Solar membrane distillation: desalination for the Navajo Nation

Abstract

Abstract Provision of clean water is among the most serious, long-term challenges in the world. To an ever increasing degree, sustainable water supply depends on the utilization of water of impaired initial quality. This is particularly true in developing nations and in water-stressed areas such as the American Southwest. One clear example is the Navajo Nation. The reservation covers 27,000 square miles, mainly in northeastern Arizona. Low population density coupled with water scarcity and impairment makes provision of clean water particularly challenging. The Navajos rely primarily on ground water, which is often present in deep aquifers or of brackish quality. Commonly, reverse osmosis (RO) is chosen to desalinate brackish ground water, since RO costs are competitive with those of thermal desalination, even for seawater applications. However, both conventional thermal distillation and RO are energy intensive, complex processes that discourage decentralized or rural implementation. In addition, both technologies demand technical experience for operation and maintenance, and are susceptible to scaling and fouling unless extensive feed pretreatment is employed. Membrane distillation (MD), driven by vapor pressure gradients, can potentially overcome many of these drawbacks. MD can operate using low-grade, sub-boiling sources of heat and does not require extensive operational experience. This presentation discusses a project on the Navajo Nation, Arizona (Native American tribal lands) that is designed to investigate and deploy an autonomous (off-grid) system to pump and treat brackish groundwater using solar energy. Βench-scale, hollow fiber MD experiment results showed permeate water fluxes from 21 L/m2·d can be achieved with transmembrane temperature differences between 40 and 80˚C. Tests run with various feed salt concentrations indicate that the permeate flux decreases only about 25% as the concentration increases from 0 to 14% (w/w), which is four times seawater salt concentration. The quality of the permeate water remains constant at about 1 mg/L regardless of the changes in the influent salt concentration. A nine-month MD field trial, using hollow fiber membranes and completely off-the-shelf components demonstrated that a scaled-up solar-driven MD system was practical and economically viable. Based on these results, a pilot scale unit will be constructed and deployed on the tribal lands.

Full text

DOI 10.1515/reveh-2014-0019 Rev Environ Health 2014; 29(1-2): 67–70
Vasiliki Karanikola*, Andrea F. Corral, Patrick Mette, Hua Jiang, Robert G. Arnoldand
and Wendell P. Ela
Solar membrane distillation: desalination for the
Navajo Nation
Abstract: Provision of clean water is among the most seri-
ous, long-term challenges in the world. To an ever increas-
ing degree, sustainable water supply depends on the
utilization of water of impaired initial quality. This is par-
ticularly true in developing nations and in water-stressed
areas such as the American Southwest. One clear example
is the Navajo Nation. The reservation covers 27,000 square
miles, mainly in northeastern Arizona. Low population
density coupled with water scarcity and impairment
makes provision of clean water particularly challenging.
The Navajos rely primarily on ground water, which is often
present in deep aquifers or of brackish quality. Commonly,
reverse osmosis (RO) is chosen to desalinate brackish
ground water, since RO costs are competitive with those
of thermal desalination, even for seawater applications.
However, both conventional thermal distillation and RO
are energy intensive, complex processes that discourage
decentralized or rural implementation. In addition, both
technologies demand technical experience for operation
and maintenance, and are susceptible to scaling and
fouling unless extensive feed pretreatment is employed.
Membrane distillation (MD), driven by vapor pressure
gradients, can potentially overcome many of these draw-
backs. MD can operate using low-grade, sub-boiling
sources of heat and does not require extensive operational
experience. This presentation discusses a project on the
Navajo Nation, Arizona (Native American tribal lands)
that is designed to investigate and deploy an autonomous
(off-grid) system to pump and treat brackish ground-
water using solar energy. Βench-scale, hollow fiber MD
experiment results showed permeate water fluxes from 21
L/m2·d can be achieved with transmembrane temperature
differences between 40 and 80˚C. Tests run with various
feed salt concentrations indicate that the permeate flux
decreases only about 25% as the concentration increases
from 0 to 14% (w/w), which is four times seawater salt
concentration. The quality of the permeate water remains
constant at about 1 mg/L regardless of the changes in
the influent salt concentration. A nine-month MD field
trial, using hollow fiber membranes and completely off-
the-shelf components demonstrated that a scaled-up
solar-driven MD system was practical and economically
viable. Based on these results, a pilot scale unit will be
constructed and deployed on the tribal lands.
Keywords: desalination; membrane distillation; micro-
porous hydrophobic membrane.
*Corresponding author: Vasiliki Karanikola, Department of
Chemical and Environmental Engineering, The University of Arizona,
1133 E. James E. Rogers Way, Harshbarger 108, Tucson, AZ 85721,
USA, Phone: +858 429-8112, E-mail: vkaranik@email.arizona.edu
Andrea F. Corral, Patrick Mette, Hua Jiang, Robert G. Arnoldand
and Wendell P. Ela: Department of Chemical and Environmental
Engineering, The University of Arizona, 1133 E. James E. Rogers Way,
Harshbarger 108, Tucson, AZ 85721, USA
Membrane distillation (MD) utilizes a microporous hydro-
phobic membrane that is in contact with aqueous solu-
tions at different temperatures and/or compositions (1).
The hydrophobic nature of the membrane prevents the
passage of liquid water through the pores while allowing
the passage of water vapor. The temperature difference
produces a vapor pressure gradient which causes water
vapor to pass through the membrane and then condenses
on a colder surface on the membranes permeate side
(Figure 1). The result is a distillate of very high purity (2).
The first MD was patented in 1963. At that time, inter-
est in this technology was low due to the growing popu-
larity of reverse osmosis (3). MD interest grew in the early
1980s with the development of improved membranes.
The fact that alternative sources of energy, such as solar
energy, could drive the process increased the attractive-
ness of MD. This interest motivated development of differ-
ent MD configurations that are described later. From the
late 1990s to now, interest has steadily increased, espe-
cially in MD powered by renewable energies.
The MD advantage relies on the fact that a lower tem-
perature and hydrostatic pressure are needed for opera-
tion. The membrane acts as a barrier to hold the liquid/
vapor interfaces at the pore entrance. Pore sizes of the
membranes used in MD lies between 10 nm and 1 μm
(4). To avoid pore wetting, the membrane material must
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68 Karanikola etal.: Solar membrane distillation: desalination for the Navajo Nation
be hydrophobic with high water contact angle and small
maximum pore size. Polypropylene, polyethylene, pol-
ytetrafluoroethylene, and polyvinylidene fluoride are
examples of membrane materials that meet these require-
ments (1). Because MD membranes are relatively loose and
chemically inert (for instance with respect to free chlorine
and low pH), their economics and robustness are favora-
ble compared to the reverse osmosis (RO) and nanofiltra-
tion membrane alternatives.
The MD driving force is the transmembrane vapor
pressure difference, which can be established in any of
the following reactor configurations (Figure 2) (3).
1. Direct contact membrane distillation: an aqueous
solution colder than the feed solution is in direct
contact with the permeate side of the membrane.
A vapor pressure difference is induced by the
transmembrane temperature difference. As a result,
water evaporates at the hot liquid/vapor interface,
crosses the membrane in vapor phase and condenses
at the cold liquid/vapor interface.
2. Air gap membrane distillation: A stagnant air
gap is interposed between the membrane and the
condensation surface. In this case, the evaporated
fluid or solute crosses both the membrane pores and
the air gap to finally condense on a cold surface inside
the membrane module.
Tf
Pf
Pp
Tp
Tpm
Diffusion
Vapor flux
Heat flux
Tfm
Convection
Convection
Figure 1 Membrane distillation process (1).
Feed in
Feed in
Feed in Feed in
Feed out Feed out
Feed out Feed out
Membrane
Permeate in
Permeate out
DCMD
Membrane Membrane
AGMD
Condensing
plate
Air gap
Coolant out
Coolant in
Product
tSweep gas in
Sweep gas out
Permeate Permeate
Condenser
SGMD
Membrane
Vacuum
Condenser
VMD
Figure 2 Different types of MD configurations (3).
3. Sweep gas membrane distillation: An inert gas (e.g.,
air) sweeps the permeate side of the membrane
carrying the vapor molecules, and condensation
occurs outside the membrane module.
4. Vacuum membrane distillation: The total applied
pressure under vacuum on thepermeate side is lower
than the saturation pressure of volatile molecules to
be separated from the feed solution. As in sweep gas
membrane distillation, condensation occurs outside
of the membrane module.
Among the advantages of MD as a separation technology,
we can mention that: i. the operating temperature of the
MD process is in the range of 60–80°C, which is a tempera-
ture level at which thermal solar collectors perform well, ii.
it is not necessary to chemically pretreat the feed water, iii.
intermittent operation of the module is possible contrary
to RO, and iv. system efficiency and high product water
quality are virtually independent of the salinity of the feed
water (4). However, MD has not yet been widely imple-
mented in the water treatment industry. The main obsta-
cles for the commercial implementation of MD are: i. the
relative low permeate flux compared to other separation
techniques like RO, ii. permeate flux decay due to tempera-
ture and concentration polarization effects, iii. membrane
and module design for MD, and iv. high thermal energy
consumption: uncertain energy and economic costs for
each MD configuration and application (3).
Research objective
The objective of this research was to develop a MD con-
figuration for desalination within the constraints that it
be ready for immediate field-scale deployment (robust,
generic, commercially available components); operate
autonomously off the energy grid; and be broadly scalable
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Karanikola etal.: Solar membrane distillation: desalination for the Navajo Nation 69
from 50 to 5000 gallon per day (gpd) product and from
50% to 98+% recovery. With this in mind, a bench-scale
hybrid, vacuum-driven sweeping gas, MD configuration
was constructed using only off-the-shelf components. The
challenge was to develop an appropriate system, deter-
mine its capabilities and constraints, and transition the
setup from benchscale to field pilotscale. Such a system
has the advantage that can be deployed in any remote
location that faces water scarcity and where the only
water available would be water of impaired quality. More
specifically this first field scale is going to be deployed
in the Navajo Nation. The community has a population
density of approximately 4 people/km2 and the only
source of water in the area is brackish groundwater. The
community currently hauls water from the nearby towns
with the closest one at approximately 70 km. The solution
proposed will reduce the cost and ease the access of water
for the community.
The bench-scale vacuum-driven sweeping gas MD
experimental setup is schematically shown in Figure 3.
It consists of three fluid circulation loops: a brine loop,
air loop, and a condenser cooling water loop. In the brine
loop, the salty water is heated, circulated past the hydro-
phobic porous membrane, and returned to the heating
tank. In the air loop, air is sucked by a compressor creat-
ing a vacuum and acting as a sweep gas to carry the vapor
to the condenser. Finally, in the condenser cooling water
loop, water flows in counter current flow with the vapor
and condenses it as purified water.
Due to the vapor pressure difference created on both
sides of the membrane between the heated brine and the
air, the feed water evaporates through the membrane pores
and the air loop sweeps the vapor from the reactor in the air
Cooling water
Air in
Vapor
Brine/hot
reservoir Compressor
Cold
reservoir
Membrane module
Hot brine
1
2
34
5
Figure 3 Bench-scale vacuum-driven sweeping gas MD experimental
setup.
Brine from 0% to 14%
T from 40˚C to 70˚C
0
0
0.2
0.4
Permeate molar flowrate, mol/min-m2
0.6
0.8
1.0
0.1 0.2 0.3
Vapor pressure, atm
0.4
Figure 4 Vapor pressure vs. water production for varying tempera-
tures and salt concentrations. The solid line denotes predicted data.
stream. As the air is cooled down to near-ambient tempera-
ture, the water vapor moves to the condenser loop where
it condenses and is finally collected as pure product. The
MD performance was determined as a function of the feed
water temperature and salinity. Bench-scale, hollow fiber
MD experiments show that permeate water fluxes increased
5 times with small changes in the transmembrane tempera-
ture between 40% and 70°C. Tests run with various feed
salt concentrations show that the permeate flux decreases
about 25% as the concentration increases from 0% to 14%
(w/w), which is four times seawater salt concentration. The
quality of the permeate water remains constant at about
1 mg/L regardless of the changes in the influent salt con-
centration. Some representative results can be shown in
Figure4.
In addition, similar research has been conducted in
Australia and more specifically at the National Centre of
Excellency in Desalination Australia, Perth. The objective
of the project is to provide clean water to a community
of 150 people located 800 km east of Kalgoorlie, called
Tjuntjuntjarra. The community is located in a very remote
location, with limited access to potable water. The only
water that can be used is hypersaline groundwater with
Total Dissolved Solids (TDS) of 47,000 mg/L. The system
to be deployed is Solar Membrane Distillation (SMD);
however, it is operated under a different configuration,
vacuum membrane distillation. The system is fully built
by MemSYS and the water is heated up by solar PV panels
provided by CoGenra (Mountain View, CA, USA). A system
schematic can be seen below in Figure 5 that was obtained
from the MemSYS (Thermal Separation company based
in Singapore and Germany) operational manual for the
Vacuum Membrane Distillation (V-MEMD) system. This
similar project is an indication of the need for alternative
ways of water purification in arid and semiarid locations.
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70 Karanikola etal.: Solar membrane distillation: desalination for the Navajo Nation
Memsys V-MEMD unit
Memsys-module Droplet
catcher
Recircu-
lation Feed Brine Distillate
Cooling
loop
Heating
loop
Vacuum
system
Steam
raiser Stage 1 Stage 2 Stage N Condenser...
Figure 5 Multistage V-MEMD MemSYS system setup.
Currently, all SMD systems have only been studied in
pilot scale studies and positive results (maximum puri-
fied water production with less cost compared to previ-
ous purification methods) would enhance the possibility
for duplication and adaptation of the system in similar
remote locations and communities in need of improve-
ment of quality of life.
Received January 16, 2014; accepted January 17, 2014; previously
published online February 19, 2014
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membrane distillation separation process. J Membrane Sci
2006;285:4–29.
4. Koschikowski J, Wieghaus M, Rommel M. Solar thermal-
driven desalination plants based on membrane distillation.
Desalination 2003;156:295–304.
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