Study On The Enhanced Strategies And Fouling Mechanism Of Membrane Distillation In Treating Urine From Environmental Control Life Support System

Environmental control life support system is a key to guarantee the daily life for astronauts in the long-term deep space exploration missions,but the treatment of urine is still a major challenge.Urine has a complicated composition and high pollutants concentration,which will increase the nitrogen load during the treatment process if it is treated together with other wastewaters.In this paper,the urine was source-separated and treated by membrane distillation（MD）,with a focus on the scaling mitigation by adding scale inhibitors and developing hydrophobic membranes.And the mechanism of membrane fouling was further analyzed.Firstly,the variation of urine during storage was investigated to reveal the concentration and composition of main pollutants in urine.The effect of pretreatment on urine storage by acid and alkali was studied.The results showed that the contents of urea,phosphate and other fouling in fresh urine were high,and urea decomposed continuously with the increase of storage time,which significantly increased the ammonia nitrogen,p H and electrical conductivity of urine.At the same time,precipitates were formed,and the properties of urine were stable after pretreatment.MD was used for fresh urine treatment to evaluate the feasibility and identified the key issues in MD process for urine treatment.Secondly,the effect scale inhibitors was used for enhanced MD.The operational efficiency of was increased by 70.34%（PAPEMP）,48.96%（PASP）,77.24%（PAA）,69.66%（DTPMPA）,and 55.17%（EDTMP-Na5）,respectively,with the addition of scale inhibitors.The key anti-scaling mechanism were the coupling,chelation,and dispersion effects,by which the formation and growth of crystal nucleus were hindered and the solubility was increased in water.Further,the hydrophobic membrane was prepared oriented to the enhanced MD by the used of iron-or magnetic nanoparticles-based additives.It was found that the use of Fe-based additives significantly improved the performance of PVDF hydrophobic membranes,and the conductivity of the permeate side was more stable.Among them,the hydrophobic film prepared with Fe Cl₂·4H₂O as additive had obvious chemical oxidation in the preparation process,and the casting solution gradually changed from light green to deep red.Nano-γ-Fe₂O₃ was easy to aggregate in the process of membrane preparation,and its dispersion was poor,resulting in a gradual decrease in flux with the increased of the addition amount.Nano-Fe₃O₄ was well dispersed and uniformly distributed on the surface of hydrophobic membrane.After drying,the surface of the membrane was grayish black.The optimum addition amount was 2.5 mmol,and the membrane flux was 16.08±0.51 L·m^-2·h^-1.Finally,the homemade hydrophobic membranes were applied in urine treatment.The results showed that the membrane modified with Fe₃O₄ showed the best performance with an initial flux of 15.59 L·m^-2·h^-1,and the flux decreased by 26.11%after 12 h of operation.The addition of nanoparticles increases the roughness of the membrane and increases the cavitation between the solid-liquid interface.Long-term experiments showed that the hydrophobic membrane had good anti-fouling performance,but the increase in cleaning frequency would deepen the risk of membrane wetting,resulting in a decrease in its membrane distillation performance.After cleaning,the flux recovery rates were 95.12%,73.06%,44.19%,and 38.42%,respectively.