A comparison of vacuum and direct contact membrane distillation for phosphorus and ammonia recovery from wastewater

Recovery of ammonia (NH₃) and phosphorous (P) from municipal wastewater is important to minimize the environmental impact of their discharge and to comply with the concept of circular economy. Technologies have been established to recover P in form of struvite (MgNH₄PO₄·6H₂O) for its use as a fertilizer. NH₃ is usually converted to molecular nitrogen and released to the atmosphere; however, NH₃ recovery in form of NH₃/H₂O mixture is relevant for numerous applications such as manufacturing of cleaning agents, sanitizers and liquid fertilizers. Membrane distillation (MD), a process driven by the partial vapor pressure across a microporous hydrophobic membrane, has demonstrated the potential for concentrating the non-volatiles while, at the same time, recovering volatiles from a solution. Among various configurations of MD, vacuum and direct contact MD (DCMD) have been extensively studied. The current study compares the performance of DCMD and vacuum MD (VMD) for simultaneous recovery of NH₃ and P from wastewater in form of NH₃/H₂O mixture and struvite, respectively. The study finds that while both the configuration are equally effective to concentrate P, the control over removal efficiency of NH₃ in the two configurations becomes the bottleneck for struvite recovery under different pH conditions. For wastewater with high pH, VMD almost completely removes NH₃ before reaching substantial P concentration, thereby eliminates the possibility of struvite formation. DCMD on other hand, only partly removes NH₃ while concentrating P, therefore, for struvite recovery from a high pH wastewater, DCMD is better option. For low pH wastewater, VMD is preferred choice as it recovers NH₃ more effectively than DCMD but still leaves ample NH₃ in the wastewater for struvite formation.