Membrane distillation is a promising emerging technology for treating brackish and some industrial wastewaters using solar heat. It uses a hydrophobic, microporous membrane to separate water from dissolved salts via a temperature difference across the membrane. This process was evaluated on a bench scale using various commercially available membranes. Results showed increasing flux with larger pore sizes but no clear trends with hydrophobicity or salt rejection. Pilot scale testing of a novel stacked flat-sheet design module is planned to treat solar panel wash waters and other wastewaters at solar energy facilities. Future work includes testing various surfactants, cleaning agents and simulated or actual site water to further evaluate membrane performance.

1. Figure 2: Schematic of DCMD5
Key Advantages
Low operating pressures
Low operating
temperatures
Waste heat or solar heat
sources can be used
Decreased energy costs
Driving force not function
of concentration
Maintains efficiency at
high salt concentrations
Emerging treatment
technology for brackish and
some industrial
wastewaters
Temperature-driven
membrane separation
process
Warm ‘dirty’ water
circulated on feed side
Cool ‘clean’ water
circulated on distillate side
Temperature difference
creates partial vapor
pressure difference (Dpvap)
across membrane
Dpvap drives transport of
water vapor through a
microporous, hydrophobic
membrane
Provides high rejection of
non-volatile contaminants
and high water recoveries
Membrane Distillation & Solar Energy – a Sustainable Partnership
Coral R. Taylor, P.E. and Sage R. Hiibel, Ph.D.
Department of Civil and Environmental Engineering ~ University of Nevada, Reno
Decreased clean water and transportation costs
‘Dirty’ wash water from cleaning solar panels can be re-used,
reducing costs to pump and treat on-site groundwater (Fig. 1)
Up to 20 gal/MWh saved by treating water on-site1
Improved efficiencies with cleaning
1.1 – 10% with large-scale photovoltaic arrays2
Up to 26% with concentrating solar power facilities3
Improved solar panel efficiency with cooling
Efficiency typically decreases by 0.4–0.5% per °C elevated4
8 – 24% improvement with water-induced cooling4
Synergistic technologies (Fig. 1)
Why Membrane Distillation + Solar?
FeedSide
DistillateSide
Membrane
Direct Contact Membrane Distillation
Results
Bench-Scale
Used to evaluate treatment performance
of commercially available membranes
Maximum water flux and contaminant
rejection
Minimum fouling and scaling
Experimental data for development and
validation of computational DCMD model
Figure 3: Schematic6 and lab setup of the bench-
scale DCMD system.
1http://www.seia.org/policy/power-plant-development/utility-scale-solar-power/water-use-management
2Massi Pavan, A. et al., (2011) Solar Energy, 85. 1128-1136.
3Vivar, M., et al., (2013) Solar Energy, 84. 1327-1335.
4Smith, M. et al., (2014) J. Sol. Energy Eng., 136. 034503-1 - 034503-4.
5Adham, S. et al., (2013) Desalination, 314. 101-108.
6Rao, G. et al., (2015) Desalination, 367. 197-205.
7Lin, P. et al., (2015) J. Membrane Science, 475. 511-520.
8Matthern, et al., (2005) Idaho Cleanup Project, App.A.
References
This material is based upon work supported by the
National Science Foundation under Grant
No. IIA-1301726.
Acknowledgements
Presence of surfactant in used water wash possible
Surfactant in feed water increases flux and decreases
membrane performance (Fig. 6)
Surfactants have been reported to cause pore wetting and
cause loss of hydrophobicity and high salt passage7
Results
Wide variety of commercially available hydrophobic, microfiltration membranes
evaluated (Table 1, Fig.5)
Increasing flux observed with increasing pore size
No correlation between flux and hydrophobicity or salt rejection
Pilot-Scale
Used to treat solar panel wash waters and
other wastewaters at solar energy
facilities
Novel stacked flat-sheet design
Increased membrane surface area with
small footprint
Improved thermal profile over hollow-fiber
systems
Modular design for adjustable treatment
throughput
Figure 4: Side view and expanded gasket
schematics of the small pilot-scale DCMD
module.
PT
PTTP
TP
DistillateFeed
Gasket
Feed
Side
Membrane
Distillate
Side
Supplier Material
Pore Size
(mm)
Contact
Angle (°)
Dist. Cond.
(mS)
Flux
(L/m2-hr)
GE Clarcor PTFE 0.11 125.5 34.8 9.8
Donaldson PP 0.20 124.5 39.3 12.1
Osmonics PP 0.20 134.3 21.7 14.7
3M ECTFE 0.20 110.3 NA 16.6
GE Clarcor PTFE 0.22 134.9 22.7 16.6
3M PP 0.20 121.2 20.1 18.1
3M PP 0.45 122.6 NA 18.1
Donaldson PTFE 0.45 125.3 34.9 18.1
Osmonics PTFE 0.45 134.2 25.8 21.1
Table 1: Hydrophobicity, distillate conductivity, and
flux of commercially available membranes. PTFE –
polytetrafluoroethylene; PP- polypropylene; ECTFE –
ethylene chlorotrifluoroethylene; NA – not available
Figure 5: Representative membrane flux data
for 35 g/L NaCl solution.
Membrane
Module
Circulating
Pumps
Feed
Chiller
Heated
Bath
Distillate
Conductivity
Meter
Flux Balance
Data Acquisition
Computer
Flow Meters
Bench- and Pilot-Scale DCMD Systems
Future Work
Surfactant type (anionic, cationic, zwitterionic) studies to
evaluate impact on flux, salt rejection, and hydrophobicity
Evaluate non-surfactant cleaning agents effects
Synthetic groundwater8 testing to simulate site water
Short-term flux and salts rejection
Extended runs to evaluate concentration capabilities and
scaling/fouling potential
Bench-scale testing with synthetic panel wash water (salts
plus dust particles)
Bench-scale testing with actual site water
Pilot-scale DCMD system assembly and field deployment
at solar facility sites
Figure 6: Membrane flux data after surfactant (Dawn™ dish soap,
0.01 wt%; Gain™ dish soap, 0.1 wt%) is added to 35 g/L NaCl solution.
Figure 1:
Treatment and
reuse of water
for cleaning
solar panels
powered by
solar heat.
Used Wash
Water
Membrane
Distillation Panel
Cleaning
Water
Treated
Water
Site
Water
5
10
15
20
25
30
0 20 40 60 80 100 120
Flux(L/m2-hr)
Time (min)
PP 0.2
QM022
QM011
3M PP 0.45
0
2000
4000
6000
8000
0
100
200
300
400
0 20 40 60 80
Conductivity(mS)
Flux(L/m2-hr)
Time (min)
Flux
Conductivity
Dawn™ Gain™