Page 381 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: GLQ Final Pages
Encyclopedia of Physical Science and Technology EN009K-419 July 19, 2001 20:57

316 Membranes, Synthetic, Applications

3
for very large (up to 200,000 m /day) desalination this purpose is Water Factory 21 located in Orange County
plants—a transition aided by sharply rising energy ontheCaliforniacoast.Inoperationsince1976,thefacility
costs, and by high-performance, yet competitively priced, treats municipal wastewater by reverse osmosis and blends
membrane systems. Energy-recovery turbines are used ex- the product with water purified by carbon absorption and
tensively in seawater RO systems to reclaim energy from from deep wells. The combined stream, which meets
the high-pressure brine stream. For the very high-salinity drinkingwaterstandards,isreinjectedintocoastalaquifers
seawaters (>4.5% TDS) found in Middle East locations, to replenish local groundwater supplies and prevent sea-
desalination systems are designed to operate at about water intrusion. At Yuma, Arizona, the world’s second
3
80 bars. largest reverse osmosis plant, treats 275,000 m /day of
Brackish waters contain between 0.05 and 1 wt % TDS. saline farmland drainage so that salinity requirements can
Their lower osmotic pressures allow reverse osmosis op- be met for Colorado River water released to Mexico.
eration between 15 and 30 bar. Less expensive pressure Liquefied and gasified coal have been considered as
equipment and energy consumption translate to more fa- an alternative to petroleum for producing energy and as
vorable water production economics than those for sea- chemical feedstock. Both liquefaction and gasification
water desalination. generatelargevolumesofwaterfromcoalwashing,slurry-
Reverse osmosis membranes can be divided into sub- ing, and the conversion process itself. These wastewaters
classes according to their solute/water selectivity and are contaminated with salts, phenol, ammonia, hydrogen
operating pressure regimes. Figure 30 shows a number sulfide, and a complex mixture of other substances. Simul-
of commercial membranes developed for seawater and taneous removal of organics (up to 98%) and salts (be-
brackish desalination, and for nanofiltration. These in- tween 80 and 95%) by reverse osmosis shows some
clude cellulose ester and polyamide asymmetric mem- promise.
branes available since the 1960s, and high-performance Reverse osmosis also serves some of the waste manage-
composite membranes developed in the 1970s. Collec- ment and resource recovery needs in the metals and metal
tively, they make it possible to produce potable water from finishing industry. Effluent streams from mining and plat-
virtually all saline water sources. ing operations containing heavy metals, acids, and other
A lingering limitation with the present generation of chemicals can be treated with reverse osmosis to recover
reverse osmosis membranes is their limited resistance to both the metal as its salt, and purified water for reuse.
chemical attack. In particular, membranes derived from For metal ion recovery from dilute solutions, however,
polyamides, polyureas, and other nitrogen-containing reverse osmosis faces competition from conventional sol-
polymers are susceptible to oxidative degradation by vent extraction, membrane-based solvent extraction, and
chlorine—the most widely used disinfectant to pretreat its variant, coupled transport (see Section V.F.3).
feed waters. Dissolved oxygen can also damage reverse An estimated 10 15 KJ are consumed annually in the
osmosis membranes when catalyzed by trace heavy United States for food processing, primarily in concentra-
metals. Successful development of oxidation-resistant tion and purification operations. Concentration by reverse
membranes will help reduce the complexity and costs osmosis is attractive because of its ability to remove water
associated with the elaborate pretreatment now required. without adding heat, and is already used for concentrat-
Water supplied to industry has to meet stringent spec- ing sugar solutions, fruit and vegetable juices, and bever-
ifications. For example, process water for the chemi- ageswhileretainingsaltsandlow-molecular-weightflavor
cal and biotechnology industries is routinely purified components. Ambient temperature processing also helps
beyond potable water standards. Boiler feed water for preserve product quality. High concentrations are reached
steam generation must contain a minimum of silica. Re- by using membranes with high rejections and operating at
verse osmosis units designed specifically for these pur- very high pressures (100 bar or above) so as to overcome
poses are in widespread use today. For example, reverse the osmotic pressures associated with increasing sugar
osmosis/distillation hybrid systems have been designed contents. Sometimes membranes with lower rejection are
to separate organic liquids. For semiconductor manufac- used to recover residual solute in the permeate, or at the
ture, reverse osmosis is combined with ultrafiltration, ion final stage of concentration where the osmotic pressure
exchange, and activated carbon adsorption to produce the is at its maximum. In these applications, reverse osmosis
extremely clean water required. and nanofiltration membranes are often deployed together
Wastewater reclamation is a logical extension of desali- to balance productivity, product specification, and cost.
nation technology. Much of the membrane system design The United States textile industry consumes over 4 bil-
3
is common to both applications, and the membranes avail- lion m of water annually. Much of the process water is
able for wastewater treatment are those originally devel- discharged together with dyes and auxiliary chemicals,
oped for desalination. The first major project designed for plus a loss of energy in the hot effluents. Reverse osmosis