Performance And Mechanisms Of Residual Sludge Treatment In Bioelectrochemical Assisted Constructed Wetland

Sludge treatment wetland(STW)can simultaneously thicken and dewater sludge,with significant advantages such as low investment and operating costs,low energy consumption and ease of maintenance and management.However,the low degradation efficiency of sludge organic matter limits its practical application.Bioelectrochemical systems(BES)can use microbial activity to convert chemical energy into electrical energy,recover electrical energy while treating sludge,and accelerate the anaerobic degradation process of sludge.In this study,a bioelectrochemical system coupled with a sludge treatment artificial wetland process(BES-STW)was developed by introducing bioelectrochemistry into a sludge treatment wetland system.The effects of substrate materials and electrode settings on the electrical production performance,organic pollutant removal and leachate water quality of the BES-STW system were firstly investigated,and the degradation and transformation patterns of organic substances in the BES-STW and STW systems were revealed with the aid of resin classification experiments.Finally,the mechanism of the enhanced sludge treatment by the bioelectrochemical system is explained from a microbiological perspective through high-throughput sequencing technology,which provides a basis for the design and operation of future sludge treatment wetlands.Substrate materials and anode electrode settings have a significant effect on the treatment efficiency of BES-STW systems.The study showed that the output voltage of the BES-STW system with graphite pellets and zeolite as the substrate material was high and stable;the output voltage of the BES-STW system with gravel as the substrate material fluctuated considerably;and the output voltage of the BES-STW system with ceramic pellets as the substrate material was the lowest.The BES-STW system with graphite pellets as the matrix material had the highest power density(0.467 W/m~3)and the highest removal rate of Total chemical oxygen demand(TCOD)(59.45%).The BES-STW system had a higher voltage output and power density when the anode electrode was set in the substrate layer,with an open circuit voltage and maximum output power density of 0.874 V and 0.498 W/m~3 respectively.STW system and the correlation between the removal efficiency of each substance and the voltage of the STW system showed a very strong and significant positive correlation between the p H of the leachate effluent and the TCOD removal rate of the accumulated sludge.The characterisation of the organic matter showed that the hydrophilic fraction(HPI)was the main component of the DOC content in EBOM(Extracellular biological organic matter)and DOM(Dissolved organic matter).Aromatic substances are mainly concentrated in hydrophobic organic acids(HPO-A)and DOC removal from the EBOM of BES-STW sludge is mainly reflected in the significant reduction of HPO-A and HPI fractions.Bioelectrochemistry promoted the degradation of aromatic substances in the sludge and leachate in the STW system;it also promoted the hydrolysis of aromatic organics in the neutral fraction of the sludge.Three-dimensional fluorescence spectroscopy showed that humic acid-like substances in the sludge EBOM and DOM of the BES-STW system were mainly present in the HPO-A fraction.Bioelectrochemistry accelerates the hydrolysis and degradation of humic acid-like and aromatic proteins in the STW system sludge.the BES-STW system preferentially removes tryptophan and fulvic acid substances.The introduction of bioelectrochemical techniques significantly increased the diversity and abundance of biofilms in the STW system.longilinea was the dominant genus in the BES-STW system,and the enrichment of longilinea and methylophilus accelerated the hydrolysis process of organic compounds in the STW system.geobacter had the highest relative abundance in the BS-F,and the BS-STW system had higher abundance of nitrifying and denitrifying bacteria.The enrichment of Thauera,Dechloromonas and Syntrophorhabdus in the BES-STW helped to accelerate the degradation of aromatic compounds in the sludge.the enrichment of Ornatilina and Anaerolinea accelerated the protein and polysaccharide degradation activities in the BES-STW system.The metabolic pathway was higher in the BES-STWBES-STW system than in the STW system in terms of relative content of amino acid metabolism,carbohydrate metabolism,energy metabolism,sugar biosynthesis and metabolism.The introduction of electricity production increased the abundance of genes related to carbohydrate metabolism,and bioelectrochemical technology facilitated polysaccharide cleavage and hydrolysis processes in the STW system.there was little difference in the types of genes in the TCA cycle pathway between the BES-STWBES-STW system and the STW system.The introduction of bioelectrochemical techniques increased the abundance of genes related to glycolytic pathways,aromatic compound metabolism,tyrosine metabolism and tryptophan metabolism in the STW system.