| As a by-product of biological wastewater treatment,wasted activated sludge(WAS)has a huge output.At present,most of the WAS hydration is directly buried in landfills in China,causing serious risk of secondary pollution.Anaerobic digestion(AD)is one of the best techniques for the treatment of WAS,which can produce bioenergy(e.g.methane)while treating pollutants.However,domestic application of AD in the treatment of WAS is still inadequate.The main reasons lie in two aspects.On one hand,the microbial cell wall of WAS is resistant to be destroyed and complex organic matter is hard to be hydrolyzed.On the other hand,the traditional AD mainly depends on the fragile interspecies hydrogen transfer(IHT)between acid-producing bacteria and methanogens.As the result,the methanogenic metabolic balance is easily broken,posing negative impact on the overall effect of AD.From previous documents,conductive materials(such as carbon materials)could improve AD of organic matters because they could mediate the electron transfer between the acid-producing bacteria and methanogens.Fe(III)oxides can enrich iron reducing bacteria(IRB)and participate in the decomposition of complex organic compounds by dissimilation of iron reduction.However,the effect of magnetite/carbon conductive materials on sludge hydrolysis,acidification and/or methanogenesis by dissimilating iron reduction or accelerating electron transfer between microorganisms has not been elaborated,hindering the technical application of this method in WAS treatment.Therefore,in this study,magnetite/carbon conductive materials were added to the sludge AD system to promote the hydrolysis and methane production by strengthening the electron transfer between acid-producing bacteria and methanogens through dissimilation of iron reduction.In addition,electric field was applied to further improve the AD performance of WAS.The technical characteristics and principles of enhanced AD of WAS with magnetite/carbon conductive materials were unveiled.The main results are as follows:(1)Carbon rods were added into anaerobic sludge digesters to investigate their effects on AD of sludge.When treating raw sludge,the methane production of anaerobic digester with carbon rods were was 10.2%higher than that without carbon rods(i.e.control group).To clarify the respective role of carbon rods in sludge hydrolysis-acidification and methanogenesis,the hydrolysis of sludge was accelerated by alkali pretreatment to form small molecular organic acids.When carbon rods were added into the anaerobic digester to treat alkali-pretreated sludge,the methane production was 8.9%higher and the acetic acid content was 7.1%lower than that of the control group.However,when the methanogenesis was suppressed by sodium bromoethyl sulfonate and heat treatment,the carbon rods had little effect on dissolved chemical oxygen demand,demonstrating insignificant effect of carbon rods on hydrolysis-acidification of sludge.Therefore,the improvement of AD efficiency of sludge could be attributed to the promotion of methanogenesis by carbon rods.In other words,carbon rods,as conductive materials,may accelerate the electron transfer between acidogenic bacteria and methanogens(i.e.syntrophic metabolism),thus speed up the decomposition of acetic acid and methanogenesis.(2)To clarify the effect of dissimilation iron reduction on sludge hydrolysis-acidification and/or methanogenesis,single conductive material and their mixture were added into anaerobic sludge digesters,respectively.Compared with the control group(no addition of conductive material),magnetite,GAC and their mixture raised the average methane yield by 7.3%,13.1%and 20.0%,respectively.The corresponding effluent total COD(TCOD)was 49.6,48.0 and 46.6 g/L,respectively,which was lower than that of the control group(53.2 g/L).Nitrogen adsorption,Fourier transform infrared spectroscopy(FTIR)and microbial analysis showed that magnetite induced dissimilation iron reduction and promoted sludge hydrolysis,providing appropriate substrate for methanogenesis.Methane production was improved by adding GAC alone and the improvement could be ascribed to high conductivity,large specific surface area and rich quinones group(contributing to large acceptance/contribution electron capacity)of GAC,which were beneficial to enhance direct electron transfer between microorganisms and as a result the methanogenic performance was improved.Meanwhile,adding the mixture of magnetite and GAC could not only increase the hydrolysis of sludge by dissimilation iron reduction,but also enhance methanogenesis by improving the electron transfer between IRB and methanogens.(3)To make full use of the electrons produced by oxidizing organic matter,the electrons were driven by applying electric field to transfer to the cathode where CO2 were reduced to methane.A carbon-paper anode modified by Fe3O4 and a carbon-paper cathode were placed in an anaerobic digester and voltage was applied between the anode and the cathode.By this way,a methanogenic single-chamber microbiological fuel cell(MEC)was constructed,which was denoted as Fe3O4/C-MEC.C-MEC was the same as Fe3O4/C-MEC except that the anode of C-MEC was not modified by Fe3O4.Compared with C-MEC,the cumulative methane production of Fe3O4/C-MEC and the TCOD removal rate were increased by 24.1%and 21.8%,respectively.Accordingly,the average Coulomb efficiency of the anode of Fe3O4/C-MEC was increased by 24.9%compared to C-MEC.The current calculation confirmed that Fe3O4-modification on the anode of C-MEC could boost sludge hydrolysis and subsequent methanogenesis,expressed by the enhancement of oxidative decomposition of complex organic matter(e.g.protein and polysaccharide)on the anode.The results of high-throughput sequencing revealed that the relative abundance of ruminococcaceae(in the sludge from the surface of the anode)and the relative abundance of Clostridiales IncertaeSedis XII(in suspended sludge)of Fe3O4/C-MEC were 10.7%and 30.1%higher than those of C-MEC,respectively.Meanwhile,the relative abundance of methanospirillum in the sludge from the surface of the anode of Fe3O4/C-MEC was increased by 30.0%.Therefore,Fe3O4-modification on the anode of C-MEC could simultaneously increase the relative abundance of IRB and hydrogenotrophic methanogens in the anaerobic sludge digester.The results showed that the electric field was beneficial to the electron transfer from IRB to methanogens,thus promoting the decomposition of sludge and methane production.(4)The low electron capture ability of CO2 is not conducive to the cathodic CO2 reduction,which affects the decomposition of sludge by anodic oxidation.Therefore,nitrate was added into the single-chamber MEC to construct a NO3-cathode-MEC.Herein,nitrate was used as cathode electron receptor to accept electrons released from the anodic sludge oxidation and thereby the decomposition of sludge on the anode is accelerated.Compared with the control MEC(no nitrate addition),the anodizing efficiency of NO3-cathode-MEC was increased by 55.9%,and the removal rate of volatile suspended solid(VSS)was increased by 21.9%.Due to the competition for electrons between nitrate reduction and methane generation through cathodic CO2 reduction,the methane production of NO3-cathode-MEC was lower than that of the control MEC in the initial ten days of nitrate addition.After that,more methane was produced in NO3-cathode-MEC and its cumulative methane production increased by 8.9%in 24 days compared to the control MEC.This is because the anodic oxidation driven by nitrate cathodic reduction has a stronger ability to decompose sludge and can provide more small molecules of organic matter for methanogenesis in bulk phase.Herein,carbon rod electrodes,as conductive materials,may accelerate the direct electron transfer between electroactive microorganisms and methanogens to boost methanogenesis.FTIR analysis indicated that enhancement of the anodic oxidation by nitrate addition accelerated the decomposition of sludge flocs and cell walls.The current calculation further elucidated that anodic oxidation driven by cathodic nitrate reduction was the main mechanism to improve the anaerobic sludge digestion. |
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