Pressure assisted fertiliser drawn osmosis process to enhance final dilution of the fertiliser draw solution beyond osmotic equilibrium

Abstract

Adequate dilution of fertiliser draw solution (DS) during fertiliser drawn forward osmosis (FDFO) desalination is important to meet nutrient concentration level for direct fertigation. The maximum DS dilution, however, occurs until the point of osmotic equilibrium between DS and feed solution (FS) thereby limiting the extent of DS dilution. Post-treatment such as nanofiltration (NF) process is required to reduce the fertiliser concentration. In this study however, pressure assisted fertiliser drawn osmosis (PAFDO) process was investigated to enhance DS dilution beyond the point of osmotic equilibrium and potentially eliminate NF post-treatment. The hydraulic pressure applied enhanced water flux significantly depending on the pressure. The applied pressure was found more effective at lower DS concentrations than at higher DS concentrations. For example, when a pressure of 10bar was applied to 10g/L NaCl FS with 0.1M (NH4)2SO4 DS, the water flux increased by 1928% against 38% with 3.0M (NH4)2SO4 DS. This additional water flux could dilute the fertiliser DS beyond the osmotic equilibrium concentrations thereby meeting the fertigation standard. PAFDO could potentially eliminate NF post-treatment significantly helping reduce the footprint and capital cost. However, the effective gain in water flux due to applied pressure at osmotic equilibrium decreased with the increase in the FS concentrations.