Osmotic dilution for sustainable greenwall irrigation by liquid fertilizer: Performance and implications

A novel osmotic dilution process using commercial liquid fertilizer for greenwall irrigation was evaluated. In this process, clean water was extracted from raw sewage by forward osmosis (FO) using a well-balanced, all-purpose commercial liquid fertilizer as draw solution. The diluted liquid fertilizer can then be used for direct sustainable greenwall irrigation. Our results show that the presence of organic matter in the liquid fertilizer draw solution did not compromise FO membrane performance. No discernible changes in water flux and key membrane transport parameters (pure water permeability coefficient, A, and salt (NaCl) permeability coefficient, B) were observed when the organic matter concentration in the draw solution was increased to 2000mg/L. Parameters influencing the osmotic dilution process were examined in terms of reverse salt flux, liquid fertilizer concentration, cross-flow rate, and feed and liquid fertilizer draw solution temperatures. The reverse flux of phosphate was much lower compared to those of ammonium and potassium as the reverse flux of these solutes were proportionally related to their hydrated radii. Cross-flow rate had no discernible impact on either water flux or reverse nutrient transport. Water and reverse nutrient fluxes increased markedly with increasing temperature, driven by higher water and solute diffusivities. More than 80% water recovery was achieved by osmotic dilution using raw sewage feed. Water production was stable and not affected by deposition of organic matter on the membrane surface. By contrast, reverse nutrient diffusion was hindered due to enhanced steric hindrance. Results reported here have significant environmental implications. Extracting clean water from raw sewage by commercial liquid fertilizers harnesses unique FO mass transfer phenomena and balances greenwall nutrient requirement, thereby sustaining the greenwall irrigation process.