A REPLACEMENT FOR THE SILT
DENSITY INDEX; PERMANGANATE
DEMAND TO PREDICT REVERSE
OSMOSIS MEMBRANE FOULING
Robert D. Binovi, Bioenvironmental Engineer
U.S. Air Force Clinic
McChord Air Force Base, Washington 98438
Riley N. Kinman, Professor
Dept. Civil and Environmental Eng.
University of Cincinnati
Cincinnati, Ohio 45221
INTRODUCTION
Reverse osmosis is a pressure driven separation process which has found widespred acceptance in
desalination, water reclamation, and the production of water of exceptional quality. Designers are
attracted to the simplicity of design, low energy requirements, and the potential for high recovery
that it offers.
Perhaps the most challenging of the problems associated with the use of reverse osmosis technology is fouling. Fouling is the reduction of permeate flux with time caused by constituents in the
feed or changes in the membrane structure. It introduces a degree of uncertainty to a planner as to
the usable life of the membrane and the need for pretreatment of the feedwater.
Unfortunately, the relationship between fouling and the normal characterizations of the feed-
water, i.e., hardness, turbidity, suspended or dissolved solids has never been quantified or hardly
explored. Currently, the results of a simple filtration test, the Silt Density Index (SDI), have been
used to determine if the process can be reasonably operated for an extended period of time without
fouling. The SDI cannot predict the expected lifespan of the membrane nor can it react linearly with
the concentration of the potential fouling agents.
The purpose of this research in light of this was to: (1) determine the relationship between fouling
and the concentration for a common foulant; (2) develop a simple test to characterize to feed water's
potential to foul; and (3) develop a predictive equation by which with reasonable accuracy an estimation of a membrane's useful life can be made.
NATURE OF MEMBRANE FOULING
Fouling often results from the precipitation and gel formation of organic and inorganic substances
on the membrane and membrane supporting structures. The organic layer has been attributed to
biological slime and humic and fulvic substances. Inorganic precipitation has been reported to have
been caused by calcium carbonate, ferric hydroxide, and amorphous silica. Dissolved organics, together with other colloidal material were reported to be the most serious foulants [1]. Winfield [2]
concluded that fouling from secondary sewage was caused by a negatively charged gel layer. The
major factor controlling the rate of fouling in that instance was the dissolved organic concentration
of the feedwater.
PREDICTING REVERSE OSMOSIS FOULING
The Silt Density Index (SDI) has come into wide acceptance as a test to predict the feedwater's
potential to foul an RO membrane or the need for pretreatment of the feedwater. It is a measure of
how fast constituents of the feedwater can plug a 0.45 micron cellulose triacetate membrane filter.
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