Performance And Mechanism Of Sewage Sludge Degradation And Hexavalent Chromium Reduction In Bioelectrochemical System With Dual-cathode

Bioelectrochemical system(BES)is a new bio-electrochemical process that utilizes electron transfer mechanism of redox at anode and cathode to achieve contaminant removal and electrical energy recovery.Using organic-rich sewage sludge as the anodic substrate of BES,it can output electrons in a long-term,sustained and stable manner with itself degraded and electricity generation.Current BES targeted at Cr(VI)reduction is generally supplied with artificially-added nutrient solution such as glucose or acetate as an electron donor,lacking the ability to provide electrons continuously and stably for a relatively long period of time.Such electron acceptors need to be frequently replaced during operation,which is not conducive to the stable reduction of the cathode Cr(VI)and the reduction of the system cost.In the BES with sewage ludge as the anodic substrate and the Cr(VI)as the cathodic electron acceptor,a stable electron flow can be generated in the anode chamber to drive Cr(VI)reduction,and the cathode has a high Cr(VI)reduction potential to increase the power density.Degradation of sludge and reduction of hexavalent chromium were achieved at the same time,but the performance and mechanism of this system still need further study.The specific research content was as follows.The performances of electricity generation,sludge degradation and Cr(VI)reduction were investigated from four aspects of system configuration including sludge property,catholyte p H and Cr(VI)concentration,and the operating conditions for each best performance were also obtained.BES with bio-chemical dual-cathode was constructed to improve the stability of Cr(VI)reduction.The changes in the distribution of microbial communities,functional genes,and system electron flux were analyzed to reveal the operational mechanism of BES.In BES with the chemical double-cathode and excess sludge as the anodic substrate at the catholyte p H of 2,BES can generate maximum power density of 9.8±0.4 W/m3 at the catholyte Cr(VI)concentration of 150 mg/L,the highest reduction rate of Cr(VI)(98.3%~99.9%)was obtained at the catholyte Cr(VI)concentration of 25 mg/L,and the highest extracellular biological organic matter(EBOM)degradation rate of 77.0±3.8% at the catholyte Cr(VI)concentration of 75 mg/L.In the BES with bio-chemical dualcathode,the maximum power density of 9.6±0.5 W/m3 was obtained when the TCOD of sludge was 22307±694 mg/L,and catholyte Cr(VI)concentration was 300 mg/L at p H of 1.The highest reduction rate of Cr(VI)(98.6%~99.5%)was obtained when the TCOD of sludge was 10769±321 mg/L,and catholyte Cr(VI)concentration was 60 mg/L at p H of 2.System configuration and sludge property affected the performance of BES with chemical cathode.Increasing the proton exchange membrane area and cathode area of BES can reduce the ohmic internal resistance and internal resistance of BES,increase the maximum power density(3.5~4.2 W/m3)and further increased the the TCOD removal rate(37.2±0.4%)of BES.The increase of membrane area and cathode area also increased the cathodic reactivity and proton transmission flux of BES,thereby accelerating the Cr(VI)reduction reaction.Since excess sludge has a higher moisture content and lower sludge concentration than dewatered sludge,lower mass transfer internal resistance and higher maximum power density(5.8~6.5 W/m3)was obtained at BES with excess sludge as anodic substrate,and the hydrolysis of organics in excess sludge was faster and more efficient than dewatered sludge,with the TCOD removal rate of 46.2±3.4%.The catholyte p H value and hexavalent chromium concentration had significant effects on the performance of BES with chemical double-cathode.When the p H value of catholyte was reduced from 4 to 2,the activity of cathode electrode was enhanced,the conductivity of cathode fluid was increased,and the internal resistance of cathode was decreased.When p H was 1,the reduction time of Cr(VI)was the shortest(96~262 h),the reduction rate(96.3%~98.0%),the maximum power density(5.8~6.3 W/m3)were both the highest,and EBOM degradation rate was the maximum(72.3±0.7%).With the increase of the catholyte concentration of Cr(VI),the proportion of electrons produced in the anodic chamber for bioelectrogenesis of BES was increased.Conversely,the the proportion of electrons for methanogenesis was increased.The increase of Cr(VI)concentration had both negative effects on the relative abundance of the exoelectrogens at the genus level and the main methanogenic enzyme genes.When the catholyte concentration of Cr(VI)was 75 mg/L,the methanogenesis process was not completely inhibited by the electrogenesis process,which can lead to the greatest remove of TCOD.The results of performance and mechanism of BES with bio-chemical dual-cathode showed that BES with dual-cathode improved the performances of Cr(VI)reduction and electricity production.The Cr(VI)reduction time of BES with dual-cathode was shorter than that of BES with chemical cathodes.In the three operating cycles of Cr(VI)reduction,the voltage was always above 0.672 V.The degradation rate of sludge TCOD was 18.2% higher than that of BES with chemical cathodes.Among factors that affecting the performance of BES with dual-cathode,the higher the sludge TCOD,the faster the Cr(VI)reduction rate and the higher the power density.The higher the catholyte Cr(VI)concentration,the faster the Cr(VI)reduction rate.The internal resistance of the system was decreased and the maximum power density was increased with decreased p H.In the study of the mechanism of BES with dual-cathode,the combined action of the biocathode and the chemical cathode facilitated the conduction of higher currents in the system,increased the electron amount used for Cr(VI)reduction,promoted sludge degradation,and increased the relative abundance of exoelectrogens dominated with Geobacter spp.in anode biofilms.The Cr(VI)reduction reaction was a first order reaction,and the kinetic constant k increased with increased Cr(VI)concentration and decreased with increased p H.Decreasing the p H of the catholyte can reduce the diffusion resistance of anode and solution resistance of cathode,the diffusion resistance and the charge transfer resistance of the chemical cathode.This study aimed at the research achievements of bioelectrochemical system with dual-cathode in sewage sludge degradation and hexavalent chromium reduction,especially the method of sewage sludge degradation and hexavalent chromium reducion in bioelectrochemical system with bio-chemical dual-cathode can provide new ideas and technical means for the treatment of sewage sludge and chromium wastewater.