Water Purification Efficiency Of Landfill Leachate Treated By Two Stage Membrane Distillation And Ammonia Recovery Efficiency

Landfill leachate is a harmful product inevitably produced waste treatment and disposal.The annual amount of landfill leachate generated in China is vast,but the treatment capacity of landfill leachate is less than half of the amount generated.The treatment capacity of landfill leachate needs to be improved urgently;meanwhile,the content of ammonia in landfill leachate is often very high,reaching 2000-3000 mg/L.Therefore,ammonia recovery from landfill leachate is a critical way to achieve high value-added development and treatment of waste water.The application of the membrane distillation process in landfill leachate treatment and ammonia recovery has been widely researched in recent years,showing efficient removal capability for various pollutants in landfill leachate.Still,the traditional membrane distillation system has the drawback of weak and efficient ammonia recovery,which leads to relatively low ammonia concentration in the recovered solution.This paper used a modified direct contact two-stage membrane distillation to treat landfill leachate to investigate the water purification efficiency and the simultaneous recovery efficiency of ammonia resources in the two-stage membrane distillation process.The study first compared the water purification efficiency of single-stage membrane distillation system and a two-stage membrane distillation system in treating landfill leachate,focusing on the simultaneous water purification and ammonia recovery efficiency of two-stage membrane distillation.The results showed that single-stage membrane distillation could remove more than 99% of COD and more than 99.5% of metal ions.Yet,the two-stage membrane distillation system could stabilize more than 99.5% of COD removal efficiency and more than99.8% of metal ion removal efficiency in waste leachate to meet high-quality water reuse of landfill leachate.In addition,two-stage membrane distillation could concentrate ammonia in landfill leachate at high concentration and achieve efficient ammonia recovery in the form of ammonium sulfate with a recovery rate of up to 75%.The above results verified the feasibility of the two-stage membrane distillation system in treating landfill leachate.To further investigate the effects of operating parameters on the water purification efficiency and ammonia recovery efficiency of a two-stage membrane distillation system for treating landfill leachate,experiments were conducted by adjusting the influent p H,reactor temperature,and influent flow rate.The results showed that increasing the influent p H from8.64 to 12 increased the ammonia flux and improved the ammonia recovery efficiency.However,this may lead to increased membrane fouling,which may be due to the complex contaminants formed by the complexation of inorganic and organic matter caused by the increase of influent p H,thus affecting the system’s stable operation.Adjusting the operating temperature difference of the reactor from 60/45/20 to 55/40/15 could increase the ammonia recovery by nearly 10% by increasing the ammonia vapor flux.However,the higher vapor flux exacerbated membrane fouling;increasing the feed water flow rate could increase the flux in the short term;however,membrane wetting may occur.It can be seen that the twostage membrane distillation system can achieve high-quality water reuse from landfill leachate.However,the efficiency of ammonia recovery from landfill leachate still needs to be improved.Finally,to further investigate the potential of two-stage membrane distillation for the recovery of ammonia from landfill leachate during long-term operation,the efficiency of the two-stage membrane distillation system was investigated for feed solution with different ammonia concentrations.The results showed no significant difference recovered ammonia’s final concentration and purity between the low(C=942 mg/L)and high(C=2000 mg/L,C=3000 mg/L)ammonia concentration groups in the two-stage membrane distillation system during the cycle operation.The low ammonia concentration group needed to replace the solution for the second cycle because the low ammonia concentration remained in the solution after 24 h of a single operation.The ammonia concentration in the feed solution of the high ammonia group was too high for the second cycle of operation.Nevertheless,the single operation cycle was extended from 24 h to 48 h,which aggravated the membrane fouling by the solution concentration and decreased the produced water flux of the two-stage membrane distillation,affecting the stable operation of the two-stage membrane distillation.In summary,the two-stage membrane distillation system has the advantages of high organic pollutant removal rate,high ion retention rate and high heavy metal removal rate in the landfill leachate treatment process.Simultaneously,it can also efficiently recover ammonia resources in landfill leachate.However,adjusting operating parameters to improve ammonia recovery efficiency would inevitably increase membrane fouling,which affected the long-term stable operation of the two-stage membrane distillation system.Specifically,expanding the feed solution ammonia concentration would shorten the stable operation time of the two-stage membrane distillation system due to the increased membrane fouling.By contrast,the stable operation can be maintained more extended at relatively low ammonia concentration.Therefore,it may be necessary to improve the produced water flux and ammonia recovery efficiency through pretreatment.Meanwhile,the membrane fouling and membrane wetting problems during long-term operation should be mitigated to ensure the long-term stable operation of the two-stage membrane distillation system in treating landfill leachate.The research in this paper can provide theoretical support for practical engineering applications.