Novel fertiliser-driven forward osmosis for agriculture

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Challenge

Forward osmosis (FO) is an innovative membrane-based process that uses concentration difference between two solutions as the main driving force to separate water from saline water sources instead of hydraulic pressure as in reverse osmosis processes. However, the energy consumption depends on the types of draw solute used and its end use applications. When a highly concentrated draw solution and a saline feed solution are separated by a special osmotic or FO membrane, the water moves from the lower concentrated feed solution towards the higher concentrated draw solution by natural osmosis due to osmotic pressure difference without the need of an external energy source. The draw solution becomes diluted but cannot be used directly for potable purposes but could be utilised for crop irrigation.

Investigation

Evaluate the potential of fertiliser-drawn forward osmosis (FDFO) as a low energy, low cost desalination technology for irrigation. Determine the performance of different fertiliser blends as draw solutions and optimise the operating parameters for brackish water treatment. Design and build the first FDFO desalination unit and field test the system.

Outcomes

A total of nine commonly used fertilisers were selected for testing, representing both single and double/complex fertilisers. Performance greatly depended on the type draw solute as it was the driving force of the process. The primary necessary characteristics were high water solubility and an osmotic pressure much higher than the feed solution. NH4Cl, KCl and NaNO3 were able to extract higher amount of water, while Ca(NO3)2 which had the largest molecular weight had a much lower water extraction capacity. Any soluble fertiliser would be capable of being used as a draw solute to some degree and the choice is dependent on compatibility with the FO membrane (physical and chemical parameters) and plant nutrient requirements.

A pilot-scale FDFO unit was constructed capable of treating up to 5 kL/day. The unit was successfully field tested at Newstan Colliery in NSW for six months. The saline water (with a TDS between 2,000 and 5,000 mg/L) was obtained from the water treatment plant which treats the mine impaired groundwater. The treated water was used to irrigate a nearby turf farm, which has to close production during drought as the groundwater is too saline to use for irrigating the turf. An economic analysis showed that use of the FDFO system, while less profitable than utilising rainwater from a dam, would enable the business to continue to operate and also be profitable during times of drought, negating the need to shut down production.

No significant fouling issues occurred during long-term operation, with a simple 30 minute hydraulic clean effectively restoring water flux by 95%.

Future Direction

Low rejection of feed salts (Na+, Cl) by the FO membrane may result in their gradual build-up in the fertiliser draw solution in a closed FDFO-nanofiltration (NF) system eventually affecting the final water quality unless it is balanced by adequate bleeding from the system through NF and re-reverse diffusion towards the FO feed brine. Therefore, a FO membrane with higher reverse flux selectivity than the membrane used in this study would be necessary for the application of the FDFO desalination process.

Technology Readiness

A field trial produced positive results. The agricultural product irrigated with the drawn water performed well and the economic analysis shows some promise. Expressions of interest are encouraged from potential commercial partners. For further information please contact the Principal Investigator directly.

Partners

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Total Value: $1.241 million (cash and in-kind contributions)

Principal Investigator: Associate Professor Ho Kyong Shon

Title: Pilot-scale fertiliser driven forward osmosis desalination of brackish groundwater (Phase 1 and Phase 2)

Length: 38 months (Phase 1) and 22 months (Phase 2)

Personnel: 18 collaborators contributing 7.85 FTE (Phase 1) and 13 collaborators contributing 7.9 FTE (Phase 2)

Further Information

FR2 UTS Fertilisers Summary Poster

Project Summary Poster – fertigation phase 1

FR2 UTS Fertiliser Reinvestment Summary Poster

Project Summary Poster – fertigation phase 2

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