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

Abstract

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 (SNH3/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, SNH3/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 SNH3/H2O of both gas-permeable and PV membranes was higher for unsteady than for steady hydraulic conditions. Furthermore, the SNH3/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 SNH3/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.