RO Pretreatment
Proper pretreatment using both mechanical and chemical treatments is critical for an RO system to prevent fouling, scaling and costly premature RO membrane failure and frequent cleaning requirements. Below is a summary of common problems an RO system experiences due to lack of proper pretreatment.
FOULING
Fouling occurs when contaminants accumulate on the membrane surface effectively plugging the membrane. There are many contaminants in municipal feed water that are naked to the human eye and harmless for human consumption, but large enough to quickly foul (or plug) an RO system. Fouling typically occurs in the front end of an RO system and results in a higher pressure drop across the RO system and a lower permeate flow. This translates into higher operating costs and eventually the need to clean or replace the RO membranes. Fouling will take place eventually to some extent given the extremely fine pore size of an RO membrane no matter how effective your pretreatment and cleaning schedule is. However, by having proper pretreatment in place, you will minimize the need to address fouling related problems on a regular basis.
Fouling can be caused by the following:
Particulate or colloidal mater (dirt,
silt, clay, etc.)
Organics (humic/fulvic acids, etc)
Microorganisms (bacteria, etc).
Bacteria present one of the most common fouling problems since RO membranes in
use today cannot tolerate a disinfectant such as chlorine and thefore
microorganisms are often able to thrive and multiply on the membrane surface.
They may product biofilms that cover the membrane surface and result in heavy
fouling.
Breakthrough of filter media upstream
of the RO unit. GAC carbon beds and softener beds may develop an under drain
leak and if there is not adequate post filtration in place the media can foul
the RO system.
By performing analytical tests, you can determine if the feed water to your RO has a high potential for fouling. To prevent fouling of an RO system, mechanical filtration methods are used. The most popular methods to prevent fouling are the use of multi-media filters (MMF) or microfiltration (MF). In some cases cartridge filtration will suffice.
SCALING
As certain dissolved (inorganic)
compounds become more concentrated (remember discussion on concentration
factor) then scaling can occur if these compounds exceed their solubility
limits and precipitate on the membrane surface as scale. The results of scaling
are a higher pressure drop across the system, higher salt passage (less salt
rejection), low permeate flow and lower permeate water quality. An example of a
common scale that tends to form on an RO membrane is calcium carbonate (CaCO3).
CHEMICAL ATTACK
Modern thin film composite membranes
are not tolerant to chlorine or chloramines. Oxidizers such as chlorine will
'burn' holes in the membrane pores and can cause irreparable damage. The result
of chemical attack on an RO membrane is a higher permeate flow and a higher
salt passage (poorer quality permeate water). This is why microorganism growth
on RO membranes tends to foul RO membranes so easily since there is no biocide
to prevent its growth.
MECHANICAL DAMAGE
Part of the pretreatment scheme should
be pre and post RO system plumbing and controls. If 'hard starts' occur
mechanical damage to the membranes can occur. Likewise, if there is too much
backpressure on the RO system then mechanical damage to the RO membranes can
also occur. These can be addressed by using variable frequency drive motors to
start high pressure pumps for RO systems and by installing check valve(s)
and/or pressure relief valves to prevent excessive back pressure
on the RO unit that can cause permanent membrane damage.
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RO Water Purifier Sales and Service.
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