| Microbial fuel cell (MFC) can directly convert chemical energy in biodegradable materials (fuels) into electricity by employing microorganisms as catalysts. However, as a stand-alone wastewater treatment technology, MFC has also sufferred from some issues, like inadequate nitrogen removal, poor effluent quality and anodic acidification and cathodic alkalization. To improve biomass energy extraction and pollutant removal, three novel MFC reactors (rotating biocathode MFC, membrane filtration biacathode MFC and sequencing polarity-invert ing MFC) were designed. Especially, the overall efficiency of nitrogen removal and the capacity of electrochemical denitrification were studied.To simplify the process for nitrogen removal, lower energy demand and reduce operational complexity, a rotating biocathode MFC was designed to unload aeration and achieve simultaneous nitrification and denitrification. Under continuous regime with a COD/N ratio of5:1, removal efficiencies of chemical oxygen demand (COD) and total nitrogen (TN) were85.7±7.4%and83.7±7.6%, respectively, and a maximum power output of584.5±29.2mW/m3was yielded. In the batch tests, TN removal efficiencies for closed/open circuit were82.1±0.5%and59.4±3.3%, respectively, which was improved by23%. The biocathode could efficiently catalyze nitrate reduction to facilitate nitrogen removal using cathode as electron donor. The denitrifying bacteria, Acidovorax sp.(99%) and/or Delftia sp.(98%) affiliated with the family Comamondaceae, predominated in the interior layer of cathodic biofilm.To obtain high effluent quality, sedimentation or filtration were still demanded in biocathode MFC. Membrane filtration biocathode MFC was developed with membrane module playing dual roles (filtering and cathode). Stainless steel mesh and carbon felt were employed to fabricate membrane modules (SS MFC and carbon MFC). Turbidity, COD and NH4+were effectively removed in both MFCs. In SS MFC, nitrate contents in cathode and effluent were close, while those in carbon MFC decreased from35.9±4.2to27.3±5.3mg-N/L, the removal efficiency was23.3±6.5%and TN removal was improved by17%. The maximum power densities of SS and carbon MFCs were121.4±0.0and1253.3±8.9mW/m3, respectively. Micrococcus and rod-shaped bacteria covered the carbon felt surface and fewer bacteria were found on SS mesh. The putative bacteria affiliated with Paracoccus genus and Pseudomonas spp. dominated in the interior biofilm on carbon felt for denitrification.To avoid the effect of pH drift between anode and cathode on MFC performance, a polarity-inverting MFC was developed to neutralize anodic acidification/cathodic alkalinization. The same electrode sequentially experienced anode, aerobic cathode and anoxic cathode like SBR, and such two SBR half cells with opposite electrode polarities formed a complete MFC. The system successfully buffered electrolyte and generated a maximum power density of623.3±4.1mW/m3. COD was completely removed. Three bacteria, Arthrobacter sp., Stenotrophomonas sp. and Sphingobacterium sp., were found on the polarity-inverting electrode, and Arthrobacter sp. was believed to be capable of transferring electrons both to and from electrode.Dosing activated sludge was performed to improve nitrification. When anaerobic/aerobic/anoxic phases were adjusted to6/4/2h, the removal efficiency of ammonium was99.0±1.3%, whereas denitrification was insufficient and the overall removal efficiency of TN was only29.1±5.8%. When anaerobic/aerobic/anoxic phases were adjusted to6/2/4h, the removal efficiencies of ammonium were100.0±0.0%in both closed and open circuits, and the overall removal efficiencies of TN were91.4±0.2%and71.7±4.2%, respectively, improved by20%in MFC mode. AOB and NOB in the sludge carried out nitrification. The main denitrification pathways in anoxic phase involved PHA denitrification by denitrifying GAO and electrochemical denitrification by exoelectrogen. Few PAO were present, which accounted for poor P removal. |
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