Membrane Fouling Mitigation For Domestic Wastewater Treatment Using An Adsorption-osmotic Membrane Bioreactor Hybrid System

Osmotic membrane bioreactor（OMBR）,as a new wastewater treatment process,has been obtained more attention due to the advantages such as no external pressure,higher effluent quality and lower membrane fouling propensity.However,salt accumulation and membrane fouling have become the main factors restricting the development and promotion of OMBR.In this paper,calcined Mg/Al layered double hydroxides with C ² as interlayer anion（Mg/Al-CO₃-CLDHs）was used as an adsorbent in OMBR to construct an adsorption-OMBR-MD hybrid system.The performence of adsorption-OMBR-MD hybrid system with that of a traditional OMBR-MD system was compared and the effect of Mg/Al-CO₃-CLDHs added into OMBR on mitigating membrane fouling and enhancing water flux was investigated.Mg/Al-CO₃-LDHs was prepared by coprecipitation method,and calcined to get Mg/Al-CO₃-CLDHs.The optimized preparation conditions were as follows:Mg/Al ratio of4:1,the reaction temperature of 30℃,the reaction time of 3h,the calcination temperature of 500℃,and the calcination time of 2h.The optimized adsorption conditions were the following:the Mg/Al-CO₃-CLDHs dosage of 4 g/L,the adsorption time of 6h,and the adsorption temperature of 30℃.The adsorption kinetics fitting results showed that the adsorption of chloride ion by Mg/Al-CO₃-CLDHs corresponded with the pseudo-second-order kinetics model.The adsorption isotherm accorded with the Langmuir isotherm adsorption model.The adsorption mechanism of Mg/Al-CO₃-CLDHs for chloride ions was the"memory effect"of hydrotalcite,that was,the layered structure of Mg/Al-CO₃-CLDHs could be restored by adsorbing anions in solution.The water flux of the adsorption-OMBR-MD hybrid system could still be maintained at 4.36 LMH after 20 days operation without membrane cleaning,while the water flux of the OMBR-MD was 2.49 LMH.Due to the higher water flux and reverse diffusion flux,the conductivity in the adsorption-OMBR-MD was also higher than that in the conventional OMBR-MD,but the specific reverse solute flux（SRSF）value in the adsorption-OMBR-MD was much lower,indicating that the sodium chloride diffused into the adsorption-OMBR-MD while one unit volume of water recovered was relatively low.However,due to the high-efficiency rejection of the FO and MD membranes,the average removal efficiency of COD,,,and TN by the adsorption-OMBR-MD were 90.27%,96.84%,99.74%,and 95.61%,respectively,and the average removal efficiency of COD,,,and TN by the conventional OMBR-MD were94.62%,96.21%,97.59%and 96.52%,respectively.Both effluents could meet the toilet flushing and vehicle flushing standards specified in"GB/T 18920-2020,The use of urban recycling water—Water quality standard of urban miscellaneous use".Combined with the results of sludge concentration changes,EPS,SMP and SOUR,it could be seen that the Mg/Al-CO₃-CLDHs added into OMBR may lead to the decrease of sludge activity without presenting strong alkalinity.Based on the osmotic-resistance filtration（ORF）model,the membrane fouling resistance was analyzed.The results showed that the FO membrane fouling resistance of the adsorption-OMBR-MD was 23.82×10¹¹ m^-1 after 20 days operation,while the FO membrane fouling resistance of the OMBR-MD was 54.15×10¹¹ m^-1.By adding adsorbent,the membrane fouling resistance decreased significantly.The scanning electron microscope images of the fouled membrane surfaces showed that the surface of FO membrane from the adsorption-OMBR-MD seemed relatively smooth,and the surface of FO membrane from the OMBR-MD was almost completely covered with pollutants.Fourier transform infrared spectroscopy results showed that the contaminants on the FO membrane surface were polysaccharides and proteins.Where the polysaccharides content on the FO membrane in the adsorption-OMBR-MD was 3.5 mg/L,and the content of proteins was 3.8 mg/L;the polysaccharides and proteins content on the FO membrane in the OMBR-MD was 23.9mg/L and 10.5 mg/L,respectively.The polysaccharides and proteins content of membrane surface in the adsorption–OMBR-MD was lower than that of the OMBR-MD.The membrane fouling mitigation in the adsorption-OMBR-MD might be ascribed to the adsorbent sweeping the FO membrane surface contaminants;adsorbing the anions in the bioreactor to mitigate the inorganic fouling on the membrane surface,adsorbing the proteins to mitigate the organic fouling;inhibiting microbial activity,and preventing microorganisms adhering and growing on the membrane surface to mitigate biofouling.