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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
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
0 0.5 1 1.5 2 2.5 3
0
2
4
x 10-4
Flu
x (m
3 /m2 da
y)
0 0.5 1 1.5 2 2.5 30
50
100
Time (days)
RH
%
Sand
0 20 40 60 80 1000
50
100
Time (days)
RH
%
0
2
4
x 10-4
Flu
x (m
3 /m2 da
y)
Sand
Experimental results – desert in a box
• Flux affected by soil type
• Relative humidity affected the flux
0 20 40 60 80 1000
50
100
Time (days)
RH
%
0
2
4
x 10-4
Flu
x (m
3 /m2 da
y)
Sand
Saline sand
0 20 40 60 80 1000
50
100
Time (days)
RH
%
0
2
4
x 10-4
Flu
x (m
3 /m2 da
y)
Sand
Saline sandTop soil
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.
Any Questions?
Tom Bond, Department of Civil and Environmental Engineering, Imperial College London, [email protected]
Pervaporative membrane filtration for subsurface irrigation
