| Microbial fuel cell (MFC) is an emerging wastewater treatment technology which employs microbes as catalytic converting chemical energy to electric energy. In recent years, MFC has been the hotspot in wastewater treatment and new energy researches. But for the reasons of high cost, low power generation and difficult to scale-up, the researches on MFCs are still stucking in lab-scale and far away from practical application. On the basis of lab-pilot-demonstration project route, this research focused on the possibility of domestic wastewater treatment using scale-uped MFC technology.In order to improve performance of the lab-scale MFC, electrode material in biocathode were optimized. After start-up with carbon fiber brushes, three types of carbon materials-graphite granules, activated carbon granule and activated carbon powder were added into cathode chambers to improve power generation. The results showed that output voltages increased rapidly after adding those carbon materials. The addition of carbon materials could increase the electrode material’s specific surface area, improve the activity of catalytic microorganism in oxygen reduction reaction, therefore, the MFC internal resistance is reduced effectively and electricity generation performance is improved. With activated carbon granules, the maximum power density and coulombic efficiency increase significantly, by 166.1% and 64.3%, respectively. Also, with the activated carbon granules optimizing the biocathode, the anode chemical oxygen demand (COD) removal was also improved, while not so obviously with the use of graphite granules and carbon powder. In series conection the output voltage of MFCs could be improved, but voltage of single cell was easy to reversal. By increasing circulation rate of catholyte, the stabilization time of MFC series could be extended.A10L scale MFC coupled A~2/O reactor was built, with the anaerobic tank as anode chamber and the anoxic tank as cathode chamber. Meanwhile, hydraulic retention time (HRT) and internal reflux ratio were optimized to improve wastewater treatment efficiency and power generation. The results showed, the coupled system could obtain best pollutants removal effect when HRT=16h and internal reflux ratio=200%. Under this operation condition, compared with control reactor, concentrations of COD and total nitrogen (TN) in the effluent of the coupled system could be reduced by 27.4% and 11.1%. And the max power density of the coupled system could achieve by 596±31mW/m3 in the meantime. Correspondingly, the optimum operation condition when the coupled system achieved highest power generation was HRT= 12h and internal reflux ratio=100%, and the max power density was 808mW/m3. The results of energy balance showed that coupled system consumed less energy than the control reactor under the same operation condition. When HRT=16h and internal reflux ratio=200%, the coupled system could save 53.7% electric energy compared with the control reactor.Scaling-up the above A~2/O reactor to 1m3, we built a pilot-scale MFC coupled A~2/O system. The results showed that performance of MFC installed in different location related to oxidation reduction potential and wastewater component distribution in the reactor. The MFC which anode placed in the head of anaerobic tank and cathode placed in the end of anoxic tank performed best power generation. The corresponding distance of proton transfer was the longest. This result was distinguished from former researches. The performance of different size of the carbon fiber brushes was also investigated. The result indicated current intensity was not increase with the increasing carbon brush size maybe due to the internal current loss in the carbon brush. When the carbon brush was 50cm in length and 5cm in diameter, the MFC could achieved best performance with max power density was 2.36 W/m3. Using a current collector to strengthen anode, the current density could be increased from 14.1A/m3 to 22.3A/m3. On the contrary, the effect on cathode strengthened by current collector was not evident. Meanwhile, the performance of cathode was more related to the growth of biofilm, and excrescent biofilm on the cathode surface should be removed when the output current drop rapidly. After carbon brushes installed in A~2/O reactor, the magnetic stirrers could be removed, thus electric energy could be saved. But the effect on sewage disposal was not obvious.The pilot-scale MFC coupled A/O system had been continuous running under natural conditions for 12 months. During low temperature stage, water quality and power generation deteriorated obviously. With the temperature rising, water quality and power generation both had be recovered. The concentrations of COD, NH+-N, and NO3--N in the effluent on the final stage were less than 35,10, and 17mg/L, respectively, all lower than these on the initial stage, indicating that the coupled system running well after long-term operation. The microbial community structures were also tracked for 12 months and 24 samples were taken. MiSeq sequencing method was used to obtain 17989 standardized sequences and 817-1202 operational taxonomic unit (OTUs) from each sample. The results showed that, the dominant phyla of anode were Proteobacteria (30.7%), Firmicutes (22.4%), Chloroflexi(13.7%), Bacteroidetes (13.5%), and Actinobacteria (7.1%), while the dominant phyla of cathode were Proteobacteria (28.1%), Chloroflexi (23.1%), Bacteroidetes (12.5%), Actinobacteria (10.1%), and Firmicutes (6.4%) on the final stage.15 genera which were well considered as electricigens were detected on the anode biofilms. The superior phyla were Clostridium, Paracoccus, Pseudomonas, Arcobacter, and corresponding max abundances were 5.7%,1.6%,0.8%,0.6%, respectively. On the other side, the denitrifying bacterium structure of cathode changed greatly. The results of Partial Mantel test indicated that temperature significantly influenced microbial community structure both on anode and cathode. Meanwhile, the power generation related with anode was more significant than that of cathode.According to the results and experience of lab-scale and pilot-scale researches, we performed an MFC demonstration project in a domestic wastewater treatment plant. Current was producing successfully but at low level in winter. This demonstration project is supported by the International Science-Technology Cooperation Program of China. |
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