Pervaporative membrane filtration
for subsurface irrigation
Tom Bond1*, May N. Sule1, Lindsay C. Todman1, Michael R. Templeton1
and Jonathan A. Brant2
1 Department of Civil and Environmental Engineering, Imperial College
London
2 Department of Civil and Architectural Engineering, University of
Wyoming
* t.bond@imperial.ac.uk
ACS National Meeting, 12th August 2014, San Francisco, California
Outline
Theory of pervaporative irrigation
• Pervaporation can be defined as a
method for separating a mixture of
liquids by partial vaporization through a
membrane.
• The tube is made from a hydrophilic
polymer and forms a semi-permeable
membrane
• Saline water is filtered
before entering the
soil
Water enters soil in the VAPOUR phase
Theory of pervaporative irrigation
Irrigation flux depends on soil
moisture conditions
Need to ensure that irrigation rate is sufficient
Objectives
• Investigate salt removal by the hydrophilic tubular pervaporative
membrane.
• Quantify water flux across the membrane in different soil types.
• Investigate impact of soil characteristics on water flux.
• Develop a mathematical model to simulate experimental data for water
flux, relative humidity and water content distribution in three soil types
(sand, saline sand and top soil).
Experimental methods – desert in a box
Aim:
To quantify the flux
into the soil and the
availability of water in
the soil
Varied:
• Soil type
Did not consider:
• Interaction with
plants
Experimental methods – salt rejection
Modelling methods
Model developed based on:
• Diffusion of vapour (Fick’s Law)
• Liquid flow through unsaturated soil
(Richards’ equation)
• Equilibrium between the liquid and vapour
phases (defined using the sorption isotherms)
Experimental results – desert in a box
Flux (m3/m2day)
-4
x 10
4
2
0
0
0.5
1
1.5
2
2.5
3
RH %
100
50
Sand
0
0
0.5
1
1.5
2
Time (days)
2.5
3
Experimental results – desert in a box
-4
-4
Flux (m
/m222day)
day)
(m33/m
Flux
-4
xx 10
10
10
44
22
• Flux affected by soil
type
00
• Relative humidity
affected the flux
RH %
100
100
50
50
00
00
Sand
Sand
Saline sand
Top soil
Sand
Saline
sand
20
20
40
40
60
60
Time (days)
(days)
Time
80
80
100
100
Experimental results – salt rejection
Rejection of NaCl (35 and 70 g·L-1) by a plugged tube in sand was > 99.8% after 170 h, based
on sand salinity. However, conductivity of deionised water surrounding plugged tube filled
with NaCl (1 M) increased over time, indicating salt permeation.
Experimental results – salt rejection
SEM images for membrane cross sections exposed to NaCl
Modelling results
REF: Todman et al. (2013b)
Discussion and further work
• It is debatable whether recorded water fluxes are sufficient to support crop
growth (would require ~200 m2 membrane surface area per m2 crop).
•However, fluxes are limited by soil humidity.
•It is plausible that plants can act as vapour sink and hence increase water flux.
There is evidence that seeds imbibe water from the vapour phase (Wuest, 2007).
•Diurnal temperature variations and the osmotic potential of fertilisers may also
be important.
Key Findings
•Rejection of sodium chloride by the membrane in sand was > 99.8%
• Salt permeation occurred when both sides of the tube were in contact with
liquid water, simulating waterlogged soils.
• Mathematical model successfully simulated experimental data for water
flux, relative humidity and water content distribution in three soil types.
•Water transport across the membrane highly sensitive to soil water content.
•Water uptake by plants can potentially drive water flux across the
membrane, highlighting the suitability of this technology for 'on demand',
water-conservative irrigation.
Acknowledgements
References
Wuest, S. (2007). Vapour is the principal source of water imbibed by seeds in unsaturated
soils. Seed Science Research, 17, 3–9.
Todman, L. C., Ireson, A. M., Butler, A. P. & Templeton, M. R. (2013a) Water Vapor Transport
in Soils from a Pervaporative Irrigation System. Journal of Environmental Engineering. 139(8),
1062–1069.
Todman, L. C., Ireson, A. M., Butler, A. P. & Templeton, M. R. (2013b) Modeling Vapor Flow
from a Pervaporative Irrigation System. Vadose Zone Journal, doi:10.2136/vzj2013.05.0079
Sule, M., Jiang, J., Templeton, M.R., Huth, E., Brant, J., & Bond, T. (2013). Salt rejection and
water flux through a tubular pervaporative polymer membrane designed for irrigation
applications. Environmental Technology 34(10): 1329-1339.
Pervaporative membrane filtration
for subsurface irrigation
Any Questions?
Tom Bond, Department of Civil and Environmental Engineering, Imperial
College London, t.bond@imperial.ac.uk