Performance Evaluation And Membrane Fouling Mechanism Of Membrane Distillation For Urine Treatment

With the rapid development of the space station and deep space exploration activities,the controlled ecological life support system has been considered.The effective treatment and reuse of wastewater is a key to the water circulation and reuse in CELSS.Especially,in the confined environment with limited load and space conditions,the treatment and reuse of urine with high concentration,high salinity and variable properties is a great challenge.Membrane distillation（MD）has attracted much attention due to its remarkable pollution resistance and efficient retention characteristics.In this study,MD process was used to treat urine to achieve water recovery and the removal of salts and organics.In order to provide technical support for source separation of urine and special space treatment of urine,the research was carried out in the systematic research on the changes of urine properties during long-term storage,performance evaluation of MD treatment of urine and pollutant reduction characteristics,and analyzed the mechanism of membrane fouling.The main conclusions are as follows:Through systematic analysis of the properties of urine,it was found that the key process of transformation was urea hydrolysis and the inorganic coprecipitation phenomenon,while the transformation of phosphate precipitation to struvite was found with the accumulation of NH₄⁺-N in the urine storage.in DOM composition,urea and protein substances decreased,while aromatic organic matter increased,low molecular weight organic matter decreased,but the concentration of macromolecular organic matter increased gradually.Among them,the acidification showed significant urine stabilization ability by inhibiting urea hydrolysis.Ceramic membrane and activated carbon can remove organic matter,but the removal of ceramic membrane for urea was higher than that of other organic components,and the removal ratio of activated carbon for aromatic organic matter was the highest.Furthermore,the effects of temperature difference and flow rate on MD performance were investigated.Under the optimal conditions,99.1%salt rejection and92%organic rejection were achieved,and the flux was 8.03%kg/m²·h,indicating that urine MD is an effective way of urine treatment,however membrane fouling occurred in the process of MD.therefore,the ceramic membrane and concentrated H₂SO₄ were used in the pretreatment of urine.Further analysis of its removal effect on key pollutants showed that MD had significant removal effect on P,DOM and N.The concentration of PO₄^3--P and TP on the original liquid side of urine showed MD-T>MD of acid pretreatment>MD,indicating that urine pretreatment can further reduce the P pollutants in urine,and the removal rate was as high as 99%.In MD-T treatment,protein pollutants were the main pollutants in the produced water.In order to further analyze the formation mechanism of membrane fouling,the morphology,composition and formation mechanism of membrane fouling were studied.In the process of MD,it is found that a large number of particles or glued pollutants were attached to the membrane surface,and there was the phenomenon of microbial growth,which indicated that there were organic-inorganic-microbial compound pollution,among which the organic pollution was mostly protein,while the inorganic pollution was salt crystallization.Chemical cleaning agent can effectively remove the pollutants on the membrane,acid solution can effectively remove the inorganic salts on the membrane surface,and alkali can mainly remove the organic pollutants such as proteins and fulvic acid.In conclusion,this paper systematically analyzed the urine MD,defined the nature of urine-MD treatment performance-membrane fouling analysis and so on,and provided effective technical support for the treatment of urine wastewater in space station.