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Innovative Technologies Increase Evaporation Pond Efficiency

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Desalination of brackish groundwater is an increasingly important option for inland communities. However, disposing of concentrated saline residual waste streams in evaporation ponds is land intensive. Large facilities might be concerned that this technology is one of the few treatment methods that offers decreasing returns to scale because of increasing boundary-layer resistance for larger ponds. This study evaluated several innovative options for improving evaporation pond performance, including fabric evaporators, wetted-boundary layer breakers, salt-tolerant plants, and droplet spraying. Cost models were developed for boundary-layer breakers and droplet spraying. Incremental costs and evaporation enhancements are compared with site-specific cost information for a wastewater treatment facility in California's Central Valley. Results indicate that boundary-layer breakers and spray technologies are cost-effective compared to a simple pond expansion. Boundary-layer breakers appear to be more cost-effective per gal of incremental capacity but have a lower evaporation enhancement capacity compared to droplet spraying (24 percent vs. 35 percent). For a new facility, an example calculation with preliminary cost information indicates that spray evaporation is more cost-effective because of avoided pond excavation and lining costs. Boundary-layer breakers as a retrofit to an existing facility are preferred if they provide sufficient additional capacity to avoid the need for pond expansion.
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74 IDA JournAl | FIrst QuArter 2010 www.IDADesAl.org
vaporation ponds are lined detention basins into
which wastewater is discharged and held to allow
evaporation to decrease the water’s volume. Be-
cause the cost of wastewater disposal depends on volume,
not concentration, it is more cost-effective to dispose of
a small amount of extremely saline water than a large
amount of slightly saline water. Evaporation ponds can
be a cost-effective alternative for dewatering compared to
other energy-intensive methods and are used primarily in
semi-arid regions where land costs are low.
Evaporation ponds have some disadvantages, including
the expense of impervious liners to prevent seepage of sa-
line water into aquifers and the need for large quantities
of land when high levels of evaporation are required (Gil-
ron et al, 2003). Research has focused on finding ways to
increase evaporation rates, which would result in reduc-
ing pond size, liner costs, and land (Ahmed et al, 2002).
This report evaluates available alternatives to enhance
evaporation from these ponds.
Literature Review
Several researchers have explored ways to enhance evap-
oration rates. For example, water’s hydrogen bonds may
be weakened by attaching foreign molecules to the sur-
face, resulting in increased evaporation rates (Kingdon,
1963). The gases examined included helium, nitrogen,
butane, and oxygen, with butane working best. Another
method used electric wind created at 5,250 V to produce
turbulence in the water, resulting in a four-fold increase
of evaporation rates (Barthakur and Arnold, 1995). Other
researchers have explored the possibility of adding dye
to maximize use of solar energy (Winans, 1967; Keyes,
1966; Bloch et al, 1951). A colored solution would absorb
more solar energy, increase temperature, lower surface
tension, increase saturation vapor pressure, and subse-
quently increase evaporation rate (Ahmed et al, 2002).
Researchers observed increased evaporation with the ad-
dition of methylene blue dye (Keyes, 1966) and recom-
mended adding 3.5 grains of dye per ft3 of brine (Bloch
et al, 1951).
Other possible methods to enhance evaporation in-
clude spraying the brine, increasing pond turbulence,
passing the brine over inclined rough surfaces, and creat-
ing airflow over the pond (Ahmed et al, 2002). Among the
various alternatives, the following options were targeted
for detailed review (Table 1):
n Wind-aided intensified evaporation (WAIV)
n Wetted floating fins
n Salt-tolerant plants
n Droplet spraying
E
Desalination of brackish groundwater is an increasingly important option for inland communities. However, disposing of concentrated saline
residual waste streams in evaporation ponds is land intensive. Large facilities might be concerned that this technology is one of the few treatment
methods that offers decreasing returns to scale because of increasing boundary-layer resistance for larger ponds. This study evaluated several
innovative options for improving evaporation pond performance, including fabric evaporators, wetted-boundary layer breakers, salt-tolerant
plants, and droplet spraying. Cost models were developed for boundary-layer breakers and droplet spraying. Incremental costs and evaporation
enhancements are compared with site-specific cost information for a wastewater treatment facility in California’s Central Valley. Results indicate
that boundary-layer breakers and spray technologies are cost-effective compared to a simple pond expansion. Boundary-layer breakers appear
to be more cost-effective per gal of incremental capacity but have a lower evaporation enhancement capacity compared to droplet spraying (24
percent vs. 35 percent). For a new facility, an example calculation with preliminary cost information indicates that spray evaporation is more
cost-effective because of avoided pond excavation and lining costs. Boundary-layer breakers as a retrofit to an existing facility are preferred if
they provide sufficient additional capacity to avoid the need for pond expansion.
Innovative Technologies Increase
Evaporation Pond Efficiency
Shamia Hoque, Terry Alexander, and Patrick L. Gurian
www.IDADesAl.org FIrst QuArter 2010 | IDA JournAl 75
Although they require more maintenance than WAIV
and wetted floating fins, salt-tolerant plants called
“halophytes” could be used to enhance evaporation.
WAIV. WAIV is a method in which water is pumped
onto fabrics to provide more surface area for evaporation.
Gilron et al (2003) conducted two WAIV-configuration ex-
periments. The first was a rooftop setup in which non-
woven geotextile and woven-netting fabric surfaces were
tested. The fabrics were constantly wetted by water sup-
plied from a head tank or feed pipe. The fabrics each had
an exposed length of about 5.74 ft (1.74 m) running from
head tank to collection tank. Several methods of imple-
menting WAIV require pumping water to an elevated stor-
age tank. The methods differed, however, in whether wa-
ter from the head tank reached the fabric strips through
capillary action or gravity. The gravity method proved
to be more efficient. From the elevated tanks, water ran
down the fabric at a constant rate. The test areas ranged
in size from 2 ft2 to 6.5 ft2. Brine used in testing had total
dissolved solids (TDS) of 16–18 g/L and was supersatu-
rated with calcium.
The second setup involved a larger evaporation sur-
face (107–430 ft2), which was stretched over a vertical
height of about 5 ft in a rectangular array. In this meth-
od, it was essential to keep the materials from drying
out. During a two-month study, one of the fabrics dried
out from lack of feedwater. Despite rewetting, the mate-
rial’s efficiency fell significantly because salts had de-
posited in the fabric and impeded water flow. To help
reduce the amount of residual salts on the fabric and
keep the material from drying out, the material had to
be rinsed with water and, if necessary, citric acid every
two months. The study concluded that woven-netting
fabric is more effective than nonwoven-geotextile fabric
because woven-netting fabric had significantly less salt
on its surface.
Tested against an open-pan evaporation pond, this
method increased the amount of evaporation by 50 per-
cent on a given area of fabric compared to an open-water
surface (Gilron et al, 2003). Overall a 13-fold increase in
performance for a given land area was reported because
it was possible to fit a large fabric surface area within a
relatively small footprint. Also, the most stringent test was
Table 1. Evaporation Enhancement Options*
Alternative Physical Basis Potential Improvement Comments
WAIV1
Water drips from an
elevated tank and
evaporates from a vertical
hanging cloth.
13-fold increase in evaporation Only tested with TDS up to 18,000 ppm (“considered
severest test”) and in small-scale applications.
Wetted floating fins
(aluminum fins)2
“Boundary layer breaker”
and adds additional area
for exchange.
24% increase in evaporation using cotton
fabric for covering. Improvement may be
greater in windier environments.
Fins must be perpendicular to wind.
No information on large-scale applications is available.
The effects of waves and differing salinity levels have
not been studied and may affect performance.
Salt-tolerant plants3 Plants absorb and filter. 30% at ~90,000 mg/L TDS, greater at
lower TDS.
Tested in an 840-gal/day application. In the study, the
plants grown were suitable for cattle to graze on
(without removing salt crystals).
Droplet spraying:
Fountain or misting
during daytime4
Spraying water droplets
into the air results in
increased air–water
interface.
Overall increase of evaporation is
30–35%. Results show that 2–15% of
water sprayed is lost to evaporation from
a single nozzle.
Overall evaporation based on 24 nozzles in an
evaporation pond of area 19,264 ft2. Most data based
on fresh/tap water.
Droplet spraying:
Cooling tower5
Water is sprayed from the
top into the air, resulting in
lowering of the water
temperature. Some water
evaporates during the
process.
Evaporation loss averages approximately
1% for each 12.6°F (10.7°C) drop in
water.
According to ASHRAE, evaporation loss averages
approximately 1% for each 12.6°F drop in water
temperature.
The focus has been on minimizing evaporation losses in
cooling towers. However, the information available can
be used to optimize a cooling tower for this application.
*Maximum evaporation from open water surface is 0.16 in./day based on observational data from NOAA using a class A evaporation pan (47.5-in. diameter and 10-in. depth) for the
month of July 2006.
1Gilron et al, 2003
2Guitierrez and Roman, 1993
3Negri et al, 2003
4Gault, 1986; Lorenzini, 2006; and Burt et al, 2005
5Quereshi et al, 2006
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conducted with water having a TDS level of 18 g/L. Higher
concentrations of salt than those tested would produce
more deposits on the fabric, which could increase the fre-
quency and cost of maintenance.
Wetted Floating Fins. Another potential method is float-
ing aluminum fins to enhance the surface area available
for evaporation (Figure 1). This method uses the material’s
absorbent properties to elevate the water vertically but
does not require pumping. Floating fins would provide an
additional area of exchange and act as a boundary-layer
breaker (Guitierrez and Roman, 1993).
A 50-cm x 50-cm cross section of aluminum was tested
in a laboratory wind tunnel. The water container, measur-
ing 100 cm x 19.5 cm x 10 cm, was thermally insulated
(Guitierrez and Roman, 1993). An electric heater was used
to heat the water from the bottom. The experiments inves-
tigated eight fins at different orientations to the wind—
three parallel and five perpendicular, measuring 88 cm x
3.8 cm x 0.32 cm and 19.5 cm x 3.8 cm x 0.32 cm, respec-
tively. The fins were covered with cotton cloth to keep
them permanently wet through the cotton’s wicking ac-
tion (Guitierrez and Roman, 1993). Temperature, humid-
ity, and wind speed were also measured.
Guitierrez and Roman (1993) defined a coefficient of
effective additional fin area, ε. Its value was determined
through experiments and mathematical analysis. The val-
ue of ε determines the equivalent free-surface area of a
unit of fin area added. For example, for a fin area of a,
the additional equivalent free-surface area would be εa.
The study showed that—in heated water (45–65oC) and
wind of 5 mi/s—for perpendicular fins ε = 0.76 and for
parallel fins ε = 0.16. Therefore, fins would have to be
placed perpendicular to wind flow to obtain maximum
advantage. When water temperature was < 20oC for per-
pendicular fins, ε varied between 0.26 for wind speeds
of 1 mi/s to 1.2 for wind speeds of 5 mi/s. A 24 percent
enhancement of evaporation could be obtained in un-
controlled ambient conditions for a value of 0.6 for ε
(Guitierrez and Roman, 1993). Fin effectiveness depend-
ed on orientation, relative humidity, and wind speed.
Guitierrez and Roman (1993) also determined that the
most effective spacing between each fin would be five
times a fin’s height.
Salt-Tolerant Plants. Although they require more main-
tenance than WAIV and wetted floating fins, salt-tolerant
plants called “halophytes” could be used to enhance
evaporation. Figure 2 shows an example of a halophyte
(Spartina alterniflora). A plant’s natural transpiration
processes (evaporation through leaf stomata) increases
the effective surface area in which evaporation occurs.
A study on salt-tolerant plants concluded that saltwater
cordgrass (Spartina alterniflora) and great bulrush
(Scirpus validus) were acceptable for treating water with
TDS greater than 100g/L and chloride of about 60 g/L
(Negri et al, 2003). Other halophyte species tested in-
cluded coastal dropseed (Sporobolus virginicus), peren-
nial glasswort (Salicornia virginica), sawgrass (Cladium
jamaicenese), and vermillion cordgrass (Spartina al-
terniflora v ar. vermillion). For dissolved solids of about
3 percent or less, several-fold evaporation rate increases
are possible. Although winter weather would decrease
evapotranspiration rates, evaporation rates would still
be higher than for open-surface evaporation (Negri et al,
2003). Additional maintenance costs could be offset by
potential sales of harvested halophytes. The water evap-
orated and salt-covered plant leaves would have to be
tested for a specific facility. In the previous study, tests
on the salt-covered leaves did not reveal significant lev-
els of toxic contaminants, so cattle were allowed to graze
on the halophytes (Negri et al, 2003). The economics of
the process should be investigated before application,
but Keyes (1966) reported the process was economically
feasible.
Droplet Spraying. Spraying water droplets into the air
can enhance evaporation by increasing the area of air–
water interface (Gault, 1986). The spray method could be
implemented by constructing a sprinkler system on the
pond or installing a cooling tower on the evaporation
pond.
Sprinkler System. To determine sprinkler system or
spray pond effectiveness, a pond 172 ft x 112 ft (52 m
x 34 m) was constructed and fitted with an array of 24
nozzles at the center of the pond (Gault, 1986). The noz-
zles sprayed at a pressure of 10 psig and a flow rate
of 8 gpm/nozzle. Low pressure was maintained to re-
duce spray drift. In the month of June in Wyoming, an
Figure 1. A schematic representation of the wetted floating
fin method
Air flow direction
Insulation
Fins
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Most studies have focused on reducing evaporative
losses in cooling towers, but this scenario focuses on
maximizing evaporation loss in towers and from ponds.
evaporation rate increase of 0.34 in. was ob-
served. Modifying sprinkler height, nozzle
angle, and flow rate yielded an overall evap-
oration rate increase of about 35 percent.
A single droplet’s evaporation loss during
flight ranged from 2 percent to 15 percent,
with smaller droplets losing more because
of their higher surface-area-to-mass ratio
(Lorenzini, 2006). Other researchers have
reported enhanced evaporation through
spraying (Burt et al, 2005). Performance is
affected by relative humidity, air tempera-
ture, rate of application, and wind loss. A
California-based study noted that evapora-
tion from stationary sprinklers could range
from 0 percent to 50 percent over short pe-
riods, and three to four times more evapo-
ration could occur during daylight hours
compared with nighttime in California’s Im-
perial Valley (Hermsmier, 1973).
Droplets traveled farther with increasing diameter
(Lorenzini, 2006). The maximum droplet diameter noted
in the research was 0.2 in. (5 mm), which traveled 66 ft
(20 m) when released from a nozzle height of 14.8 ft
(4.5 m) at a flow rate of 0.008 gpm (5.5 x 10-4dm3/s) and
at a jet inclination of 25° (Lorenzini, 2006). However, in
most cases the distance traveled for droplets from nozzle
angles ranging from 0° to 10° was within 15 ft depending
on nozzle height (4–13 ft) for droplet diameters ranging
from 0.02 in. to 0.1 in. (0.5–2.5 mm) (Lorenzini, 2006). The
data were based on freshwater or tap water, not brine.
Cooling Tower. The cooling tower method has not been
researched in detail with regard to evaporation loss. How-
ever, installing a cooling tower would ensure that spray
droplets are contained and high wind velocities will not
be a concern.
A cooling tower is a heat-rejection device that releases
heat into the atmosphere by cooling a water stream to a
lower temperature. The heat raises the air temperature and
relative humidity. During cooling, a small amount of water
evaporates, causing overall temperature to decline further.
Cooling towers can be cross flow, counter flow, or parallel
flow, depending on air flow direction. Water is pumped
to the top, flows to the bottom, and is usually sprayed or
dripped through internal fill material to increase air and
water mixing efficiency (NCDENR, 2006).
Water can be lost in a cooling tower through drift,
blowdown, and, most significantly, evaporation (Qureshi
et al, 2006). Blowdown occurs when water is removed
from the recirculating cooling water to reduce contami-
nant buildup in tower water (NCDENR, 2006). Drift is
loss of water from the cooling tower in the form of mist
carried out of the tower by an air draft (NCDENR, 2006).
Drift usually occurs within a range of 0.002–0.2 percent
of the water circulation rate (Qureshi et al, 2006). Accord-
ing to ASHRAE (1979), evaporation loss averages about
1 percent for each 12.6oF (10.7oC) drop in water tempera-
ture (Qureshi et al, 2006). Another estimate puts the loss at
1.2 percent of the rate of flow of recirculating water pass-
ing through the tower for every 10oF decrease in water
temperature (NCDENR, 2006). Cooling tower perfor-
mance is governed by the difference in temperature of
inlet and outlet water flows, the difference in temperature
of the dry bulb and wet bulb (i.e., humidity), and the
maximum possible temperature difference observed in a
cooling tower.
Most studies have focused on reducing evaporative
losses in cooling towers, but this scenario focuses on
maximizing evaporation loss in towers and from ponds.
A study on the performance of a cross-flow cooling tower
installed in southern Tunisia showed the total water lost
annually by evaporation represented 4 percent of the total
flow rate (Kairouani et al, 2004).
Evaluating Alternative Approaches
Each of these methods has advantages and limitations.
Although the potential of each method is evaluated, not
enough information is available to prepare a comprehen-
sive design and cost analysis.
WAIV. The WAIV study (Gilron et al, 2003) used
brine supersaturated with calcium and TDS concentra-
tion of 16–18 g/L. However, the authors did not per-
form a parametric study exploring the influence of brine
Figure 2. Salt-tolerant plant, Spartina alterniflora
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concentration on system efficiency. To design this method
would entail determining an optimum area of fabric and
developing a design for overhead tan ks (size and num-
ber) for each pond. The study also fails to explore the
influence of different weather conditions—humidity and
wind speed—on the evaporation rate. To determine the
practicality of the method, controlled experiments must
be conducted. Contacting the company that created this
method and a patent search provided no new informa-
tion.
Wetted Floating Fins. Applying this method depends
on optimizing the number and dimensions of fins. Based
on the study previously described (Guitierrez and Ro-
man, 1993), a spacing of five times the fin height was
recommended, with such spacing estimated to enhance
evaporation 24 percent. Material for fin covers must be
chosen to achieve the best hydrophilic/hydrophobic rela-
tionship to maximize evaporation. However, a tradeoff be-
tween the fabric’s wicking ability and rate of evaporation
from the fabric is likely. Calculations based on literature
show the capitalized cost of evaporative capacity through
addition of 700 solid fins is $0.64
per gal/yr when using a cotton fab-
ric for additional evaporation of 24
percent. Cost was reduced to $0.26
per gal/yr of additional evaporative
capacity when an aluminum screen
was used instead of a flat pane
(Table 2).
Calculations were based on a fin
height of 3 in.—2 in. of which was
above water. A higher fin height
would allow more of the boundary
layer to be accessed by the fin. A
small experiment conducted in our
laboratory indicates that conven-
tional fabrics have enough wicking
ability to allow greater fin height
to be wetted. One in. of cotton fab-
ric was dipped in a 3.5-in.-deep
pan (27 in. x 16 in.). A height of
12 in. was reached after 24 hours.
The experiment was repeated for
3 hours with a fast-drying fabric
(82 percent polyester, 18 percent
spandex), which attained a height
of 8 in. Cost and water evaporation
from the fabric would further influ-
ence fabric choice.
The experiments conducted by
Guitierrez and Roman (1993) used
freshwater without added salts or dissolved solids. Incor-
porating the influence of solids on the wicking, as well
as required maintenance, would result in higher, but still
acceptable, costs. Further investigation is warranted.
Salt-Tolerant Plants. Keyes (1966) increased evaporation
30 percent for an 840-gal/day application with 100,000
mg/L of TDS and about 60,000 mg/L of chloride. If low
TDS effluent streams can be identified and selectively
treated with this method, higher removals may be pos-
sible. Overall, this approach is attractive, possibly gen-
erating revenue if a suitable market is available for the
plants grown. Using halophyte as a potential feed prod-
uct must be judged to ensure future revenue. Weather
conditions conducive to halophyte growth must also be
considered.
Droplet Spraying. The spray method is an energy-
intensive, active system that requires saline water to be
pumped into nozzles or cooling towers. Considering
2 percent evaporation from a single nozzle, the cost of
electricity for pumping (60 psi and 60 percent efficiency)
to achieve additional evaporation capacity per gal/yr is
Table 2. Capitalized cost for the wetted-fin and spray evaporation methods
Parameter Wetted-Fin Method Spray Method
Evaporation rate (in./day) 0.13 0.13
Pond surface area (ft2)189,000 189,000
Total evaporation (ft3/day) 2,048 2,048
(ft3/year) 747,000 747,000
Evaporation rate increase (%) 0.24 0.35
Additional evaporation (in./day) 0.031 0.0455
Added evaporation (ft3/day) 491 717
(gal/yr) 1,340,000 1,960,000
Fin dimensions
L (ft) × H (ft) × W (ft) 270 × 0.25 × 0.04 Fraction of pumped
water that is evaporated 0.02
Height above water surface (ft) 0.17 Gal pumped/day 268,000
Spacing between each fin (ft) 0.831Acre-ft pumped/day 0.82
Number of fins 840 Pumping cost/acre-ft 23.583
Exposed area of each fin (ft2)45
Total area of fins (ft2)56,700 $/day 19.39
Cost/fin ($) 77.32
$/year 7,079
Total fin cost ($) 29,200
Cost of cotton/yd ($) 1.00 Interest rate 5%
Total cotton cost ($) 6,300
Total Capital Cost ($) 35,500 Capitalized cost ($) 142,000
Cost ($)/gal/yr 0.026 Cost ($)/gal/yr 0.072
1
Spacing is five times the fin height
2
Cost per 300 ft for 600 ft of screen
3
60 psi at $0.1/kW·h and 60% efficiency
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The WAIV process has the greatest potential to
increase evaporation, with 13-fold increases reported
over open water on an equivalent footprint basis.
$0.072 (Table 2). Lower pump pressures are probably
feasible. Sprinkler system optimization depends on nozzle
height, number of nozzles, and spray diameter. The cool-
ing tower method would require greater capital for items
such as tower structure and higher-capacity pumps but
should offer greater droplet containment. A cooling tow-
er with a water circulation rate of 14 gpm, 15oF cooling
(from 105oF to 90oF), and wet-bulb temperature of 74oF
costs $820 for a commercial cooling tower. For a water
circulation rate of 90 gpm, costs increase to $2,990 for a
similar commercial unit.
Experiments detailed in the literature were conducted
on water without TDS. In the presence of salt and other
minerals, however, sprinkler size and maintenance must
be considered to avoid blockage. In a large facility, the
chances of spray drifting out of the pond are minimal,
except during high winds. Therefore, a higher pumping
rate and larger diameter spray could be considered for
both systems. For cooling towers, the amount of water
circulated and the extent of cooling would have to be op-
timized to achieve the best balance between maximum
evaporation and minimum cost. Higher circulation rates
would incur higher pumping and cooling tower installa-
tion costs but would also increase evaporation. Cooling
tower maintenance and installation costs would also play
a significant role.
The Bottom Line
A literature review identified four options—WAIV, wet-
ted floating fins, salt-tolerant plants, and droplet spray-
ing—for enhancing evaporation pond performance. Only
small-scale experimental results are available for WAIV
and wetted-fins processes. Full-scale data are available
for the salt-tolerant plant and spray methods. This litera-
ture review cannot identify a preferred alternative for a
particular application with any degree of confidence, but
some conclusions can be drawn based on available infor-
mation.
The WAIV process has the greatest potential to increase
evaporation, with 13-fold increases reported over open wa-
ter on an equivalent footprint basis. However, this method
requires a large amount of fabric, a pumping system, and
continuous electricity. A cost-effectiveness evaluation has
not been reported in the literature for this system. How-
ever, evaporation is reported to increase by 24 percent
for wetted fins, 30 percent for salt-tolerant plants, and 35
percent for spray evaporation. Increases could be signifi-
cantly greater for the salt-tolerant plant method if lower
TDS waste streams could be treated selectively. These per-
formance differences are small compared with the degree
of uncertainty in actual, full-scale performance at a given
site. Therefore, selection among these processes may be
driven by other factors, such as implementation complex-
ity and cost-effectiveness.
With the exception of the WAIV process, use of salt-
tolerant plants is probably the most complicated of the
processes described, because it requires at least some
agricultural expertise. The economics of the process
were not evaluated here, but it is reported to be cost-
effective (Keyes, 1966). Use of wetted floating fins ap-
pears to be the simplest process, with spray evapora-
tion falling between this method and the salt-tolerant
plant method in complexity. The cost-effectiveness of
the wetted-fins approach and of electricity required for
the spray method appeared favorable based on prelimi-
nary cost models.
Cost will rise somewhat as additional factors are in-
cluded in the models, but an effort was made to identify
major costs. Based on the available data and literature, it
seems the wetted-fins approach is attractive as a retrofit
option, because it has the lowest cost-per-incremental
improvement in evaporation ($0.026/gal). This assumes
the estimated 24 percent increase in performance is
sufficient to meet design requirements for a retrofit. If
these methods are implemented for a new facility, the
economic analysis must account for capital costs that
are avoided by reducing the facility’s required footprint.
Therefore, costs should be based on the combined cost
of pond construction and the evaporation enhancement
method:
Facility cost =
Qconv. × Costconv. + Fenhance × Qconv. × Costenhance
where Qconv. is the pond’s annual evaporative capac-
ity without any enhancement; Fenhance is the fraction by
which the enhancement increases capacity; and Costenhance
and Costconv. are the capitalized unit costs of the enhance-
ment and conventional pond, respectively. The overall unit
cost is the facility cost divided by total capacity:
Unit cost = Qconv. × Costconv. + Fenhance × Qconv. × Costenhance
Qconv. + Fenhance × Qconv.
which simplifies to:
Unit cost = Costconv. + Fenhance × Costenhance
1 + Fenhance
In the following examples, it is assumed that pond
construction costs $1/gal/yr annual capacity. Using the
preliminary cost estimates as an example, implementing
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80 IDA JournAl | FIrst QuArter 2010 www.IDADesAl.org
the fin evaporation method costs an additional $0.026/
gal/yr annual capacity and enhances capacity by 24 per-
cent:
Unit cost = 1 + 0.026 × .24
1 + 0.24 = 0.81
The spray method is estimated to increase evaporation
by 35 percent and cost $0.072/gal/yr annual capacity:
Unit cost = 1 + 0.072 × .35)
1 + 0.35 = 0.76
This analysis suggests the spray method is favored
for new ponds. Although it is more expensive than the
wetted-fins approach on a per unit treatment capacity ba-
sis, it provides greater enhancement to the pond’s capacity,
which avoids the cost of additional pond area. This analysis
is an example only; information is too preliminary to de-
termine which approach would be most cost-effective in
practice.
This research has identified several options for enhanc-
ing evaporation pond performance. Preliminary estimates
suggest these methods may be cost-effective. Further re-
search, ideally using a combination of physical experi-
ments and mathematical modeling methods, is required to
assess performance of alternative designs and identify an
appropriate conceptual basis of design.
Acknowledgments
Financial support from Musco Olives Inc. is gratefully acknowledged.
About the Authors
Shamia Hoque (sh338@drexel.edu), Terry Alexander, and Patrick L.
Gurian are with the Department of Civil, Architectural, and Environmental
Engineering, Drexel University, Philadelphia.
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Editor’s Note
This article is a peer-reviewed, updated version of a paper that was
presented at ACE08, AWWA’s Annual Conference and Exposition, June
8–12, 2008, Atlanta.
... This footprint is inversely proportional to the evaporation rate, which is inherently low due to the passive nature and inefficient use of solar energy in these ponds. To reduce the environmental impact (that is, smaller areal footprint) and capital costs, evaporation enhancement in brine disposal ponds is essential, and different approaches have been implemented in this regard 11,12 . ...
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Chapter
Produced water is a salty brine generated during the production of oil and gas. Brine disposal is typically achieved by reinjection in deep wells, and the cost is highly dependent on volume. We investigated, evaluated, and developed the principal components of a cost-effective, engineered bioreactor system that will, through natural plant processes of transpiration and salt tolerance, reduce the volume of produced water. After a screening of plant effectiveness at the greenhouse scale, the system was tested at the pilot scale at a gas and oil field in Oklahoma. Saltwater cordgrass (Spartina alterniflora) and great bulrush (Scirpus validus) were chosen to treat the wastewater with chloride levels between 30 000 and 60 000 milligrams per liter in a two-compartment bioreactor. These tests confirmed greenhouse data and demonstrated volume reductions of 75 percent in less than 5 days. Compared to evaporation from an open surface, plants achieved 30 percent more volume reduction in the same time. Cost savings between 36 and 70 percent are feasible under full field conditions.