PRE TREATMENT SOLUTIONS
Below are some pretreatment solutions for RO systems that can help
minimize fouling, scaling and chemical attack.
MULTI MEDIA FILTRATION (MMF)
A Multi-Media Filter is used to help prevent fouling of an RO
system. A Multi-Media Filter typically contains three layers of media
consisting of anthracite coal, sand and garnet, with a supporting layer of
gravel at the bottom. These are the medias of choice because of the differences
in size and density. The larger (but lighter) anthracite coal will be on top
and the heavier (but smaller) garnet will remain on the bottom. The filter
media arrangement allows the largest dirt particles to be removed near the top
of the media bed with the smaller dirt particles being retained deeper and
deeper in the media. This allows the entire bed to act as a filter allowing
much longer filter run times between backwash and more efficient particulate
removal.
A well-operated Multi-Media Filter can remove particulates down to
15-20 microns. A Multi-Media Filter that uses a coagulant addition (which
induces tiny particles to join together to form particles large enough to be
filtered) can remove particulates down to 5-10 microns. To put this in
perspective, the width of a human hair is around 50 microns.
A multi media filter is suggested when the Silt Density Index
(SDI) value is greater than 3 or when the turbidity is greater than 0.2 NTU.
There is no exact rule, but the above guidelines should be followed to prevent
premature fouling of RO membranes.
It is important to have a 5 micron cartridge filter placed
directly after the MMF unit in the event that the under drains of the MMF fail.
This will prevent the MMF media from damaging downstream pumps and fouling the
RO system.
MICROFILTRATION (MF)
Microfiltration (MF) is effective in removing colloidal and
bacteria matter and has a pore size of only 0.1-10µm. Microfiltration is
helpful in reducing the fouling potential for an RO unit. Membrane
configuration can vary between manufacturers, but the "hollow fiber"
type is the most commonly used. Typically, the water is pumped from the outside
of the fibers, and the clean water is collected from the inside of the fibers.
Microfiltration membranes used in potable water applications usually operate in
"dead-end" flow. In dead-end flow, all of the water fed to the
membrane is filtered through the membrane. A filter cake that must be
periodically backwashed from the membrane surface forms. Recovery rates are
normally greater than 90 percent on feed water sources which have fairly
high quality and low turbidity feeds.
ANTISCALANTS AND SCALE INHIBITORS
Antiscalants and scale inhibitors, as their name suggests, are
chemicals that can be added to feed water before an RO unit to help reduce the
scaling potential of the feed water. Antiscalants and scale inhibitors increase
the solubility limits of troublesome inorganic compounds. By increasing the
solubility limits, you are able to concentrate the salts further than otherwise
would be possible and therefore achieve a higher recovery rate and run at a
higher concentration factor. Antiscalants and scale inhibitors work by
interfering with scale formation and crystal growth. The choice of antiscalant
or scale inhibitor to use and the correct dosage depends on the feed water chemistry
and RO system design.
SOFTENING BY ION EXCHANGE
A water softener can be used to help prevent scaling in an RO
system by exchanging scale forming ions with non scale forming ions. As with a
MMF unit, it is important to have a 5 micron cartridge filter placed directly
after the water softener in the event that the under drains of the softener
fail.
SODIUM BISULFITE (SBS) INJECTION
By adding sodium bisulfite (SBS or SMBS), which is a reducer, to
the water stream before an RO at the proper dose you can remove residual
chlorine.
GRANULAR ACTIVATED CARBON (GAC)
GAC is used for both removing organic constituents and residual
disinfectants (such as chlorine and chloramines) from water. GAC media is made
from coal, nutshells or wood. Activated carbon removes residual chlorine and
chloramines by a chemical reaction that involves a transfer of electrons from
the surface of the GAC to the residual chlorine or chloramines. The chlorine or
chloramines ends up as a chloride ion that is no longer an oxidizer.
The disadvantage of using a GAC before the RO unit is that the GAC
will remove chlorine quickly at the very top of the GAC bed. This will leave
the remainder of the GAC bed without any biocide to kill microorganisms. A GAC
bed will absorb organics throughout the bed, which is potential food for
bacteria, so eventually a GAC bed can become a breeding ground for bacteria
growth which can pass easily to the RO membranes. Likewise, a GAC bed can
produce very small carbon fines under some circumstances that have the
potential to foul an RO.
Yazh Pure Life
RO Water Purifier Sales and Service.
9786569973
8098094111
We thanks to Pure Tech
No comments:
Post a Comment