| Due to the increasingly serious situations of water pollution,more and more attention have been paid to the decentralized water supply.Regarding to the shortages of large investments,high energy consumptions,complicated operation/maintenance and strict requirements for the operators,current typical drinking water treatment technologies are not suited to the decentralized drinking water supply.Promisingly,gravity-driven membrane(GDM)filtration performed under ultra-low pressure without any cleaning procedures is developed,endowed with many inherent merits,such as low maintenance,low energy consumptions,simple operation and sustainable performance.GDM is regarded as the most suitable technologies for decentralized drinking water treatments.This study aimed at the impacts of different biological activity and raw water characteristics on the flux stabilization,and tried to obtain a deep understanding of the mechanisms of flux stabilization in GDM filtration.Furthermore,a separated biological pre-treatment + GDM process and an integrated granular activated carbon(GAC)/GDM system were developed,which can significantly improve the stabilized flux and removal performance of GDM filtration.In present study,a non-chemical,low pressure & maintenance ultrafiltration technology was first proposed.The results indicated that during long-term filtration,a heterogeneous and porous structure of bio-cake layer developed on the membrane surface and the flux of GDM would remain stabilized at levels of 7-9 L m-2 h-1.When sodium azide was added into GDM system to inhibit the biological activity,a complex and homogeneous bio-cake layer was observed on the membrane surface,and extracellular polymer substances(EPS)accumulation within the bio-cake layer increased significantly.As a result,the flux decreased gradually and can not remain constant.By contrast,when cycloheximide was employed to inhibit the protozoal and metazoal activity,a flux stabilization was observed,while its stable flux was apparently lower than GDM control,with a decrease of 48%.Likewise,when acetate and humic acids were respectively added into GDM systems to change the raw water characteristics,flux stabilizations could be still found in these two GDM systems.However,the roughness and porosity of the bio-cake layer in the GDM systems with addition of acetate and humic acids declined significantly,and the EPS concentration of the bio-cake layer increased by 17-28% and 67-125%,resulting in a decrease of stable flux of 34% and 15%,respectively.It’s also found that acetate exhibited a slightly higher influence on the flux stabilization of GDM filtration compared to humic acids.Unexpectedly,the addition of Ca2+ would support the formation of heterogeneous and porous structures of bio-cake layer and result in partial detachments of bio-cake layer,contributing to a slightly increase of stable flux compared to the GDM control.Further investigations indicated that for one hand the bio-cake layer would cause significantly membrane fouling and result in flux reduction,for another hand both the biological hydrolysis of foulants rejected on the membrane surface and the formation of heterogeneous structures of bio-cake layer would efficiently alleviate the membrane fouling and contribute to the flux recovery,both of which led to the flux stabilization during GDM filtration.The bio-cake layer can not only play a fundamental role in the flux stabilization,but also exhibited an efficient prefiltration effect to enhance the removal performance of GDM with an increase of removal of assimilable organic compounds(AOC)by 20%.Nonetheless,the bio-cake layer of GDM also exhibited a negative effect on the DOC removal due to bio-hydrolysis of rejected foulants(e.g particular organics and high-MW compounds)on the membrane surface,which resulted in the increase of DOC concentration in the permeate compared to feed water,risking the safety of water supply.In order to improve the permeate quality and stabilized flux,biological pre-filtrater+GDM process with the complementary effects between the biological pre-treatments and GDM filtration was developed,which efficiently improved the removal performance and enhanced the potentials in resisting the shock load of feed water quality.Besides,biological pre-treatments can significantly reduce the EPS concentration within the bio-cake layer and improve the stable flux.An apparent correlation between EPS concentration and stable flux was observed,which highlighted the vital role of EPS in the flux stabilization in GDM filtration(R2>0.9).Biological pre-treatments can not only efficiently pre-remove the foulants to improve the characteristics of feed water prior to GDM filtration and reduce their accumulations on the membrane surface,but also effectively consume the nutrients(e.g.AOC)to decrease the biomass and EPS secretion within the bio-cake layer,both of which can significantly contribute to improvements of stabilized flux.In addition,in hope to enhance the biological predation within the bio-cake layer of GDM based on the kinds of protozoa and metazoans colonizing in the GAC filter,an integrated GAC/GDM process was developed.Promisingly,the coupling GAC layer will not impact the flux stabilization,but the stabilized flux was significantly improved by 30-50%,as well as the removal performance(e.g.DOC and AOC with removals of > 80%).In the initial filtration(GAC with efficient adsorption capacities),the GAC would efficiently adsorb the foulants prior to GDM filtration to alleviate their accumulation on the membrane surface and reduce the bio-growth within the membrane bio-cake layer by largely consuming the nutrients prior to GDM filtration,leading to a higher stable flux relative to GDM control.In addition,another coupling GAC layer in which the GAC adsorption was saturated,was employed to investigate the influence of GAC layer on the long-term filtration of GAC/GDM system.It’s found that though the GAC adsorption was saturated,the high stabilized flux achieved in the initial filtration can be still maintained during long-term filtration of GAC/GDM system.The improved flux level of OGDM(40-50%)compared to GDM control was mainly related to that GAC layer can function as a protozoal and metazoal incubator to naturally add higher organisms into the bio-cake layer to enhance their predation and result in heterogeneous and porous structures of bio-cake layer.Besides,compared to the separated the GAC+GDM process,the integrated GAC/GDM conferred similar removal performance of TOC,DOC and AOC,and no significant difference of stable flux was observed between them,promisingly with lower footprint and operation & maintenance.Therefore,in practice it’s proposed to adopt the GDM process based on the considerations of local conditions and feed water characteristics,with the ultimate goals of improving the reliability of water supply.In this study,based on the systematic analysis of influence factor and mechanisms of flux stabilization in GDM long-term filtration,accessable approaches to higher stabilized flux,including improvements in both the feed water characteristics and biological predation within the bio-cake layer,were proposed in GDM practice.Furthermore,biological pre-treatments+GDM process and integrated GAC/GDM system were developed accordingly.Our findings are expected to contribute to the widespread application of GDM technologies in decentralized drinking water supply. |
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