| Currently there is a very high risk of water safety in remote areas due to the lack of appropriate water treatment facilities and the inability to solve some water quality problems.Gravity-driven membrane(GDM)was developed to deal with this problem.This system is driven by gravity,simple to operate,maintenance-free,low-cost and highly reliable,but the organic removal efficiency is low and unstable in the treatment of surface water,and it is not clear whether it can effectively deal with the high concentration of ammonia in the feed water,besides the low flux makes its application conditions further limited.Based on the adsorption capacity of activated carbon and zeolite for different pollutants,a combined adsorption-GDM process was developed by pre-depositing adsorbents on the surface of GDM membranes,and the effect of dosing amount on the flux stabilization characteristics and pollutant removal efficiency of GDM was investigated to explore a new strategy to enhance the performance of GDM.Through static adsorption experiments,the removal capacities of activated carbon and zeolite for organic matter and ammonia,respectively,were verified.In a short-term ultrafiltration process driven by air pressure,the activated carbon deposition layer increased the ultrafiltration organic removal rate by 60% while increasing the effluent flux,and the zeolite deposition layer increased the ultrafiltration ammonia and nitrogen organic removal rate by 82% but reduced the effluent flux.Short-term experiments demonstrated the feasibility of adsorbents to enhance the removal efficiency.The effects of deposition layers formed by different activated carbon and zeolite dosing on the removal efficiency of long-term GDM systems were compared.It was found that the organic removal rate of the GDM system alone was only 19%,and the organic removal rate increased to 50% and 75% with the addition of activated carbon,while the zeolite deposition layer had no effect on the organic removal.The GDM system alone could maintain a removal rate of 83% in the face of high influent ammonia concentration,and the addition of activated carbon and zeolite increased the removal rate by 5% and 15%,respectively.The effect of the dosing amount was not significant.What’s more,the zeolite layer provided ammonia adsorption capacity at the beginning of operation,but was slowly released after the system produced nitrifying bacteria.Overall the presence of both adsorbent deposition layers improved the GDM pollutant removal efficiency to different degrees.With the increase of activated carbon dosing the GDM stable flux decreased by12% and 2%,which was not significant,while the increase of zeolite dosing caused the stable flux to decrease by 24% and 43%,indicating that the zeolite deposition layer aggravated the membrane fouling.Both adsorbent deposition layers reduce the membrane pore narrowing resistance and facilitate the cleaning of the membrane.Further analysis of the fouling mechanism showed that both activated carbon and zeolite additions increased the biomass and SMP and EPS content in the cake layer,and the cake layer was denser and rougher,but the effect of activated carbon additions was smaller and the zeolite additions were higher,indicating that both activated carbon and zeolite deposition layers aggravated the biological fouling in the cake layer to different degrees,but most of the fouling in the activated carbon deposition layer occurred on the surface,while the fouling in the zeolite deposition layer occurred inside the deposition layer and on the membrane surface,which was related to the particle size of both adsorbents.In addition,the presence of activated carbon and zeolite increased the richness and diversity of microbial communities within the cake layer,while the abundance of bacterial genera associated with nitrification was also enhanced,which explains the lower ammonia concentration in its effluent.This paper investigates the effect of deposition layer formed by activated carbon and zeolite as reinforcement on the pollutant removal efficiency and flux stabilization characteristics of GDM system,and also reveals the membrane fouling mechanism of GDM in the presence of two adsorbent deposition layers with different properties and particle sizes,which can help to further explore the methods of GDM performance reinforcement and improve the decentralized water treatment technology. |
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