Optimizing Gravity-Driven Membrane Filtration For The Treatment Of Antibiotic-Containing Water And The Action Mechanism Of Biofouling Layers

With the aggravation of water crisis and water pollution,the problem of decentralized water supply security has become more prominent.The conventional water purification process is complicated in operation and high in energy consumption,which makes it difficult to popularize and use in decentralized water supply areas.As a good decentralized water supply system,gravity-driven membrane（GDM）filtration system has the advantages of simple operation,low energy consumption and maintenance,and can operate stably for a long time.However,with the development of animal husbandry and aquaculture,the phenomenon of antibiotic pollution of water sources in distributed water supply areas,such as rural areas,is widespread,which puts higher requirements for GDM system.This study firstly investigated the effect of typical environmental factors and process conditions on the performance of the GDM system,and then proposed the method of introducing active bacteria to regulate the biofouling layer to elucidate the synergistic effect of active bacteria-biofouling layer on the removal of antibiotics.On this basis,the Mn-GDM coupling system was constructed to enhance the activity of microorganisms in the system,and the mechanism of removing antibiotics and antibiotics resistance gene（ARGs）in GDM system was analyzed.Firstly,the effects of different temperatures（5°C,10°C,20°C and 30°C）,different illumination（outdoor illumination vs.indoor illumination）and different powdered activated carbon（PAC）dosing strategy（batch dosing,batch dosing+intermittent operation,one-time dosing and control group）on the performance of GDM system were investigated.The results showed that the biofouling layer with loose structure,large floc particles and well-developed pore structure was beneficial to obtain high flux.However,the morphology and structure of the biofouling layer have little effect on the removal of pollutants.The thickness of the biofouling layer did not have a significant effect on the flux,while higher roughness and porosity correspond to higher fluxes.The concentrations of soluble microbial products（SMP）and extracellular polymeric substances（EPS）in the biofouling layer will increase when microorganisms are adversely affected.The increase in concentration may aggravate membrane fouling,but it has little effect on the removal of pollutants.The biomass of the biofouling layer had no significant relationship with pollutant removal.Pollutant removal rates are higher when microbial activity in the biofouling layer is higher（measured by Adenosine Triphosphate,ATP concentration）.It can be seen from this,increasing the roughness and porosity of the biofouling layer can effectively improve the stable flux,while increasing the microbial activity can enhance the removal of pollutants.By regulating the biofouling layer,its function can be strengthened and the performance of the GDM system will be improved.The biofouling layer was regulated by introducing active bacteria,and the effect of active bacteria-biofouling layer on the removal of sulfamethoxazole（SMX）and other pollutants was investigated.The results indicated that the degradation of SMX occurred rapidly after the introduction of active bacteria,and the SMX concentration in the effluent was close to 0μg/L after only one week of filtration.In the control group,however,SMX concentration decreased over 20 days of operation.Compared with the control group,the stable removal rate of SMX in the GDM reactor with the introduction of active bacteria increased by 9.83%,and the system could better cope with the SMX shock load.The removal rates of dissolved organic carbon（DOC）and NH₄⁺-N were improved by 14.71%and 5.62%,respectively,and the removal of fluorescent pollutants was also significantly improved.When the active bacteria were introduced,the sludge was used as a carrier,the concentration of tightly bound extracellular polymeric substances（TB-EPS）increased from 0.56 g/m² to1.09 g/m²,and the thickness and compactness of the biofouling layer increased,which led to the slight reduction of the flux.The active bacteria significantly increased the diversity and abundance of bacterial communities in the biofouling layer,but had no obvious effect on eukaryotic communities.After the inoculation,the system contained more readily biodegradable carbon sources.Therefore,more abundant metabolic pathways of SMX and higher mineralization rate were realized,which produced more small molecular metabolites.The formation of manganese oxides is driven by microorganisms,which can enhance the removal of SMX by the biofouling layer.The Mn-GDM coupling system was constructed to explore the influence of manganese oxides on the GDM system.The study found that compared with the control group,the stable removal rates of DOC,NH₄⁺-N,SMX and fluorescent fractions for the Mn-GDM system were improved by 6.02%,11.48%,10.88%and 10.10%～223.52%,respectively.Manganese oxides has a loose and porous structure,after the manganese oxides was formed in the GDM system,the thickness（1774.88μm vs.775.54μm）and the porosity（64.93%vs.41.24%）of the biofouling layer increased significantly.The increase in porosity has a positive impact on the stable flux of the GDM syste m.The stable fluxes in the Mn-GDM and Control-GDM were 5.14-5.71 and 4.62-5.33 LMH,respectively.Moreover,the concentration of loosely bound EPS（LB-EPS）in the Mn-GDM system decreased after manganese oxide production.The formation of manganese oxide increased the richness and diversity of the bacterial communities in the biofouling layer,and improved the activity of SMX-degrading bacteria.Therefore,the metabolic pathways of SMX were more abundant with higher mineralization rate in the Mn-GDM system.Compared with the control group,the concentrations of antibiotics resistance gene（ARGs）int I1,sul1 and sul2 in the Mn-GDM system were reduced by 35.45%,18.46%and 38.13%,respectively.This is because manganese oxide can strengthen the linkage between organisms and reduce SMX concentration,thus reducing the expression of ARGs.This paper aimed to clarify the effect of the biofouling layer properties on the performance of the GDM system.Based on this,the biofouling layer was regulated to strengthen the microbial activity and improve the performance of the GDM system.This is of great significance to the decentralized water supply,and it can also promote the wider application of the GDM system.