Process intensification through enzymatic decomposition of urea with simultaneous recovery of ammonia

Urea in the human body and in municipal wastewater discharges above safe limit can impair the human health and adversely affect the environment. Innovative technologies for sustainable urea recovery are widely recognized as a necessity. In this study, multifunctional enzymatic composite membranes comprised of zeolite nanoparticles dispersed in sulfonated polysulfone (S2PSf) ultrafiltration membranes and urease enzyme deposited by layer-by-layer self-assembly method on the rear surface and in the pores have been fabricated. The proposed architecture offers biocatalytic reaction, ion exchange, and filtration mechanisms sequentially to first degrade urea by the urease catalyzed reaction and recover the released ammonium cations by the zeolite nanoparticles (NPs) within the membrane under continuous flow mode. Zeolite (NaX) NPs were synthesized with environmentally friendly approach and then characterized in detail. The batch mode of adsorption results revealed that the NaX NPs had an uptake of 62.3 mg/g for NH4+-N. It was also found that the mixed matrix membrane (MMM) was more effective than the powdered NaX under similar batch operation. The hydraulic permeability of the MMM compared to the PSf membrane (0.3 L/m2.h.bar) was significantly improved to 335 L/m2.h.bar, but the addition of polyethyleneimine and urease in subsequent modifications reduced the permeability to 152 L/m2.h.bar. The continuous removal of ammonium cations during filtration resulted in a higher catalytic activity of the S2PSf-10Z-PEI-URE membrane compared to the batch mode under similar conditions. However, due to the short residence time in the flow-through mode, the sorption capacity of the membrane was greatly reduced. This study demonstrates a practical tool for the complete recycling of urea contained wastewater, and further development for the applications in hemodialysis and recycling of the astronaut urine during long-term space flight. [Display omitted] •Urease immobilized mixed matrix membrane with hierarchical structure was prepared.•Catalytic activity improved under continuous flow mode compared to static condition.•40 % initial activity at the end of the 8th cycle was preserved.•Easy ammonia recovery through NaCl filtration.•Comparable filtration, reaction and adsorption rates need to be elaborated.