Selectivity of vacuum ammonia stripping using porous gas-permeable and dense pervaporation membranes under various hydraulic conditions and feed water compositions

Recovery of ammonia (NH₃) from residual waters offers various reuse opportunities, such as the production of fertilisers and the generation of electricity and heat. However, simultaneous evaporation of water (H₂O) during NH₃ stripping under vacuum results in diluted recovered NH₃ gas with high H₂O contents. Whereas porous gas-permeable membranes are already used for vacuum NH₃ stripping, the use of non-porous silica-based pervaporation (PV) membranes showed promising results in recent literature, with respect to more selective transfer of NH₃ compared to H₂O. In this study, we assessed the selectivity of NH₃ over H₂O transfer (S_NH3/H2O) for different types of membranes, under various hydraulic conditions and feed water compositions. The three following membranes were tested: a porous gas-permeable polytetrafluoroethylene (PTFE) membrane, a hydrophilic (Hybrid Silica PV) membrane and a hydrophobic polydimethylsiloxane PV (PDMS PV) membrane. For the PTFE and the Hybrid Silica PV membrane, S_NH3/H2O ranged between 0.1 and 0.4, indicating that the transfer of NH₃ was consistently less preferred compared to the transfer of H₂O. The preference for H₂O over NH₃ transfer through the membranes at various hydraulic conditions and feed water compositions can be assigned to the similarity in polarity and kinetic diameter of NH₃ and H₂O and the low relative concentration of NH₃ in the used feed waters (approximately 0.1–1.0 wt%). The PDMS PV membrane showed negligible NH₃ transfer and deteriorated rapidly during the NH₃ stripping experiments. The S_NH3/H2O of both gas-permeable and PV membranes was higher for unsteady than for steady hydraulic conditions. Furthermore, the S_NH3/H2O of the both PTFE and the Hybrid Silica decreased when the ionic strength of the feed water increased from 0.0 to 0.8 mol∙L⁻¹ and when the NH₃ feed water concentration increased from 1 to 10 g∙L⁻¹. According to the results, the used PV membranes did not show selectivity of NH₃ over H₂O transfer. In fact, the used PV membranes consistently had a lower S_NH3/H2O than the PTFE membrane. Hence, the dense silica-based PV membranes did not allow for the recovery of gaseous NH₃ from water, with lower H₂O content in the recovered gas, compared to porous PTFE membranes.