Study On The Urine Treatment In Environmental Control And Life Support System By Adsorption-membrane Distillation Process

With the development of aerospace industry,the requirements of deep space exploration technologies are urgent.Wastewater treatment and resource recovery and reuse are highly needed in space station and long-term manned space mission,which has been a key part of regenerative life support system like ECLSS（Environmental Control and Life Support System）.Urine presenting high concentration of organic matters,high salinity and complicated composition,which has been a huge challenge for its treatment and reuse in ECLSS.Thus,it makes significance difference for develop the effective technologies for urine treatment and recovery for the long-term manned spaceflight.In this study,activated carbon and iron oxide were selected for urine pretreatment,and membrane distillation（MD）process was used for advanced treatment of urine.The performance and mechanism of synergistic adsorption of activated carbon and iron oxide were investigated;the effect of MD treatment and the long-term performance of the whole process were evaluated;the formation mechanism of MD membrane fouling under different pretreatment conditions were also investigated,and the effective control strategy of membrane fouling control was proposed.The main conclusions are as follows:（1）The concentration of pollutants in fresh urine is high,and would hydrolyze easily,which induce the significant variation for water qualities.With the increase of time,the concentration of TN,PO₄^3--P,TP decreased,NH₄⁺-N,pH and conductivity increased.Acid pretreatment（pH=2）can prevent the hydrolysis of urine,so the TN,NH₄⁺-N,PO₄^3--P,TP、pH and conductivity in urine were stable,i.e.,it reduces the formation of NH₄⁺-N volatilization and precipitation effectively.（2）The adsorption capacity of activated carbon for urine follow this sesquence:coconut shell activated carbon>wood activated carbon>fruit shell activated carbon,the coconut shell activated carbon showed the best performance for its high specific surface area,as the removal rates of TOC,PO₄³-P,TP,NH₄⁺-N,and TN were 35.02%,8.17%,11.98%,39.42%and 14.79%,respectively.The removal rate of phosphorus was significantly increased by more than 14%as iron oxides was added,and the acidification pretreatment would further increase TP removal efficiency by 9.29%.The activated carbon and urea decomposition were key reasons for TOC remove,while the struvite precipitation was the main reason for N remove.P would be adsorbed by hydroxy groups on the surface of iron oxide,which was believed to be the key reason for phosphorus removal and the struvite precipitation of phosphate was another reason.Acidification pretreatment would promote the protonation between hydroxy groups on iron oxide and phosphate,which would further enhance the removal rate of phosphorus.（3）Theα-Fe₂O₃ and activated carbon showed excellent adsorption performance for the removal efficiency of TOC,PO₄^3--P and TP in urine was 39.51%,71.03%and 76.79%as the iron carbon ratio was 0.6.The adsorption of TOC byα-Fe₂O₃ coupled with coconut shell carbon is controlled by diffusion rate.The adsorption of PO₄^3--P fitted the Elovich model,and the adsorption equilibrium of TOC and PO₄^3--P could be reached within 24 hours.TOC and PO₄^3--P adsorption fitted the Redlich-Peterson adsorption isotherm model.Acid pretreatment of urine was beneficial to the adsorption of PO₄^3--P in synergistic adsorption,but the adsorption of organic matters was unaffected.The removal rate of CDOM organics byα-Fe₂O₃ and coconut shell activated carbon was72.16%,and the adsorption of tyrosine aromatic protein,tryptophan aromatic protein and soluble microbial metabolites by acidic pretreatment largely reduced.（4）The membrane flux of urine decreased significantly within 24hours in MD,and pretreatment could increase the membrane flux and prolong the running time.The membrane flux could maintain stable in the50 hours continuous operation,the membrane flux of different pretreatment process follow this sequence:activated carbon>α-Fe₂O₃/activated carbon adsorption>no pretreatment>ceramic microfiltration（MF）.More than 80%of inorganic ions in particularly99%of PO₄^3--P and TP could be removed by MD,and the removal rate of TOC was over 96%.Theα-Fe₂O₃/activated carbon-MD process showed the best performance,the desalination rate is 87%⁹0%,and the removal rate of TOC and TP is 99%.（5）The organic fouling,inorganic scaling and biofouling happened in urine treatment by MD.Na、Mg、P、Ca were main elements in foulants layer,while tyrosine-like,tryptophan-like,fulvic acids,humic acids and SMPs were main DOM.At the level of phylum,bio-fouling mainly contained Firmicutes and Proteobacteria,and at the level of genus,bio-fouling were composed of Sinibacillus,vagococcus and bacillus,etc.Organic fouling and in organic fouling could be removed effecyively by acid solution and alkaline solution respectively.Pretreatment process could reduce P、Mg、Ca,andα-Fe₂O₃/activated carbon adsorption could reduce P best.