Pervaporative membrane filtration for subsurface irrigation Tom Bond 1 *, May N. Sule 1, Lindsay C. Todman 1, Michael R. Templeton 1 and Jonathan A. Brant 2 1 Department of Civil and Environmental Engineering, Imperial College London 2 Department of Civil and Architectural Engineering, University of Wyoming * ACS National Meeting, 12 th 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 affected by soil type Relative humidity affected the flux

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 m 2 membrane surface area per m 2 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: /vzj 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):

Any Questions? Tom Bond, Department of Civil and Environmental Engineering, Imperial College London, Pervaporative membrane filtration for subsurface irrigation