Feature Article
BR EN T AL S PAC H AN D G R A HAM J U BY
Cost-Effective ZLD Technology
for Desalination Concentrate
Management
LIQUID DISCHARGE
TECHNOLOGY ILLUSTRATES
THE POTENTIAL FOR
AFFORDABLE USE AT
DESALINATION PLANTS
ACROSS THE COUNTRY IN
THE FORTHCOMING AWWA
MANUAL OF PRACTICE M69.
I
ncreasing demand for freshwater resources among a variety of beneficial
uses (i.e., municipal, agricultural, industrial, and environmental) is
straining available supplies to an unprecedented degree. In the United
States, competition for these supplies was intensified by the recent multiyear droughts that afflicted California, Texas, and the desert Southwest.
Consequently, the use of alternative, lower-quality sources, such as brackish
groundwater, saline surface water, and recycled wastewater, are rapidly becoming integral potential components of long-term water supply portfolio planning. Nearly all of these alternative supplies require desalination; however, in
many areas, particularly inland locations, concentrate management options
may be limited because of the lack of a suitable receiving body, environmental
regulations, and/or permitting restrictions. In these cases, the use of zero liquid
discharge (ZLD) technology can overcome all of these barriers.
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Layout imagery by Shutterstock.com artists: XONOVETS, teamplay
A FRESH LOOK AT ZERO
37
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With the demand for desalination at
a historic level that continues to trend
upward, expanded funding for ZLD
research has yielded a number of
novel and innovative approaches.
Although these new approaches have
significantly reduced the cost of ZLD,
many are boutique strategies with
applicability that is limited to specific
concentrate water quality characteristics, thus benefitting only a small
subset of desalination facilities. By
contrast, the more conventional ZLD
approach—the combination of brine
concentrators and crystallizers—is
not constrained by water quality and
can be deployed to treat concentrate
of almost any quality. The primary
limitation of conventional ZLD has
been very high life cycle costs, which
are largely attributable to substantial
specific energy requirements. As a
result, despite recent advances and a
growing need for versatile, more
environmentally friendly concentrate
management options, the mainstream
adoption of conventional ZLD has
remained elusive.
However, a fresh examination of
conventional ZLD demonstrates that
in some cases the process can be
applied at costs that are comparable
to other alternative sources of supply, such as desalinated seawater.
potential for affordable use at desalination plants across the country.
OVERVIEW OF ZLD
The concept of ZLD represents
the complete (or near complete)
elimination of liquid waste from a
water treatment residuals stream.
For desalination processes, ZLD further concentrates the highly saline
liquid residuals stream, ultimately
generating solids or dense slurries
that can be transported offsite for
disposal or recycling. However, the
idea of ZLD is not exclusive to
desalination; surface water treatment plants (i.e., those treating lowsalinity sources) with advanced
residuals management can likewise
achieve ZLD, albeit using much different processes. Because the literature may not clearly make this distinction, and because ZLD
associated with low-salinity water
treatment is generally much more
economical and therefore more
common, it is important to understand the context in which the term
is applied. For the purposes of this
article, the discussion of ZLD is limited to desalination applications.
Because there is no formal definition of the concept, what constitutes
ZLD is somewhat fluid; therefore,
With the demand for desalination at a historic
level that continues to trend upward, expanded
funding for ZLD research has yielded a number
of novel and innovative approaches.
This article, which is adapted from
the forthcoming AWWA Manual of
Practice M69: Inland Desalination
and Concentrate Management, discusses the results of this examination
in detail. Focusing on implementation, operations, and economics, the
objective of this article is to enhance
the practical understanding of conventional ZLD throughout the
industry and demonstrate the
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ALSPACH & JUBY | JANUARY 2018 • 110:1
there is no widely recognized threshold recovery beyond which a desalination plant constitutes a ZLD facility. Accordingly, the literature may
represent plants operating at recoveries near 98.5% as using ZLD or nearZLD. However, accomplishing nearly
100% recovery is not necessarily a
rigorous prerequisite for ZLD, as
some water may be incorporated into
the final hydrated solids or thickened
slurries. In such examples, the plant
does not discharge any strictly liquid
residuals, although technically 100%
recovery is not attained. In any case,
the ZLD label is less important than
achieving the goal of eliminating the
discharge of liquid residuals.
There are two primary advantages associated with eliminating
liquid discharge. The first advantage is maximizing the yield of
treated/desalinated water. The second advantage is providing a means
of concentrate management for
cases in which there are no other
viable options. The benefit of
increasing treated water production
could be a significant consideration
in any location, potentially driven
by issues of water rights, the cost of
developing new sources, the availability of additional supplies, procuring land for new treatment facilities, and other factors. However,
the utility of ZLD is particularly
acute for inland desalination locations, which often have fewer concentrate management alternatives
than coastal regions. For example,
• an inland area may lack a suitable receiving body for desalination concentrate;
• the concentrate water quality
may not permit discharge to any
existing receiving bodies;
• there may be a lack of a centralized wastewater collection system for discharge, or if such
infrastructure is present, the
municipal effluent discharge
permit may not allow for higher
salinity flows;
• water quality, geology, and/or
regulations may limit deep well
injection; and
• the evaporation rate and/or
availability of land acreage for
storage ponds may be insufficient to accommodate the concentrate flow.
If all of these conditions are true,
then ZLD may be the only alternative for concentrate management
and thus the only means of enabling
the use of desalination treatment.
Unlike most other concentrate
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management strategies, conventional
ZLD (defined for the purposes of
this work as the combination of a
brine concentrator and crystallizer)
can be deployed almost anywhere
and has relatively few environmen­
tal, regulatory, or practical limita­
tions that restrict feasibility; how­
ever, the cost may be prohibitive.
Conventional ZLD treatment. Con­
ceptually, ZLD can be con­sidered as
a series of two progressive treatment
steps, including advanced concentra­
tion followed by solids generation.
Although these two steps may not
necessarily be distinct, depending on
the specific processes used, the
underlying concept is useful for
understanding ZLD treatment.
Accordingly, the discussion of ZLD
treatment in this article focuses on
the combination of brine concentra­
tors (advanced concentration) and
crystallizers (solids generation).
While there may be many varied
approaches to achieving ZLD, the
combination of brine concentrators
and crystallizers has the important
advantage of not being limited by
water quality. For example, Locke et
al. (2015) describe a groundwater
desalination plant owned by the City
of Palm Coast, Fla., that achieves
ZLD by applying a combination of
precipitative softening, ultra­
filtration, sludge thickening, and
dewatering to its reverse osmosis
(RO) concentrate. However, the suc­
cessful implementation of this tech­
nique is predicated on a raw water
supply with relatively low total dis­
solved solids (TDS) levels composed
predominantly of hardness. This
same approach would not be suit­
able for the concentrate of a seawater
desalination plant, consisting largely
of sodium and chloride that do not
readily precipitate. By contrast, the
coupling of brine concentrators and
crystallizers can accomplish ZLD
with almost any saline supply, inde­
pendent of TDS concentrations or
the relative proportion of its con­
stituent dissolved solids.
The literature contains numer­
ous descriptions of both brine
FIGURE 1
Process schematic for a typical brine concentrator
Feed brine
Concentrated brine
Steam vapor
Distillate/condensate
Vent
Deaerator
Feed tank
Heat
exchanger
Feed pump
Distillate
tank
Distillate pump
Brine
evaporator
Mechanical
vaper
compressor
Recirculation
pump
© General Electric Company. All rights reserved. Reprinted with permission.
Use of this image does not signify endorsement by General Electric Company.
con­centrators and crystallizers that
include detailed process discussions
(Ciszewski 2015, Yallaly 2015, Juby
et al. 2008, Mickley 2006). The
respective descriptions that follow
provide a concise general overview of
brine concentrator and crystallizer
technology, focusing on the points
most critical from a planning-level
perspective. Although these descrip­
tions are framed in the context of
employing RO as a primary desalina­
tion process, this assumption does not
necessarily limit the dis­cussion from
applying to the use of brine concen­
trators and crystallizers to reduce or
eliminate the concentrate of other
desalination technologies.
Brine concentrators. Unlike RO,
which employs pressure-driven
membrane separation to accomplish
desalination, brine concentrators
use mechanical vapor compression
technology to separate water from
dissolved solids via thermal evapo­
ration. A process schematic for a
typical brine concentrator is shown
in Figure 1.
The influent “feed brine” shown in
Figure 1 represents the concentrate
from a primary RO process. Acid
may be added to the brine concen­
trator feed to lower the pH, thereby
converting bicarbonate to dissolved
carbon dioxide, which is subse­
quently removed in the deaerator in
order to prevent corrosion. The
stripping of carbon dioxide also
removes alkalinity from the system,
thus reducing the potential for cal­
cium carbonate scaling onto heat
transfer surfaces, which reduces effi­
ciency. It is also common to seed cal­
cium sulfate crystals into the recircu­
lated brine slurry (shown in Figure 1
as the red “concentrated brine” pipe
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TABLE 1
Influence of feed TDS and maximum concentrated brine
effluent TDS on brine concentrator recoverya,b
Brine
Concentrator
Feed TDS
mg/L
Brine Concentrator Recovery at a Maximumc Concentrated
Brine Effluent TDS mg/L
%
200,000 mg/L
250,000 mg/L
300,000 mg/L
97.5
98.0
98.3
5,000
10,000
95.0
96.0
96.7
20,000
90.0
92.0
93.3
30,000
85.0
88.0
90.0
40,000
80.0
84.0
86.7
50,000
75.0
80.0
83.3
TDS—total dissolved solids
aAssumes
brine concentrator distillate TDS of 10 mg/L
brine concentrator recoveries are independent of feed flow.
limited by assumed scaling potential
bCited
cAs
loop driven by the recirculating
pump) to serve as precipitation
nuclei, preventing scaling by facilitating crystal growth. Scale inhibiting chemicals are also routinely
added to the feed to control silica
and other sparingly soluble species.
The “distillate/condensate” flow in
Figure 1 is the treated water produced by the brine concentrator, and
TDS concentrations are typically
below 10 mg/L. If a brine concentrator is used as the first step in a ZLD
process (as is the case in this discussion), then the bleed flow from the
concentrated brine shown in Figure 1
(red pipe and arrow at the bottom
right) serves as the crystallizer feed
flow. TDS levels in this residuals
stream generally vary from approximately 180,000 to 250,000 mg/L
depending on the scaling potential of
This facility employs both brine concentrator (two units at right) and crystallizer (one unit at
left) processes. Photo credit: ©General Electric Company. All Rights Reserved. Reprinted with
permission. Use of this image does not signify endorsement by General Electric Company.
40
alspach_npr.indd 40
ALSPACH & JUBY | JANUARY 2018 • 110:1
the constituent dissolved solids; concentrations can potentially be
increased to ~300,000 mg/L by
incorporating a softening pretreatment process.
Water recovery across the brine
concentrator (i.e., the percent of
influent feed flow that is converted
to distillate) varies with the feed TDS
concentration and its associated scaling potential. These influences are
demonstrated in Table 1, which
shows the brine concentrator recovery at feed TDS levels ranging from
5,000 to 50,000 mg/L for each of
three cases in which the effluent “concentrated brine” (as referenced in
Figure 1) is limited to 200,000 mg/L,
250,000 mg/L, and 300,000 mg/L,
respectively, as a result of scaling. As
indicated by these example cases,
feed water with lower TDS and less
scaling potential (i.e., higher TDS
limits in the “concentrated brine”
effluent) results in higher brine concentrator recoveries. It is im­portant
to underscore that the recoveries
shown in Table 1 are for the brine
concentrator process only; if a brine
concentrator and crystallizer are
used as successive steps in a ZLD
treatment process, the overall combined recovery is assumed to be
effectively 100% (i.e., an insignificant volume of liquid residuals).
Although the context of this discussion is the combination of brine
concentrators and crystallizers as
“conventional ZLD” that can be
applied largely independent of feed
water chemistry and flow, the use of
a crystallizer is not always a prerequisite for producing crystalline solids
for disposal. If the flow of concentrated brine from the brine concentrator is sufficiently low, discharge
to a lined evaporation pond may be
feasible (potential limitations of
land area and evaporation rate
notwithstanding).
Brine concentrator specific energy
consumption typically ranges from
60 to 90 kW·h/1,000 gal of water
recovered (i.e., treated water flow),
with higher degrees of concentration
(e.g., targets of 200,000 versus
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250,000 versus 300,000 mg/L, and
so on) requiring more energy per
unit volume (Ciszewski 2015, Juby
et al. 2008, Mickley 2006). By contrast, seawater desalination using
RO (SWRO) typically requires
10–15 kW·h/1,000 gal of permeate.
It is important to underscore that
SWRO and brine concentrators are
not functionally equivalent; the former is a primary desalination process that produces concentrate on
the order of about 70,000 mg/L,
while the latter would be applied to
the SWRO concentrate and yield a
residuals stream with TDS levels
roughly two to four times higher.
This comparison is provided only to
illustrate the large increase in specific energy necessary to achieve
concentrated solutions significantly
higher than those produced by a primary desalination process. The values for brine concentrator specific
energy consumption provided in this
discussion assume the use of electric
power and do not reflect the contribution of any available waste heat
or other thermal advantage.
Aside from the high energy requirements, the disadvantages of brine
concentrators are primarily associated with operations and maintenance (O&M). In addition to the
system complexity, brine concentrators respond more slowly to flow
changes than a primary RO process,
thus necessitating appropriately sized
equalization tanks for the feed flow
(as illustrated in Figure 1). Moreover,
a supply of steam is necessary for
brine concentrator start-up, including
after every maintenance event, both
complicating and extending the duration of the start-up process relative to
many other treatment technologies.
This steam may be provided by a
dedicated boiler or supplied by
another steam-generating process
onsite (e.g., in the case of co-location
with an applicable industrial manufacturing or power facility).
Brine concentrator O&M challenges can be especially problematic for potable water treatment
system operators who may not
FIGURE 2
Process schematic for a typical crystallizer
Feed brine
Concentrated brine
Steam vapor
Distillate/condensate
Crystallizer
vapor body
Brine heater
Mechanical
vapor
compressor
Recirculation
pump
© General Electric Company. All rights reserved. Reprinted with permission.
Use of this image does not signify endorsement by General Electric Company.
have experience with the complex
thermal desalination systems that are
uncommon in municipal applications outside of the Middle East.
Thus, a rigorous operator training
program is essential. Aesthetic concerns may also be a drawback,
depending on the location of the
Crystallizers. Like brine concentrators, crystallizers use mechanical
vapor compression to desalinate
highly concentrated brines, generating a slurry of solids. A process schematic for a typical brine concentrator
is shown in Figure 2. Continuing
through the treatment train of a
Conceptually, ZLD can be considered as a series
of two progressive treatment steps, including
advanced concentration followed by solids
generation.
treatment facility. Because brine concentrators (and crystallizers) bear
more resemblance to industrial equipment (see the photograph on page 40)
than typical drinking water treatment
plant processes, public acceptance
may be an important consideration.
conventional ZLD treatment scheme,
the “feed brine” shown in Figure 2 is
the concentrated residuals stream
produced by the brine concentrator.
Because the primary purpose of the
crystallizer is to produce precipitated
solids, neither acid nor scale inhibitors
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crystallizer operating costs is shown
in Table 2. The higher flows associated with RO concentrate would also
substantially affect the capital cost of
a crystallizer process, as the required
capacity would be approximately an
are used for pretreatment; however,
antifoaming agents may be added.
Concentrate from a primary RO process could also be treated with a crystallizer without a preceding brine
concentrator, although the costs
If the feed water has high concentrations of
species that more readily precipitate out of
solution, the specific energy requirements will
be lower.
order of magnitude higher than
would be the case for a treatment
train that included brine concentration before crystallization.
Water quality can also have an
influence on the specific energy
requirements for crystallizers. If the
feed water has high concentrations
would be much greater due to the
extremely high specific energy
requirements, which can range from
about 180 to 250 kW·h/1,000 gal
(Ciszewski 2015, Mickley 2006). An
example of the influence of the higher
RO concentrate flows (versus brine
concentrator residuals flows) on
TABLE 2
Example of influence of flow on crystallizer operating costs
Crystallizer
Specific Energy Crystallizer
Requirement
Feed Flow
kW·h/1,000 gal
mgd
Feed Water
200
Brine concentrator
residualsc
Primary RO concentrate
Annual
Energy
Usagea
106 kW·h
Annual
Energy
Costb
$
0.1
7.3
730,000
1
73
7,300,000
RO—reverse osmosis
aAssumes
bAssumes
continuous operation
$0.10/kW·h
brine, as depicted in Figure 2
cConcentrated
TABLE 3
Sample ZLD treatment cost summary
Capital Cost
O&M Cost
Case
Capacityb
mgd
millions
of $
$/gpd
millions of
$/year
ZLD-1
1.0
41.11
41.11
7.05
19.11
10.63
29.12
ZLD-2
0.5
28.38
56.77
3.78
20.29
6.25
34.25
ZLD-3
0.25
21.53
86.13
2.16
23.67
4.04
44.24
ZLD-4
0.125
16.64
133.11
1.28
28.07
2.73
59.87
$/1,000 millions $/1,000
gal
of $/year
gal
Source: Juby et al. 2008 (for “Brine B”)
O&M—operations and maintenance, ZLD—zero liquid discharge
aAmortization
bAssumes
42
alspach_npr.indd 42
Total Amortized
Costa
over 20 years at a 6% annual interest rate
100% recovery (i.e., ZLD feed flow = ZLD treated water flow)
ALSPACH & JUBY | JANUARY 2018 • 110:1
of species that more readily precipitate out of solution, the specific
energy requirements will be lower;
however, significant concentrations
of highly soluble species such as
nitrate salts require more energy
per unit volume of distillate produced. Also, as discussed in conjunction with brine concentrators,
the 180–250 kW·h/1,000 gal range
for crystallizer specific energy consumption provided in this discussion
is a conservative estimate for planning purposes that assumes the use of
electric power and does not reflect the
contribution of any available waste
heat or other thermal advantage.
The “distillate/condensate” flow
in Figure 2 is the treated water
produced by the crystallizer, and
TDS concentrations are typically
30–50 mg/L, a range that is somewhat higher than brine concentrator
distillate (Ciszewski 2015). The
recovery of the crystallizer process is
effectively 100% (i.e., the volume of
liquid waste is insignificant),
although this includes the use of a
centrifuge to dewater the solid slurry
residual stream (i.e., the “concentrated brine” in Figure 2) to produce
crystalline solids for landfilling or
beneficial use (if feasible). The centrate generated by centrifuge is typically returned to the crystallizer feed
flow (not shown in Figure 2). Crystallizers have many of the same disadvantages as brine concentrators,
including operational complexity,
the need for a supply of steam for
each system start-up, and aesthetics.
Crystallizers also respond relatively
slowly to flow changes, and although
not shown in Figure 2, equalization
storage is likewise recommended for
the crystallizer feed flow.
Residuals management. Although a
true ZLD treatment scheme does not
generate liquid residuals, the process
does produce solids (or a highly concentrated solid slurry) that must be
properly managed for either beneficial use or disposal. Because these
solids may contain significant levels
of regulated metals or other contaminants, it is important to conduct a
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hazardous waste characterization.
Solids that cannot be recycled for
beneficial use should be directed to a
landfill (or similar facility) that is
appropriately permitted to receive the
residuals in accordance with their
charac­terization. ZLD residuals are
typically transported by truck, and
the cost of this transportation, as well
as the comprehensive cost of residuals
management, must be incorporated
into any economic analysis of ZLD.
Any economic advantage from beneficial use would also be a factor in
this analysis.
Sample RO treatment cost summary at 80% recovery
TABLE 4
Capacity
Case
Feed
mgd
Permeate
mgd
Concentrate
mgd
Capital
Costa
$/gpd
O&M
$/1,000 gal
Total
Amortized
Costb
$/1,000 gal
Costa
RO-1
5
4
1
0.58
0.30
0.44
RO-2
2.5
2
0.5
0.80
0.40
0.59
RO-3
1.25
1
0.25
1.11
0.60
0.87
RO-4
0.625
0.5
0.125
1.54
0.70
1.07
O&M—operations and maintenance, RO—reverse osmosis
aAdapted
from Bergman and Elarde 2003 (escalated to 2008 dollars)
over 20 years at a 6% annual interest rate
bAmortization
COST CONSIDERATIONS
Cost comparison and normalization. As
previously discussed, the specific
energy consumption for both brine
concentrators and crystallizers is
much higher than for seawater
desalination, which is among the
most energy-intensive processes used
in potable water treatment. These substantial energy requirements translate
into high ZLD treatment costs. Examples of ZLD costs for systems with four
different capacities are summarized in
Table 3, which is adapted from a 2008
US Bureau of Reclamation (USBR)
study, Report No. 149 in the series
prepared under the Desalination
and Water Purification Research
and Development Program (Juby et
al. 2008). The costs in Table 3
include both brine concentrators and
crystallizers, and 100% recovery is
assumed such that the feed and
treated water flows are equivalent;
capital costs were amortized over 20
years at a 6% annual interest rate. The
ZLD costs in USBR Report No. 149
include landfilling solids at $50/ton;
this cost could increase if testing
determines that the crystallized solids constitute a hazardous waste.
As shown in Table 3, amortized
ZLD unit costs range from approximately $30–$60/1,000 gal over
capacities spanning 1 mgd down to
0.125 mgd. For the purposes of comparison, AWWA M61 (Desalination
of Seawater) cites amortized unit costs
for SWRO that vary from approximately $2.50/1,000 gal–$5/1,000 gal
TABLE 5
Sample combined treatment cost summary for RO + ZLD
at 80% RO recovery
Case
Capacitya
mgd
Capital
Cost
$/gpd
O&M
Cost
$/1,000 gal
Total Amortized
Costb
$/1,000 gal
RO-1 → ZLD-1
5.0
8.68
4.10
6.18
RO-2 → ZLD-2
2.5
11.99
4.46
7.32
RO-3 → ZLD-3
1.25
18.11
5.21
9.54
RO-4 → ZLD-4
0.625
27.85
6.17
12.83
O&M—operations and maintenance, RO—reverse osmosis, ZLD—zero liquid discharge
aAssumes
flow)
100% recovery over the treatment plant (i.e., feed flow = RO permeate flow + ZLD distillate
bAmortization
over 20 years at a 6% annual interest rate
over a very wide range of capacities (AWWA 2011). Even at unit
costs that are double these figures
(i.e., $5–$10/1,000 gal), SWRO—
one of the most expensive water
shown in Table 4. The example costs
in this table are adapted from
Bergman and Elarde (2003), who
developed best-fit curves of cost figures from operating desalination
Utilities evaluating ZLD may opt to rely on
performance-based equipment specifications
rather than detailed mechanical and process
design conducted by a third party.
treatment processes—is much less
expensive than ZLD.
The difference in costs is even
more pronounced for the desalination of brackish water, which is more
characteristic of inland locations, as
plants. For the purposes of comparison with the ZLD costs published in
USBR Report No. 149, the costs in
Table 4 are escalated to 2008 dollars
(as applicable) and amortized using
similar terms (i.e., 20 years at 6%
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Sample RO + ZLD treatment cost comparison at 80% RO recovery
TABLE 6
RO System
ZLD System
Combined Treatment
Case
Permeate
Flow
mgd
Total Amortized
Costa
$/1,000 gal
Distillate
Flow
mgd
Total Amortized
Costa
$/1,000 gal
Treated Water
Production
mgd
Total Amortized
Costa
$/1,000 gal
RO-1 → ZLD-1
4.0
0.44
1.0
29.12
5.0
6.18
RO-2 → ZLD-2
2.0
0.59
0.5
34.25
2.5
7.32
RO-3 → ZLD-3
1.0
0.87
0.25
44.24
1.25
9.54
RO-4 → ZLD-4
0.5
1.07
0.125
59.87
0.625
12.83
RO—reverse osmosis, ZLD—zero liquid discharge
aAmortization
over 20 years at a 6% annual interest rate.
interest). The cases shown in Table 4
are designed to align with those in
Table 3, such that the concentrate of
case RO-1 becomes the feed to the
ZLD-1 case. A recovery of 80% was
used for the example RO systems,
which were assumed to be treating
brackish water with a TDS concentration similar to the raw water supply
used to generate the ZLD costs in
USBR Report No. 149: ~2,300 mg/L
treated with RO, with the concentrate feeding the ZLD process.
A comparison of the amortized
unit costs in Tables 3 and 4 indicates
that ZLD treatment is roughly 1 to
2 orders of magnitude more expensive than typical brackish water
Sample RO treatment cost summary at 90% recovery
TABLE 7
Capacity
Permeate
mgd
Concentrate
mgd
Capital
Costa
$/gpd
O&M
Costa
$/1,000
gal
Total
Amortized
Costb
$/1,000 gal
Case
Feed
mgd
RO-1A
5.0
4.5
0.5
0.55
0.25
0.38
RO-2A
2.5
2.25
0.25
0.76
0.35
0.53
RO-3A
1.25
1.125
0.125
1.05
0.60
0.85
O&M—operations and maintenance, RO—reverse osmosis
aAdapted
from Bergman and Elarde, 2003 (escalated to 2008 dollars)
over 20 years at a 6% annual interest rate
bAmortization
TABLE 8
Sample RO + ZLD combined treatment cost summary at 90%
RO recovery
Case
Capacitya
mgd
Capital Cost
$/gpd
O&M Cost
$/1,000 gal
Total Amortized
Costb
$/1,000 gal
RO-1A → ZLD-2
5.0
6.17
2.29
3.77
RO-2A → ZLD-3
2.5
9.30
2.68
4.90
RO-3A → ZLD-4
1.25
14.26
3.35
6.75
O&M—operations and maintenance, RO—reverse osmosis, ZLD—zero liquid discharge
aAssumes
flow)
100% recovery over the treatment plant (i.e., feed flow = RO permeate flow + ZLD distillate
bAmortization
44
alspach_npr.indd 44
over 20 years at a 6% annual interest rate
ALSPACH & JUBY | JANUARY 2018 • 110:1
desalination using RO. The figures
in Table 4 are not intended as precise estimates of RO costs, but
rather representative figures for the
purposes of illustration; even significant but reasonable variation of
the total amortized costs for RO in
Table 4 will not render these costs
comparable with those for ZLD
shown in Table 3. Cost comparisons
such as these are common in literature discussing ZLD, and they are
valid for illustrating the starkly
higher expense of ZLD treatment.
However, this exercise is not a truly
equitable comparison, given that the
unit costs for ZLD and RO are normalized for the flows treated by each
process—distillate and permeate,
respectively. In practice, an inland
desalination facility employing ZLD
for concentrate management would
have both RO and ZLD processes,
and the total cost of treatment (i.e.,
RO + ZLD) should be normalized by
the total treated water flow (i.e., RO
permeate + ZLD distillate). The
impact of properly accounting for
the capital, operating, and total
amortized costs of a combined RO
(from the example cases in Table 4)
and ZLD (from the example cases in
Table 3) is shown in Table 5.
The total amortized costs of RO
(from Table 4), ZLD (from Table 3),
and combined RO + ZLD (Table 5)
systems are summarized in Table 6
for direct comparison. Although
the total amortized costs for the
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12/11/2017 4:34:31 PM
combined treatment are generally an
order of magnitude higher than for
RO alone, they are also roughly an
order of magnitude less expensive
than ZLD alone.
Because minimizing the flow to the
ZLD process can significantly reduce
treatment costs, the impact of increasing RO recovery in a combined RO +
ZLD system can have a substantial
financial impact. Tables 7, 8, and 9
replicate Tables 4, 5, and 6, respectively, assuming the RO recovery has
been increased from 80 to 90%. Thus,
the RO-1 system concentrate flow
changes from 1 to 0.5 mgd (case
RO-1A), which then aligns with ZLD
flow in case ZLD-2. The RO-2 concentrate flow decreases from 0.5 to
0.25 mgd (case RO-2A), which aligns
with the ZLD flow in case ZLD-3.
Likewise, the RO-3 concentrate is
reduced from 0.25 to 0.125 mgd (case
RO-3A), aligning with the ZLD flow
in case ZLD-4. (Because the cost data
provided in USBR Report No. 149 are
for discrete and specific flows, there is
no companion ZLD example for the
case in which the RO-4 system concentrate decreases by half.)
Note that this example of increasing the RO recovery to 90% neglects
the resulting change in concentrate
salinity. Although the TDS levels in
the RO concentrate would roughly
double in cases in which the recovery was increased from 80 to 90%
(i.e., halving the concentrate flow
while assuming a constant 100%
salt rejection for the purposes of
illustration), the impact on ZLD
treatment costs due to this change
in salinity would be very small relative to the influence of the de­creased
flow; however, both the decreased
recovery is substantial, reducing the
total amortized unit cost of treated
water by 30 to 40% for the examples
cases shown. Overall, this analysis
underscores three important points to
Because minimizing the flow to the ZLD process
can significantly reduce treatment costs, the
impact of increasing RO recovery in a combined
RO + ZLD system can have a substantial
financial impact.
flow and increased TDS would
result in lower ZLD treatment costs.
The influence of increasing the
recovery of a primary RO process
from 80 to 90% is summarized in
Table 10, including the respective
total amortized costs and the percent
cost reduction associated with the
recovery increase. These examples are
not representative of installed facilities making adjustments to increase
RO recovery from 80 to 90%; in
these cases the capacity of the ZLD
system would be oversized by a factor
of two, skewing the capital costs.
Instead, this example illustrates the
comparison of options in the planning stage and the impact on total
amortized cost if 90 versus 80% primary RO recovery can be achieved in
conjunction with a ZLD system sized
appropriately for each case. As shown
in Table 10, the impact of increased
understand about conventional ZLD
systems (i.e., the combination of brine
concentrators and crystallizers).
(1) Economies of scale are significant. As illustrated in Table 3, economies of scale significantly affect the
unit cost of water produced by a
ZLD system. This impact likewise
heavily influences the unit cost of
desalinated water in a combined RO
+ ZLD system, as summarized in
Table 10. Thus, to the extent that
larger capacity systems are feasible
and affordable (i.e., within budget),
it is economically advantageous to
maximize the size of a planned RO +
ZLD desalination plant.
(2) Maximizing primary RO
recovery is important for ZLD
affordability. For a given total available flow of water (e.g., the rights to
5 mgd of a brackish water supply), it
is economically advantageous to
Sample RO + ZLD treatment cost comparison at 90% RO recovery
TABLE 9
RO System
Case
Permeate
Flow
mgd
ZLD System
Total Amortized
Costa
$/1,000 gal
Distillate
Flow
mgd
Total Amortized
Costa
$/1,000 gal
Combined Treatment
Treated Water
Production
mgd
Total Amortized
Costa
$/1,000 gal
RO-1A → ZLD-2
4.5
0.38
0.5
34.25
5.0
3.77
RO-2A → ZLD-3
2.25
0.53
0.25
44.24
2.5
4.90
RO-3A → ZLD-4
1.125
0.85
0.125
59.87
1.25
6.75
RO—reverse osmosis, ZLD—zero liquid discharge
aAmortization
over 20 years at a 6% annual interest rate
ALSPACH & JUBY | JANUARY 2018 • 110:1 | JOURNAL AWWA
alspach_npr.indd 45
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12/11/2017 4:34:49 PM
TABLE 10 Comparison of RO + ZLD combined treatment costs at 80 and 90% RO recovery
RO + ZLD at 80% RO Recovery
Treated Water
Capacitya
5.0
Case
Total Amortized Costb
$/1,000 gal
RO-1 → ZLD-1
6.18
RO + ZLD at 90% RO Recovery
Case
Total Amortized
Costb
$/1,000
Unit Cost
Reduction
%
RO-1A → ZLD-2
3.77
39
2.5
RO-2 → ZLD-2
7.32
RO-2A → ZLD-3
4.90
33
1.25
RO-3 → ZLD-3
9.54
RO-3A → ZLD-4
6.75
29
RO—reverse osmosis, ZLD—zero liquid discharge
aAssumes
100% recovery over the treatment plant (i.e., feed flow = RO permeate flow + ZLD distillate flow)
over 20 years at a 6% annual interest rate
bAmortization
maximize primary RO permeate
flow (e.g., 4.5 versus 4 mgd) and
minimize the flow to the ZLD pro­
cess (e.g., 0.5 versus 1 mgd).
(3) The unit costs of seawater de­­
salination and inland desalination
with ZLD treatment are roughly
comparable. The properly amor­
tized unit costs of RO + ZLD treat­
ment summarized in Table 7 are on
the same order of magnitude as the
$2.50–$5/1,000 gal figures indi­
cated for SWRO in AWWA’s M61
Desalination of Seawater (2011), as
suppliers, these utilities do have the
economically com­parable alterna­
tive of brackish water desalination
in combination with ZLD treat­
ment. The latter case is limited only
by the accessibility of brackish
water supplies, not by either pro­
hibitively disproportionate econom­
ics or the lack of a viable concen­
trate management strategy.
Other cost considerations. Because
of the large specific energy con­
sumption (kW·h/1,000 gal) associ­
ated with both brine concentrators
The specific energy consumption for both brine
concentrators and crystallizers is much higher
than for seawater desalination, which is among
the most energy-intensive processes used in
potable water treatment.
previously discussed, and they com­
pare very well for the RO + ZLD
cases at 90% RO recovery. Account­
ing for the RO + ZLD system econ­
omies of scale, as demonstrated by
the declining amortized unit cost
trends with increasing treated water
capacity shown in Table 10, larger
RO + ZLD systems would likely
compare even more favorably with
SWRO. Thus, although SWRO is
not an option for inland water
46
alspach_npr.indd 46
ALSPACH & JUBY | JANUARY 2018 • 110:1
and crystallizers, the unit price of
energy ($/kW·h) exerts a substantial
influence on the cost of desalination
for cases in which conventional ZLD
treatment is used for concentrate
management. Likewise, for these
same cases, because of the high unit
cost of treating water using ZLD (as
shown in Table 3, expressed as
$/1,000 gal), minimizing the ZLD
flow can significantly reduce the
overall cost of desalination. Thus, as
applied to a primary RO process,
minimizing the concentrate (i.e.,
ZLD feed) flow reduces both the
required ZLD capacity and the
associated energy expended, result­
ing in lower capital and operating
costs, respectively.
Although ZLD capital costs are
not sensitive to changes in water
quality, the operating cost is influ­
enced by the concentrations of both
TDS and relative levels of the con­
stituent components making up the
TDS. For example, in comparing two
cases in which ZLD feed flows are
similar, the flow with the higher TDS
requires less concentration, reducing
the specific energy consumption,
which in turn lowers the cost. In
terms of constituent dissolved solids,
greater proportions of highly soluble
species will increase the specific
energy necessary to accomplish ZLD
treatment; nitrates are among the
most notable of these soluble species.
The economy of scale associated
with ZLD treatment is demonstrated
in Table 3, with higher unit costs for
smaller capacity systems. Thus, in
terms of the cost per unit volume of
water produced, larger ZLD systems
are more economical than small facil­
ities. However, the economy of scale
is limited by the size of discrete brine
concentrator and crystallizer units for
which components can be trans­
ported; currently this limit is achieved
for ZLD systems with a capacity in
the range of ap­­proximately 1,000 to
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12/13/2017 8:25:17 AM
1,200 gpm (Ciszewski 2015). Beyond
this limit, larger-capacity ZLD systems consist of increasing numbers of
discrete treatment units (i.e., rather
than larger units), negating further
economies of scale.
INSTALLATIONS AND
INSTITUTIONAL KNOWLEDGE
Although ZLD treatment is used
in many industrial applications, at
present there is only one known
municipal water treatment
in­s tallation in the United States
using a brine concentrator to treat
the residuals from a primary RO
process, and the concentrated brine
is directed to an evaporation pond;
there are no known municipal
applications of crystallizer treatment. Thus, institutional knowledge regarding ZLD remains
extremely limited in the potable
water and reuse sectors. However,
despite the dearth of municipal
installations, the two processes
described as conventional ZLD in
this paper (i.e., brine concentrators
and crystallizers) are proven technologies that are well understood.
Consequently, utilities evaluating
ZLD may opt to rely on performance-based equipment specifications rather than detailed mechanical and process design conducted
by a third party. In addition,
because of the complexity of ZLD
systems, the use of contract operation may be a strategic consideration.
SUMMARY AND CONCLUSIONS
Although the conventional ZLD
combination of a brine concentrator
and crystallizer is an expensive
option for concentrate management,
it offers several critical advantages
that broadly enhance the viability of
desalination for treating alternative
water resources:
• Feasible deployment virtually
independent of concentrate
water quality
• Lack of environmental and regulatory permitting constraints
that inhibit many other concentrate management options
• Overall costs (including primary
RO + ZLD) roughly comparable
to seawater desalination
However, unlike SWRO, the use
of RO in conjunction with ZLD is
potentially accessible for utilities
in nearly any location, whether
coastal or inland. Thus, a potable
water purveyor at any inland location with a saline source of supply
could employ RO to augment its
portfolio of resources at a cost no
more significant than that associated with the desalination of seawater that is available to its coastal
counterparts.
ABOUT THE AUTHORS
Brent Alspach (to
whom
correspondence
may be addressed)
is director of
applied research at
Arcadis, 2175 Salk
Ave., Ste. 130, Carlsbad, CA 92008
USA; brent.alspach@arcadis.com.
He is an AWWA Water Quality &
Technology Division trustee and
chair of the AWWA 2018
International Symposium on Potable
Reuse. He also serves on the Opflow
editorial advisory board and is the
2014 recipient of AWWA’s Golden
Spigot award. Alspach is also the
sitting president of the American
Membrane Technology Association.
Graham Juby is senior project
manager with Carollo Engineers in
Costa Mesa, Calif.
https://doi.org/10.5942/jawwa.2018.110.0005
REFERENCES
AWWA, 2011 (1st ed.). Manual of Water
Supply Practices M61: Desalination of
Seawater. AWWA, Denver.
Bergman, R.A. & Elarde, J.R., 2003. The Cost
of Membrane Softening and Desalting
for Municipal Water Supplies. Proc.
AWWA Membrane Technology
Conference, Atlanta.
Ciszewski, D., 2015. Personal
communications, February–April. Global
commercial and process engineering
leader, GE Water & Process
Technologies, Feasterville-Trevose, Pa.
Juby, G.; Zacheis, A.; Shih, W.;
Ravishanker, P.; Mortazavi, B.; &
Nusser, M.D., 2008. Evaluation and
Selection of Available Processes for a
Zero-Liquid Discharge System for the
Perris, California, Ground Water
Basin, Report No. 149. US Bureau of
Reclamation Desalination and Water
Purification Research and
Development Program, Washington.
Locke, P.; Greiner, F.; Popko, R.; & Cooke,
N., 2015. ZLD Process Utilizes
Ultrafiltration to Achieve 98.5% RO
Recovery. Proc. American Membrane
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Orlando, Fla.
Mickley, M.C., 2006. Membrane
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Washington.
Yallaly, B., 2015. Vice-president, Carollo
Engineers Inc. Personal
communication.
AWWA RESOURCES
• Consider Brackish
Groundwater Supplies for
Desalination. Maliva, R.G.;
Manahan, W.S.; & Guo, W., 2014.
Opflow, 40:2:22. Product No.
OPF_0079602.
• Forward Osmosis: Novel
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& Cath, T.Y., 2014. Journal AWWA,
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JAW_0079287.
• Seawater Desalination: New
Challenges Mark a Changing
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These resources have been
supplied by Journal AWWA staff.
For information on these and
other AWWA resources, visit
www.awwa.org.
Journal AWWA welcomes
comments and feedback
at journal@awwa.org.
ALSPACH & JUBY | JANUARY 2018 • 110:1 | JOURNAL AWWA
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