Performance Of Low-level Radioactive Wastewater Treatment With Vacuum Membrane Distillation By Hollow Fiber Membrane

With the vigorous development of clean energy in China,the use of nuclear energy has become an inevitable trend.However,while nuclear energy is developing,the volume of radioactive wastewater generated has been rising,and low-level wastewater accounts for a large part of it.The wastewaters contain not only radio-nuclides,but also inorganic salts and organic matter,and their negative impacts on the environment and human beings are becoming increasingly serious.It can be seen from the experience of past nuclear accidents that radioactive wastewater generated during the operation of nuclear facilities and during decommissioning must be treated in time.At present,China's low-level wastewater treatment is mainly based on the"flocculation-evaporation-ion exchange"three-stage process.Although this process is widely used,it is easy to produce secondary pollution and the purification effect is not ideal.Therefore,it is necessary to seek a new efficient and convenient processing technology.Membrane distillation technology is a physical separation technology that uses a hydrophobic micro-porous membrane and uses the temperature difference between the two sides of the membrane as a driving force,so that the hot side volatile substances condense through the membrane pores and accumulate on the cold side.Membrane distillation technology has the advantages of good treatment effect,high treatment efficiency and low energy consumption.It is used to treat low-level wastewater and has high feasibility.In this study,uranium-containing simulation and real wastewater were used as research objects.PTFE hollow fiber membrane modules were used to carry out vacuum membrane distillation?VMD?treatment simulation and real low-level uranium-containing wastewater.First,the operating parameters of the membrane module were optimized.The effects of feed temperature,velocity and vacuum on flux and retention effect were investigated,and the polarization effect of temperature difference in the VMD process was discussed.Then,the production of nuclear fuel elements by VMD treatment was studied.Subsequently,the feasibility of VMD treatment of low-level uranium-containing waste liquid in the production process of nuclear fuel elements was studied.PTFE hollow fiber membrane was used to treat uranium-containing wastewater.The results showed that the temperature of the feed,the flow velocity of the feed liquid and the degree of vacuum did not affect the purification effect of membrane distillation.The degree of vacuum had a greater effect on the permeate flux.As the vacuum of the feed increased,the permeate flux increased significantly;the permeate flux increased as the feed flow velocity and feed temperature increased.Under laboratory operating conditions,the rejection rate of uranium by the membrane distillation unit reached more than 99.99%,the decontamination coefficient could reach more than 10⁴,and the initial permeate flux of the membrane module was 2.2 L m^-2 h^-1.Under the best operating conditions of the equipment,the maximum permeate flux was 11.3 L m^-2 h^-1,indicating that the membrane distillation unit had a good treatment effect on low-level wastewater.As the feed velocity increased,TPC increased,while CPC decreased slightly.As the temperature and vacuum of the feed liquid increase,TPC decreased and CPC increased accordingly.Membrane distillation technology treated low-level uranium-containing wastewater generated in the manufacture of nuclear fuel components.The initial conductivity of this wastewater was 36100?s cm^-1.Studies had shown that when the conductivity of the feed liquid was concentrated to 180400?s cm^-1?the concentration ratio was 9 times?,the conductivity of the distillate remained at around 17?s/cm,and the conductivity removal rate exceeded 99.9%.With the increase in the concentration of the feed liquid,the residual pollutants in the wastewater were not affected,the salt removal rate reached 96.87%,the uranium interception rate was always greater than 99.99%,and the uranium concentration in the distillate was less than 1?g/L,which meet the national emission standards?GB23727-2009?.The increase of feed concentration had little effect on the depth removal effect of membrane distillation,but the permeate flux decreased with the increase of feed liquid concentration.The continuous operation results of real wastewater showed that within 1400 minutes after the experiment,the conductivity of the ammonia nitrogen content gradually decreased to 3.3mg L^-1 throughout the experiment,and there was no sudden increase during the process;the average COD content gradually decreased from the previous 1038 mg L^-1until it stabilized at around 22 mg L^-1,which indicated that membrane distillation had good stability in the treatment of low-level wastewater.The FT-IR and SEM characterization analysis results of the experimental showed that there was no obvious contaminant aggregation on the surface of the membrane,and the quality and efficiency of the experimental process were not significantly affected.Combined with the above research,VMD had a high-efficiency purification effect on the complex system low-level wastewater generated in the production process of nuclear fuel components,and could provide new ideas and data references for the deep purification of low-level wastewater in the nuclear industry.