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the pH of the feed water is raised
to pH 10.5 to increase the solubil-
ity of the silica and destroy biologi-
cal organisms in the feed water.
The feed water then enters the RO
system (4).
Benefits and Costs
The HERO system provides several
benefits. For example, it reduces
scaling of the RO membrane.
Dissolved silica is one of the ma-
jor causes of membrane scaling.
Increasing the pH of the water
entering RO greatly increases the
solubility of silica, thus eliminating
its potential for scaling. Increasing
the pH also destroys many bacte-
ria and creates a slightly negative
charge that repulses colloids; both
reduce fouling of the membrane.
The typical water recovery of
the HERO process is 95%-99%
(depending on site-specific charac-
teristics) in comparison to a typical
recovery rate for conventional RO
of approximately 75%. In addition,HERO produces purer water than
conventional RO processes do,
and the operating flux for HERO is
nearly double that of conventional
RO.
Because the pretreatment process
eliminates many of the causes of
membrane fouling and scaling,
HERO does not use antiscalant
chemicals and requires less clean-
ing and maintenance than con-
ventional RO does. HERO also has
limited self-healing properties, so
small problems tend not to cause
major system disruptions. In addi-
tion, there are the environmental
benefits of saving water and energy
and the ability to use reject water
from HERO (which is soft water at
high pH) for acidic gas scrubber
makeup or acid waste neutraliza-
tion.
Capital costs for HERO are general-
ly higher than those of convention-
al RO for small (25-50 gallons per
minute [gpm]) systems because
of the pretreatment requirements.However, for systems larger than
50 gpm, capital costs for HERO are
generally about 15% lower because
of the higher flux rate, which
allows fewer RO membranes to
be used. Because of higher water
recovery rates and reduced fouling
and scaling of the RO membranes,
HERO operating and energy costs
are also typically less than those
of conventional RO. Case studies
suggest that, depending on the
characteristics of the feed water
and water costs, operating a HERO
system costs about 20%-40% less
than operating a conventional RO
system.
Reverse Osmosis/
Electrodeionization
Reverse osmosis/EDI is another
technique for purifying process
water (Figure 4). Feed water is first
treated by single- or double-pass
RO.Then, the water undergoes
EDI to deionize it.Figure 4. Schematic of EDI.
FEDERAL ENERGY MANAGEMENT PROGRAM - 3
Anode [+]
Cation Membrane
Anion Membrane
Q r*Ultrapure
Feed - - +- + Water
Water ._ + Cation Membrane
++ + + + Anion Membrane
Cathode [-]
O CI- (DOH- OH+ QNO+ *Ion Exchange Resin
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Saving Energy, Water, and Money with Efficient Water Treatment Technologies, text, June 1, 2004; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc1412061/m1/3/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.