Tapping into salty water to grow healthy food

Water scarcity is a global risk owing to the fact that, despite our planet being over 70% water, sources of pure fresh water required to sustain many economic activities are limited across much of the globe. Therefore, alternative sources of fresh water are urgently needed and one plausible alternative is to desalinate brackish water.

Brackish water has low salt content, as compared to sea water, hence adding some economic sense to subject it to desalination. Desalination is an expensive process, although Forward Osmosis (FO) desalination has been reported to yield promising results. However, the immaturity of this technology coupled with some technical shortfalls hinders its application. Worse off, to date, limited academic research has been conducted on brackish water FO desalination and there has been no pilot-scale assessment conducted by any industry due to the high operating costs associated.

FO technology utilizes the osmotic flow produced when a concentrated draw solution pulls water across a semi-permeable membrane from a contaminated feed water. The draw solution can then be regenerated by a secondary system to produce clean water or used directly for other purposes, depending on what solution is used.

The FO technique has potential to fertigate agricultural lands economically should the forward osmosis system be improved. In this view, researchers from the Centre for Water Advanced Technologies and Environmental Research at Swansea University: Wafa Suwaileh (PhD candidate), Dr. Daniel Johnson and led by Professor Nidal Hilal critically evaluated a Fertilizer Drawn Forward Osmosis (FDFO) system for desalinating brackish water and converting it into fertilizer solutions suitable for the fertigation of agricultural land. Specifically, they investigated the performance of the FDFO using four fertilizer based draw solutions at various concentrations and a polyamide thin film composite (TFC) FO membrane for brackish water desalination. Their work was recently published in the research journal, Desalination.

In their study, combinations of fertilizer compounds as draw solutions were used in conjunction with a polyamide (PA)-TFC membrane commercially manufactured specifically for FO processes as a potential method for improving the water permeability and reducing the reverse solute flux. Moreover, the effect of polyamide (PA)-TFC material on the water flux and reverse solute flux (RSF) was investigated. The composition and salinity concentration of the brackish water (BW) feed was also examined to highlight its effect on the membrane and fertilizer draw solutions performance.

The results revealed that KCl fertilizer draw solution achieved the highest water flux and an acceptable reverse salt flux as compared to other fertilizer draw solution. Additionally, it was reported that the mixture KCl + KNO₃ and KH₂PO₄ fertilizer draw solution generated the lowest water permeation and reverse salt flux. KH₂PO₄ draw solute promoted the growth of salt scaling which affected the membrane productivity. The negative charge of the membrane surface was responsible for precipitation of salt on the selective layer.

In summary, the study by Professor Nidal Hilal (internationally recognized as a world-leader in desalination) and his research team presented an in-depth assessment of brackish water desalination by fertilizer drawn forward osmosis system. Results showed that the membrane Zeta potential indicated that the negatively charged surface could induce reverse salt flux, leading to scaling on the surface. FDFO does not require a recovery step to re-concentrate the draw solution, instead using diluted draw solution as a supplement to irrigation water via fertigation.

“Water, this most precious of natural resources, is often taken for granted despite many campaigns reminding us that one in ten of the world’s population has no access to safe drinking water. If you don’t have clean water, you can’t grow healthy food and you’re more likely to be sick and unable to work or attend school. This in turn leads to people being trapped in poverty, with all the social and security issues that poverty brings. As such, improving the water/food sustainability is of major importance for significantly improving the lives of 100s of millions of people worldwide”. Said Professor Nidal Hilal the lead author in a statement to Advances in Engineering