Water regeneration from human urine by vacuum membrane distillation and analysis of membrane fouling characteristics

As a potential high-efficiency water circulation technique in some fields, such as space station that is abundant with vacuum and cooling resources and solar power, vacuum membrane distillation (VMD) process was employed for water regeneration from human urine. Particularly, membrane fouling characteristics were investigated. The separation performances including the membrane flux as well as the chemical oxygen demand (COD), ammonia-nitrogen concentration, and conductivity value of distillates were studied experimentally. The results indicated the flux increased with increasing feed temperature, but decreased with time. In the distillates, the COD reduced with time and went up with increasing feed temperature, the ammonia-nitrogen and conductivity increased gradually with time and rose up with increasing feed temperature due to the thermal decomposition of urea. The VMD processes of 500 mL urine feed after 16-h operation had high COD removal rates of 99.3-99.5%, and the removals of ammonia-nitrogen reached 40.6-75.1%. The corresponding yields of water were 31.9-48.6%. Furthermore, the effects of vacuum degree in the permeate side, cross-flow flowrate and feed temperature on membrane flux were experimentally evaluated. The results showed that the critical fouling operating conditions (CFOCs) existed in the urine VMD processes. From the scanning electron microscopy photographs, it was confirmed that organic fouling accompanied by salt crystals on membrane surface can be restrained effectively when the urine VMD processes were carried out within the CFOCs. The organics, salt ions, and ammonia-nitrogens could be removed effectively under appropriate VMD conditions, such as relatively low temperature, sufficient membrane area and short heating time.