5
corrosion on metallic parts or scaling internal components upstream from the membrane, and
likely the membrane itself. Keep in mind that reverse osmosis lowers the pH of the feed water,
by up to 1.0, depending on the water chemistry. This will be a consideration for the end use of
product water and for the vessels and plumbing system that will be used to store and control it.
•
Turbidity:
This is the term to describe suspended particulate in the raw water. Turbidity is treated
by the sediment cartridges that your RO system has been equipped with. Reduction of turbidity
is necessary to avoid fouling your membrane and damaging your drive pump. Make sure that
your pre-filters are changed regularly and that quality Excelpure 5-micron cartridges are used as
replacements. The SDI (silt density index) on feed water reaching the membrane must be kept
< 5 to assure long membrane life.
2.2. Plan Your Pre-treatment Strategy
Pre-treating feed water can lengthen the life of membrane elements, improve the quality of the water
produced and reduce the amount of maintenance and cleaning a system requires.
The importance of feed water conditions is evident when you examine how crossflow membranes
work. In simple terms, a crossflow filtration system separates an influent stream into two effluent
streams - the permeate and the concentrate. The permeate is the portion of the fluid that has passed
through the semi-permeable membrane. The concentrate stream, on the other hand, contains
constituents that have been rejected by the membrane.
An inherent advantage of crossflow filtration is its ability to continuously operate in a self-cleaning
mode. It's self-cleaning because suspended solids and rejected solutes are constantly swept away
from the membrane surface. Membrane fouling occurs when materials from the feed stream collect
on or near the membrane surface and restrict water permeation. Fouling may occur as layers of
deposition on the surface of the membrane (cake fouling), a hardened layer on the membrane
surface (scale), particle insertion into the pore channel or entrance (pore blockage), or chemical
attachment of particles to the membrane (adsorption).
You will note that many of the prescribed treatments for various raw water conditions will be effective
in treating more than one problem. Where multiple pre-treatment requirements are necessary,
consult with a water treatment professional to design a pre-treatment system that meets the
necessary needs most economically.
Use the pre-treatment summary below to prescribe the necessary pre-treatment where your water
analysis indicates parameters outside of the recommended range:
