Fertiliser Drawn Forward Osmosis Desalination: Achieving Water Quality Requirements for Direct Fertigation by Integrating Nanofiltration

The prevailing desalination technologies are energy intensive and therefore uneconomical for large-scale applications such as irrigation. Forward osmosis (FO) has been recently recognised as an emerging technology for low energy desalination. In particular, FO process offers a promising scope for applications, when a draw solute is used that adds value to the extracted water. In the fertiliser drawn FO (FDFO) desalination process, a concentrated fertiliser is used as draw solution (DS) in which the final diluted fertiliser DS can be directly used for fertigation. However, because of the process limitations posed to the recovery rate by the osmotic equilibrium between the feed solution (FS) and the DS, the nutrient concentration in the final FDFO product water usually exceeds acceptable nutrient concentrations (NPK or nitrogen, phosphorous, potassium) requiring additional fresh water for dilution before fertigation. Such a situation is not preferable given the very purpose of desalination as a means to serve as alternate source of freshwater.

In this study, we evaluated the integration of nanofiltration (NF) with FDFO desalination process as either pre-treatment or post-treatment in order to achieve acceptable nutrient concentrations for direct fertigation. NF has highly selective rejection, high water flux and requires much lower pressure than RO process. The study was conducted through experimental investigation using bench scale FO unit (commercial tri cellulose acetate FO membrane from HTI) and NF units (NE 90 from Woongjin Chemicals, Korea) as well as through theoretical simulation. Eleven commonly used single fertilisers and two NPK blended fertilisers (NPK ratio customised to suit for fertigation) were investigated as DS with simulated brackish groundwater (BGW) as FS. The maximum N/P/K nutrient concentrations on target for direct fertigation were 200/50/300 mg/L.

NF as pre-treatment was able to reduce the TDS of the BGW feed by up to 85% removing majority of the divalent ions and moderate removal of monovalent ions. The feed TDS reduction ultimately enhanced the amount of water a unit mass of fertiliser DS can extract from a feed with a particular TDS consequently decreasing the nutrient concentrations in the final FDFO product water. However, when high TDS feed is used, the final FDFO product water could not meet the nutrient concentrations indicating that post dilution is necessary before fertigation. The other advantage of NF pre-treatment is that, it results in high quality feed water that can enhance the performance of the FDFO process, especially by removing scaling ions that could cause internal scaling especially when FDFO process is operated with FO membrane support layer facing FS.

When NF is used as post-treatment, it serves two objectives: reduce nutrient concentrations in the diluted DS, reconcentrate the excess fertiliser nutrients, and recycle fertiliser draw solutes for further extraction of water. NF post-treatment of diluted fertiliser DS from FDFO desalination produced product water that meets the fertigation standards especially when DS containing high rejecting divalent ions such as (NH₄)₂SO₄, NH₄H₂PO₄, (NH₄)₂HPO₄, KH₂PO₄ and Ca(NO₃)₂ are used. However, fertilisers with monovalent ions such as NH₄Cl, KCl, NaNO₃, NH₄NO₃, KNO₃ having lower rejection mostly required second pass before the final product water can be used for direct fertigation. For blended fertiliser DS, the variable rejection by NF post-treatment resulted in completely different NPK ratio in the final product water requiring redesign of the initial NPK ratio of the DS as per the rejection efficiency of NF in presence of multiple ionic compounds.