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Concentrating RO Reject
Streams with VSEP
Guest article by Dr. Brad Culkin, New Logic Research, Inc.
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Introduction
 The problem of dealing with the
concentrate streams from reverse osmosis (RO) membrane processes
is well known. Spiral wound membrane systems are limited in the
overall recovery attainable with thin film composite membranes.
The reason recoveries are limited to between 50% and 90% is the
fact that spiral membrane systems cannot tolerate any suspended
solids, whether they are already present in the feed or they are
appearing by chemical precipitation inside the modules.
Elaborate pretreatment is used to
eliminate particulate solids from RO spiral feeds. The
removal of suspended solids must be absolute. Even trace amounts
of suspended solids will kill a spiral module application.
Anti-scalants are added to suppress the kinetics of crystal
nucleation thus delaying formation of suspended colloids inside
the modules.
The current model assumes that
mineral fouling, whether by colloids in the feed landing on the
membrane surface, or by precipitates forming on and in the outer
thin film layer, is capable of reducing flux to near zero.
Many minerals can be periodically dissolved away in acid but in
the case of salts whose solubility is independent of pH, like
gypsum, or those which are nearly insoluble such as silica, this
fouling is deemed intractable. This explains the current
practical limits to recovery in RO systems.
The inability to dispose of this
high solids product from reverse osmosis systems leads to the
need for huge evaporation ponds, high capacity injection wells,
and energy intensive crystallizers, each with its own set of
problems. Without such two step methods, however, it is
often impossible to get the required operating permits because
discharge of the liquid reject stream of membrane systems is
often not allowed. What is needed is a membrane technology
capable of functioning as a crystallizer or evaporator.
A New Technology - VSEP
 A new type of membrane separator
which used vibration rather than cross flow to keep the
membranes from caking up with solids was developed in the late
1980's. Originally the idea was to use the technique,
called Vibratory Shear Enhanced Processing, or VSEP, to dewater
viscous slurries like paints and pigments. A great number
of systems used this technology for micro filtration and ultra
filtration of latex polymer, oily water wastes, kaolin clay,
precipitated calcium carbonate, ground calcium carbonate,
colloidal silica, and many other ultra fine colloidal or “white
water” suspended solids streams.
The extension of this VSEP
technique to doing direct crystallization using RO membranes was
delayed for two reasons. First, the machinery had to be
modified to take the high pressures associated with RO Second,
the accepted model for fouling of thin film composite membrane
by mineral precipitation suggested that even the high shear of
the VSEP process might not remove the tenacious scale thought to
be formed on and within the membrane. By 2001, conditions
were ripe for testing both a new high pressure design and this
theory of mineral fouling.
It turns out that, given enough
shear, the process of crystal formation at the membrane surface
does not form a tenacious continuous film of scale. To the
contrary, the minerals precipitate as a colloidal particle which
is roughly spherical and which is attached only at the point of
tangency between each particle and the membrane surface.
In other words, the particles
forming by precipitation are similar to those which are already
present in the feed. Since spiral wound membrane modules
cannot handle particulate solids in the feed, it should be no
surprise they cannot handle identical particles appearing by
precipitation either. It is not necessary to invoke a
separate model involving super tenacious scale forming models to
explain fouling of spirals by precipitation. Indeed, such
models are apparently wrong.
Continuous Crystallization
Since VSEP is inherently good at
handling colloidal separations by design, it should be no
surprise that we now are routinely using this technique for
continuous crystallization of dissolved minerals as part of a
continuous reverse osmosis system. The typical process
uses conventional spirals to reach the point of saturation with
respect to calcium, silica, barium, or some other mineral, and
to feed the concentrate from the spirals directly to a VSEP
module which runs as a continuous crystallizer.
The VSEP process runs to an end
point determined by the rheology of the suspended solids sludge
reaching a “pumpability” limit, or the osmotic pressure due to
sodium salts reaching the design pressure limit of the machine
(as much as 2,000 psi). For typical applications this
corresponds to volume reductions of 1000 to 1 from spiral feed
to VSEP concentrate.
To be clear, the VSEP membrane
module is fitted with RO membranes. As the on-spec
permeate passes through the membrane, dissolved solids
precipitate as ultra fine suspended solids. These solids
join other suspended solids which may be present in the feed.
All suspended solids are then swept away from the membrane
surface by the high shear VSEP process.
Once a given ionic species begins
to drop from solution, its solution concentration remains
constant at the saturation concentration of that salt. For
this reason, the permeate does not deteriorate as it would in
the case of infinitely soluble materials, and so permeate
quality with respect to hardness is excellent. Only highly
soluble sodium salts climb in concentration, and the recovery
eventually becomes comparable to the rejection of the membrane;
permeate becomes higher in sodium salts only.
Applications
Since 2001, dozens of high
pressure applications using VSEP modules fitted with RO and
nanofiltration membranes have been successfully deployed.
Applications include:
- VSEP is being used at full
scale to dewater raw pig manure lagoon water through RO
membranes. There is no pretreatment ahead of the VSEP
process other than coarse screening.
- VSEP is used to remove BOD.
from pulp mill end of pipe waste using a 50% salt reject
nanofiltration membrane.
- VSEP is used for organics
removal from a chemical plant waste stream with RO
membranes.
- In the largest refinery in
the world, VSEP is used to remove selenium from stripped
sour water at a design flow rate of 700 gpm. The concentrate
from VSEP is 5 gpm of concentrated black water which goes to
an evaporator.
- A very large VSEP reverse
osmosis system will be delivered to an oil field customer.
Eventually 3,000 gpm of high pressure boiler feed will be
produced at 95 percent recovery.
- The VSEP has been piloted
for crystallization of hardness from municipal RO reject.
For the engineering community
this is great news. There is a proven, mature machine
technology which has been successfully adapted to solving the
recently topical problem of RO reject. Moreover, this new
process allows membranes to function as a crystallizer, opening
the door to many new and exciting applications for process
engineers working in the chemical, environmental and water
treatment industries.
About our author
Dr. Brad Culkin is the Chief
Technical Officer of New Logic Research, Inc. In addition
to founding New Logic and inventing VSEP, he holds numerous
patents in diverse areas including filtration, loudspeakers and
flat panel displays.
Before joining New Logic, Dr.
Culkin worked as chief engineer for Dorr-Oliver, Inc., where he
developed several new separation technologies. Culkin
earned his bachelors degree in chemical engineering from the
University of Pennsylvania, master's degree in theoretical
mathematics from Johns Hopkins and a PhD in chemical engineering
from Northwestern.
For more information contact
Dr. Brad Culkin
Chief Technical Officer
New Logic Research, Inc.
1295 67th Street
Emeryville, CA 94608
Telephone: 510.655.7305
Fax: 510.655.7307
Web site:
http://www.vsep.com/
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