Solutions
to Membrane Fouling
By: Gil K. Dhawan Ph.D. P.E.,
Applied Membranes, Inc.
Note: Click Italicized
Underlined words for product information.
Introduction to Membrane Fouling
All membranes lose their performance with time. One of the major
causes for the loss of performance with time. One of the major causes for
the loss of performance is due to substances that deposit on the membrane
surface. Although the term fouling is used for deposit of any
materials on the membrane, the coating of the membrane surface can be due
to the following:
Fouling
Fouling of membranes is due to the suspended or emulsified materials
that may be present in the feed water to the reverse osmosis system.
Examples of such materials are: silica, oil, clay, iron, sulfur and
humic acids. These substances can be present in a very fine or
colloidal form. Even the typical 5 micron cartridge filters used
upstream from a reverse osmosis system may not completely remove these
foulants.
Membrane Fouling and Scaling
The concentration of all materials in the feed water - dissolved and
suspended - is highest near the membrane surface. As permeate is
removed through the membrane, all impurities are left behind near the
membrane surface. The layer of water next to the membrane surface
(boundary layer) gets more and more concentrated in the dissolved and
suspended materials. These concentrations reach a certain steady
level depending on feed velocity, element recovery and membrane permeate
flux (gallons per square foot of permeate produced per day).
It is important to follow membrane manufacturers' recommendations on
minimum feed flow, maximum element recovery and maximum element flux.
These recommendations are based on the element size and quality of feed
water being treated. The concentrations of the dissolved and
suspended solids in the boundary layer control the performance of the
membrane. Higher concentrations mean higher osmotic pressure, higher
tendency of suspended solids to coagulate and coat the membrane surface,
and higher likelihood of scaling to take place. Maintaining proper
operating conditions for the membrane is the key preventative step to
minimize membrane fouling.
Antiscalant Injection
For non-residential systems, another option to avoid calcium carbonate
and calcium sulfate scaling is by the use of
antiscalants. These are
injected directly into the feed water upstream from the cartridge filter.
Dosage of antiscalant depends on the feed water analysis but usually is
between 2 to 5 ppm. In simplified terms, the antiscalants delay the
scale formation process. This delay is sufficient to avoid
precipitation of calcium carbonate and calcium sulfate on the membrane
surface. As this delay is for a finite period, scaling can take
place in systems on shut down. For this reason, it is a good
practice to flush the membranes with permeate or feed water at shut down.
By this flush, the concentrated solution in the membrane is displaced by
the permeate or feed water.
Dispersant Injection
For suspended or colloidal materials, a
dispersant can be injected in
the feed water. The usual dosage for a dispersant is 10 ppm.
Dispersants keep fine suspended solids from coagulating and coming down on
the membrane surface. Proper use of dispersants can minimize fouling
due to problem particulates that are difficult to prefilter.
Acid Injection
Adjusting the pH of the feed water is another way to control calcium
carbonate scaling. The net effect of lowering the feed pH with acid
injection is to convert bicarbonate alkalinity to carbon dioxide and
thereby prevent the formation of calcium carbonate. For reasons of
handling and safety, acid injection is not used for residential or small
commercial systems.
Reduce Recovery
Membrane recovery is defined as the ratio of permeate flow to feed flow
for that membrane. Recovery can be reduced by increasing the feed
flow. Another way to reduce recovery is to decrease the operating
pressure. Lower operating pressure produces a lower amount of
permeate. If the feed flow can be maintained near the original
value, then a lower recovery is obtained.
The effect of lower recovery is to reduce the overall concentration of
all substances in the reverse osmosis system. More favorable
boundary layer conditions are also achieved by reducing the system
recovery.
Membrane Cleaning
Even with all the preventative care given to a reverse osmosis system,
some fouling of the membranes will take place.
Cleaning
of the membranes can improve membrane performance. Membranes can be
cleaned using
Cleaning Solutions approved by the membrane manufacturer.
It is not economical to clean membranes used in the residential reverse
osmosis systems.
Details of membrane cleaning can be found on this site as well -
Click
Here to link.
Conclusions
Membrane fouling and scaling can be minimized by proper design and
operating conditions. Important variables that control the membrane
fouling must be considered in designing an operating the reverse osmosis
system. |
