Fouling Control Of Forward Osmosis-membrane Distillation(FO-MD) Integrated Process For Landfill Leachate Treatment

Forward osmosis-membrane distillation(FO-MD)integrated process has significant advantages in the field of water treatment,such as high treatment efficiency and excellent quality effluent.However,due to the high concentration of contaminants in the feed and the limitations of the membrane itself,a challenge of the application of forward osmosis(FO)is serious FO membrane fouling,when FO is employed as the pretreatment unit of the FO-MD integrated process for the treatment of highly difficult wastewater.For the FO membrane inorganic and organic scaling/fouling of FO-MD integrated process for treatment of landfill leachate with high salt and high concentration organic foulants,both strategies of process optimization and membrane modification were used to alleviate the inorganic scaling of FO membrane and improve the antifouling performance of FO membrane.For the serious inorganic fouling of FO membrane in the treatment of landfill leachate by FO-MD integrated process,an organic phosphonic acid scale inhibitor,hexamethylene diamine tetra(methylene phosphonic acid)(HDTMPA),was added in draw solution to control the inorganic scaling.For the separate FO process,when the simulated inorganic leachate and the actual leachate were used as the feed solution,the influence of adding HDTMPA scale inhibitor in draw solution on the performance of FO and the mechanism of membrane scaling control were systematically explored.The results showed that the water flux,reverse salt flux and rejection rate of FO were significantly improved,while HDTMP was added to draw solution.The reverse diffusion HDTMPA chelated with Ca2+ to prevent the nucleation and growth of gypsum scale crystals and alleviate FO membrane surface scaling.For FO membrane cleaning,the water flux recovery rate was greater than 90%after three scaling-cleaning cycles owing to the formation of deformed crystals and loose scaling layer via addition of HDTMPA.At the same time,the mechanism of membrane fouling in FO process was deeply explored.Moreover,the scale inhibitor HDTMPA added to draw solution can be recovered by membrane distillation(MD)process.Further research shows that the addition of HDTMPA can significantly improve the coordination of FO and MD unit and equilibrium of its water fluxes,indirectly prevent wetting of the MD membrane and then promote the efficiency and stability of the integrated FO-MD system.For the serious organic fouling of FO membrane in the treatment of landfill leachate by FO-MD integrated process,the antifouling performance of the membrane can be improved by surface modification.The polyamidoamine(PAMAM)dendrimer with much terminal amine groups and carboxylated cellulose nanocrystals(CCN)with a large number of hydrophilic groups were used as grafting materials,which were sequentially grafted onto the surface of polyamide FO membrane via coupling reaction.The morphology,roughness,surface potential and hydrophilicity of the modified FO membrane were systematically characterized.The results showed that PAM AM and CCN were successfully grafted onto the surface of FO membrane.Surface characterization revealed that the modified membrane exhibited lower surface roughness,higher hydrophilicity,and better electronegativity compared to the raw membrane.During the FO experiment,simulated organic leachate and actual leachate were used as feed solution,the water flux decline rate and water flux recovery rate of the modified membrane were apparently improved.It can be seen that the modified membrane has better robust antifouling capacity,mainly because of the grafted hydrophilic polymer chains can prevent foulants from adsorption onto the membrane surface through the steric repulsion and electrostatic repulsion.In the FO-MD integrated experiment,it was proved that the modified FO membrane promoted the balance of water transfer rate between FO and MD units,and significantly improved the coordination and sustainability of leachate treatment by FO-MD system.