| Under the dual pressures of environmental pollution and energy shortage, it is obvious that conventional energy-intensive wastewater treatment processes do not meet the requirements of sustainable development any longer. Microbial fuel cell (MFC) is an emerging technology that directly converts chemical energy stored in wastewater to electricity, purifying wastewater and generating electricity simultaneously, which is considered as an attractive option for sustainable wastewater treatment. So far, however, most of the MFC research has been focused on organic wastewater treatment. Since nitrogen pollution is becoming a serious environmental problem, it is significant to develop MFC that can simultaneously remove nitrogen and recovery electricity from wastewater.In this research, the anodic denitrification MFC (AD-MFC) and the anaerobic ammonium oxidation MFC (ANAMMOX-MFC) were developed and their performances, operational parameters and working mechanisms were studied to achieve high-efficient nitrogen removal and electricity generation. The main results are as follows:1) The AD-MFC was constructed and its performances of simultaneous denitrification and electricity production were investigated.It was proved that the AD-MFC could be successfully started up with denitrifying bacterial enrichment culture as inoculum. The AD-MFC had good performances of nitrogen removal and electricity generation. In batch mode, when the initial nitrate and COD concentrations were100.22±0.62mg/L and500.40±1.67mg/L, the maximum denitrification rate, the maximum output voltage and the maximum power density were0.31±0.01kg N/m3·d,602.80±5.42mV and908.42±0.07mW/m3, respectively. The voltage curves of the AD-MFC showed three-phase characteristics of descend phase, ascend phase and redescend phase, which could be attributed to the succession of denitrification, methanol degradation, endogenous respiration and cell hydrolysate fermentation as dominant reactions in the anolyte. This phenomenon has never been reported before in literatures. The AD-MFC also had sensor function. The Pearson correlation coefficients between the voltage loss and the consumed nitrate and the consumed COD were0.9964and0.9917, respectively, which suggested that the voltage variation was closely related to the change of substrate concentration and the voltage could indicate the denitrification course.2) The influence of substrate concentration on the performance of the AD-MFC was explored, and kinetic characteristics of substrates degradation and electricity generation in the AD-MFC were revealed.It was discovered that both of nitrogen removal and electricity generation performances were closely related to the substrate concentration. Nitrogen removal rate and power generation could be promoted with increasing substrate concentration in a certain range (50-2000mg NO3--N/L), but they would be inhibited at high substrate concentrations (over2000mg NO3--N/L). Han-Levenspiel model could well describe the kinetic characteristics of the AD-MFC. Based on Han-Levenspiel model, the maximal value (rmax), the half-saturation coefficient (Ks) and the critical inhibitory concentration (Sm) for nitrate degradation, COD degradation, output voltage and power density were1.27kg N/m3·d,351.63mg/L and4301.25mg/L;5.14kg COD/m3·d,1950.21mg/L and20050.69mg/L:1030.53mV,203.25mg/L and4950.36mg/L;1386.39mW/m3,293.47mg/L and4649.03mg/L, respectively. According to the kinetic model, the half-saturation coefficient and the critical inhibitory concentration for nitrate were more than200mg/L and4300mg/L, respectively. The results demonstrated that AD-MFC was tolerant to high strength nitrate-containing wastewater. When the initial nitrate and COD concentrations were1999.95±2.86mg/L and10058±1.26mg/L, the maximum denitrification rate, the maximum output voltage and the maximum power density could be as high as1.26±0.01kgN/m3·d,1016.75±4.74mV and1314.41±24.60mW/m3, respectively. The volumetric nitrogen removal rate was much higher than that reported in literatures. 3) The substance transformation, microbial functional space, electron transfer pathway and functional microbial community were studied to reveal the working mechanism of the AD-MFC.It was found that electricity genaration was coupled to denitrification in the AD-MFC. Methanol or nitrate could not be effectively degraded as sole substrate, and electricity production capacity was very low. Only when methanol and nitrate coexist, could the AD-MFC achieve simultaneous denitrification and electricity production. Both the biofilm on the anode and the suspended sludge in the anolyte had the ability for denitrification and electricity production. The functional space of the anode biofilm and the suspended sludge was smaller than the total functional space of the AD-MFC, which suggested the anode biofilm and the suspended sludge both contributed to denitrification and electricity production. The suspended sludge was predominant in the functional space of the AD-MFC, and the functional spaces of the anode biofilm and the suspended sludge for denitrification and for electricity production were41.90%and67.98%,52.26%and69.03%, respectively. The electron transfer pathways of the anode biofilm and the suspended sludge were different. The electron transfer of anode biofilm was mainly realized by the direct contact to the electrode, while the suspended sludge relied on endogenous mediators for electron transfer. There was a great difference between the functional microbial populations. The seed sludge involved large amounts of cocci, bacilli and filamentous bacteria, while the anode biofilm mainly involved bacilli and filamentous bacteria, and the suspended sludge mainly involved cocci and bacilli. It was found that succession of bacterial communities occurred along with power generaion process, and the population diversity in the anode biofilm and the suspended sludge was significantly fewer than that in the seed sludge. The predominant bacterial populations in the AD-MFC belonged to y-proteobacteria,(3-proteobacteria, Bacteroides and Ignavibacteria, and the functional bacteria were mainly denitrifying bacteria.4) The ANAMMOX-MFC was constructed and its performances of simultaneous anaerobic ammonium oxidation and electricity production were investigated.It was proved that the ANAMMOX-MFC could be successfully started up with ANAMMOX bacterial enrichment culture as inoculum. The ANAMMOX-MFC also had good performances of nitrogen removal and electricity generation. Under continuous operation, when the initial ammonia and nitrite concentrations increased from25mg/L and33mg/L to250mg/L and330mg/L, the removal efficiencies of the total nitrogen, ammonia and nitrite were always above90%,90%and80%, respectively. Meanwhile, the maximum nitrogen removal rate, the maximum output voltage and the maximum power density were3.01±0.27kg N/m3·d,225.48±10.71mV and1308.23±40.38mW/m3, respectively. The volumetric nitrogen removal rate was the highest reported value so far. It was found that the polarization resistance of the anode was significant, accounting for60%of the total ANAMMOX-MFC internal resistance, which was the bottleneck for electricity production in the ANAMMOX-MFC. The ANAMMOX-MFC also had sensor function. The output voltage changed linearly with the variation of ammonia concentration in a certain range (25mg/L-250mg/L), which might mainly caused by the changed ammonia oxidation rate.5) The influences of temperature, pH and exogenous mediators on the performance of the ANAMMOX-MFC were explored, and the critical operational parameters for the ANAMMOX-MFC were optimized.It was discovered that temperature could significantly affect the performances of both nitrogen removal and electricity generation in the ANAMMOX-MFC, and the optimum temperature was about30℃. When the temperature was above or below30℃, the nitrogen removal rate and output voltage would decrease at the same time. The change of biological reaction responding to the changed temperature was the primary cause of the varied power output. The pH could also significantly affect the performances of both nitrogen removal and electricity generation in the ANAMMOX-MFC and the optimum pH was about7-8, but the changed degrees of nitrogen removal rate and output voltage were not so consistent compared with the temperature influence. The changes of biological reaction and anode potential responding to the changed pH was the common causes of the varied power output. The addition of exogenous mediators could enhance the electricity generation in the ANAMMOX-MFC. In the range of low concentrations (<0.01mmol/L), exogenous mediators such as neutral red,2-Hydroxy-1,4-naphthoquinone and phenothiazine which corresponding to the front of the electron transport chain, with small molecular weight and simple molecular structure would effectively reduce the anode charge transfer resistance and enhance power production in the ANAMMOX-MFC. While exogenous mediators such as brilliant cresyl blue and heme which corresponding to the end of electron transport chain, with large molecular weight and complex molecular structure, the enhancement effect of power production was not so obvious. However, when the concentrations of exogenous mediators were too high (0.01-0.02mmol/L), the biological reaction would be inhibited and the output voltage in the ANAMMOX-MFC would fell rather than rose.6) The substance transformation, microbial functional space, electron transfer pathway and functional microbial community were also studied to reveal the working mechanism of the ANAMMOX-MFC.It was proved that electricity genaration was coupled to ANAMMOX in the ANAMMOX-MFC. When ammonia or nitrite was as the sole substrate, ANAMMOX enrichment culture would hydrolyzed and the nitrogen removal and electricity production processes could not be sustained. The ammonia oxidation was coupled to the nitrite reduction, only when ammonia and nitrite coexist, could ANAMMOX-MFC sustain simultaneous anaerobic ammonium oxidation and electricity production. Both the biofilm on the anode and the suspended sludge in the anolyte had the ability for ANAMMOX and electricity production, and they both contributed to ANAMMOX and electricity production processes. The anode biofilm and the suspended sludge dominated different functional spaces of the ANAMMOX-MFC, and the functional spaces of the anode biofilm and the suspended sludge for ANAMMOX and for electricity production were30.14%and53.43%,59.52%and47.87%, respectively. The electron transfer pathways of the anode biofilm and the suspended sludge were also different in the ANAMMOX-MFC. The electron transfer of anode biofilm was mainly realized by the direct contact to the electrode, while the suspended sludge relied on endogenous mediators or nitrite as the potential mediator for electron transfer. There was certain difference between the functional microbial populations in the ANAMMOX-MFC. The ANAMMOX bacteria on the anode had larger anammoxosome, more iron particles, more heme c and less extracellular polymeric substances (EPS), which would facilitate the extracellular electron transfer. It was found that the microbial species in the suspended sludge were similar to that in the seed sludge, but the microbial species in the anode biofilm was significantly different from that in the seed sludge. The predominant bacterial populations in the ANAMMOX-MFC belonged to β-proteobacteria, y-proteobacteria, Acidobacteria, Ignavibacteria and Planctomycetes, and the functional microbial communities were consisted of ANAMMOX bacteria, denitrifying bacteria and a variety of other bacteria. |
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