5
Step 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 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:
!
TEST THAT THE PRE-TREATMENT SYSTEM IS DELIVERING RO FEED WATER TO
NECESSARY SPECIFICATIONS BEFORE PLUMBING THE RO SYSTEM ONLINE.
!
ALWAYS ALLOW A NEW SOFTENER INSTALLED TO PRE-TREAT AN RO SYSTEM TO
START ON THE BACKWASH/REGENERATION/RINSE CYCLE. OPERATE THE SOFTENER
OFFLINE UNTIL WATER RUNS CLEAR BEFORE USING IT TO PROVIDE RO FEED WATER.
THIS PROCEDURE ALSO APPLIES TO NEW RESIN BED INSTALLATIONS. COLOUR
THROW FROM NEW RESIN BEDS MAY DAMAGE OR FOUL RO MEMBRANES
