Desalination by pervaporation for sub-surface irrigation in arid regions

1. Desalination by pervaporation for sub-surface irrigation in arid regions Dr Michael Templeton Department of Civil and Environmental Engineering Imperial College London

2. What is the problem? • Clean fresh water is an increasingly scarce resource • Groundwater supplies are being abstracted faster than they are being replenished and are becoming salted • Agricultural land is being polluted by irrigation water itself and by agri-chemicals • There is an increasing demand for food crops to support growing populations • There is an increasing demand for non-food crops (e.g. biofuel crops)

3. What is pervaporation? • Specially constructed material – non-porous hydrophilic polymer • Water permeates across the surface and condenses on the opposite side as moisture • Virtually all non-water constituents are rejected (e.g. salts, microbes, organics) • When partial pressures are balanced, water transfer stops • The new idea - to construct irrigation piping out of this material • Partnership with Design Technology & Irrigation Ltd (DTI Group), based in Brighton

4. How does it work?

5. What does a pervaporative material look like?

6. Does it work?

7. Does it work?

8. Does it work? • Tested in the UK (Eden project), Middle East, USA, and South America • Trial crops have included peppers, tomatoes, strawberries, radishes, beans, lettuce, potatoes, grass, vines, sunflowers, and various types of trees (cherry, banana, Acacia) • Major trial in Abu Dhabi currently – 200 Prosopis trees growing in 45 ºC watered with highly saline untreated groundwater (140,000 ppm!) • Possibly better crop uniformity and yield - i.e. one trial yielded higher radish biomass

9. What are the advantages? • Allows the use of otherwise unusable water resources (e.g. brackish water, seawater) • By definition, it is an efficient water delivery process – impossible to over-water; potential for significant reduction in water use • Easy to operate and manage • No requirement for high pressure input (unlike other membrane filtration processes)

10. What are the challenges? • It is not possible to provide nutrients to the plants through this system • Some plants have shown better aptitude for this method than others • Currently more expensive than drip irrigation (but it is more efficient and allows the use of water resources that would be otherwise unusable) • Disposal of the reject water must be considered

11. What are some research questions? • What are the limits of water quality that are feasible? • How do different soil characteristics influence water transfer rate? • How can fouling and salt accumulation best be managed? • What are the limits of pipe diameter and thickness? • When is this technology more favourable/appropriate compared to drip irrigation or other irrigation techniques? • What is the pattern/rate of crop growth when a new irrigation project is started with this technology? • Do water-stressed plant roots develop differently and have different requirements than plants under un-stressed conditions? • Are plants grown in this way as healthy / productive as those grown by alternative irrigation methods?

12. Interested in collaborating? • Dr Michael Templeton, Imperial College London Tel: +44 (0)207 594 6099 Email: m.templeton@imperial.ac.uk Web: www.imperial.ac.uk/people/m.templeton