessels with sand or other loose media
are widely used in industrial filtration applications.
During the cleaning cycle, called
"backwash", the bed is lifted (or "fluidized") to loosen
the filter media and release trapped dirt which is removed in the backwash
flow.
After the backwash cycle, the bed is allowed to
settle before the filter is returned to service (i.e., normal flow).
A "filter-to-waste" cycle is used
following the settling to assure the filtration media has
sufficiently re-stratified and that any loose dirt is removed from the
underdrain / collectors.
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