Study On Volume Reduction Of Waste Activated Sludge By Bio-graphene Hydrogel And The Role Of Electroactive Microorganisms

Waste activated sludge(WAS)is a secondary pollutant in the biological wastewater treatment process,which is mainly composed of proliferating bacteria.The excess activated sludge owned a rich microbial community and the water content of can reach 99%,which makes WAS difficult to collect and dispose the bacteria,so dewatering and volume reduction were firstly required in WAS treatment,which is also a current technical challenge.Graphene materials and their derivatives owned excellent properties of optics,electricity and high specific surface area.They are widely used in the fields of energy storage,sensors,and biomedicine,but the application in the environmental governance sector is not in-depth.Therefore,this study is based on the characteristics of the reduction of graphene oxide(GO)by microorganisms and the spontaneous agglomeration of them to form bio-graphene-hydrogel(BGH)and developed a new WAS dehydration and volume reduction separation technology by enriching bacteria through BGH.Besides,we analyzed the mechanism of BGH formation through 3 strains of Shewanella bacteria.The main research results are as follows:In this study,the graphene oxide(GO)solution was first prepared by the modified Hummers method,then the density of GO was measured.WAS extracted from the secondary sedimentation tank was acclimated for 7 days,after stewing for 30 min the sediments were collected.Then,according to the mass dry weight ratio WAS/GO=0.4,WAS and GO were mixed into a 20 min-aerated medium,After stewing for 3 days,a black hydrogel is formed,which is named waste activated sludge-graphene-hydrogel(WGH)to distinguish it from ordinary BGH.Chemical characterization methods such as Fourier Infrared(FTIR),X-ray spectroscopy(XPS),Raman spectroscopy(Raman)and X-ray diffraction(XRD)proved that GO was reduced by microorganisms in WAS.The volume of WGH was reduced to about 12%comparing with the initial WAS,and the dewatering rate of WGH gel reached 98%,indicating this method can achieve excellent sludge reduction and dewatering results.The rheological test proves that the mechanical properties of WGH was excellent,which made the collection of sludge after reduction more convenient.Same weight of WAS were selected to prepara BGH and kept as control,respectively.The results showed under the same weight,The drying time of WGH is reduced by 60%compared with WAS at 104℃.Thermogravimetric analysis(TGA)showed that WGH will release more energy than WAS during the heating process.Besides,by measuring,the heat value of WGH and WAS were 6718.57 J/g and 5,244.731 J/g,respectively.Thermal tests indicated that WGH owned excellent thermal properties and is a potential combustion-supporting material.Compared with the traditional centrifugal disposal method,the method in this article has advantages in the difficulty of collection,disposal effection,and subsequent application.Three typical electroactive microorganisms in activated sludge:Shewanella xiamenensis BC01,Shewanella putrefaciens CN32 and Shewanella oneidensis MR-1 were selected to cultivate with GO.The results were that BGH formed in 5 days,BGH formed in 6 days,and BGH can not form,respectively.Scanning electron microscopy(SEM)proved that the morphology of the three bacteria were almost the same.Through FTIR,XPS,Raman and XRD characterization,it is found that the degrees of reduction of GO were different by three kinds of bacteria and the reduction ability is BC01>CN32>MR-1.which indicated the reduction ability of bacteria is positively correlated with the results of BGH formation.After adding a cytochrome C inhibitor,BC01 can not continue to form BGH with GO,which proved that the EET process is closely related to the BGH formation.After choosing to add two kinds of electron shuttles:riboflavin(RF)and potassium ferricyanide(Fe(CN3)6-)were chose and added in the BGH formtion process.MR-1 was able to form BGH in the reaction with GO,which proved the promotion of EET is also related to the formation of BGH.It is proved that the ability of extracelluar electron transfer(EET)of bacteria is also positively related to the formation process of BGH.Electrochemical test:Cyclic Voltammetry(CV)and Differential pulse voltammetry(DPV)tests show that cytochrome C inhibitor can greatly inhibit the electrochemical activity of Shewanella and the electron shuttles can effectively enhance the electrical properties of bacteria,which further confirms the connection between EET and BGH formation.In addition,the Extracellular Polymeric Substances(EPS)of the three kinds of bacteria were also investigated,and it was found that the hydrophilic and hydrophobic properties of them were different.The hydrophobicity of them were BC01>CN32>MR-1,which indicated that the formation of BGH is related to the combined effect of electrical properties and EPS hydrophobicity.By explaining the mechanism of BGH formation by Shewanella and GO,it can provide us a theoretical basis and technical support for a better understanding and applied of WGH to achieve better WAS dewatering and volume reduction in the future.