The Role Of Cathode Configuration And Operation Strategies On The Performance Of Microbial Fuel Cell-Constructed Wetland

Microbial fuel cell-Constructed wetland(MFC-CW)is a kind of novel wastewater treatment system.MFC-CW achieves both wastewater treatment and bio-electricity generation by integrating MFC and CW.The basic principle of MFC-CW is the similarity of profiles of electronical potential in MFC and CW.The most popular configuration of MFC inside MFC-CW was single anode chamber installed with air-cathode.CW acts mainly as the anode section of MFC.The anode is embedded inside substrate of CW.The interface of water level and the air is cathode section of MFC.Thus,the cathode is situated on the interface.Combination of oxygen in the air and protons in liquid into water is the dominant reaction in cathode section.Therefore,cathode has to be contacted with air and water at the same time.Cathode and anode are connected with an external resistor.As wastewater was fed,part of pollutants was removed by effects of CW while others were digested by effects of MFC.Thus,MFC could simultaneously achieve bio-electricity generation and promoted pollutants removal.The anode of MFC in MFC-CW was embedded inside the substrate of CW and kept immersed all the time.This created a stable anaerobic environment for anode.Furthermore,anode was fully contacted with pollutants,which was favorable for anodic reaction.Therefore,working conditions of anode were relatively well.Exoelectrogens were situated on and around the anode.Released electrons were collected by anode and then utilized by external circuit.Therefore,electrons were flowed from anode to cathode.Protons were generated by exoelectrogens in the process of releasing electrons.Protons travelled from anode section to cathode section through CW.In the cathode section,electrons and protons from anode section,oxygen from the air were reacted into water on cathode.As oxygen was from the air and protons were from water,it was necessary to consider the contact between air and water on cathode in order to guarantee cathodic performance.Therefore,investigating the role of cathode working condition on operation of MFC-CW was very significant.The Chapter 1 was background section which gave the general information of bio-electrochemical system(BES)and CWs.The Chapter 2 was the literature review.The content of Chapter 3 was investigating the key factors which were determining the cathode performance in a pilot-scale MFC-CW.The MFC-CW was established in a container with 330 mm in length and width,800 mm in height.The dewatered alum sludge cake was the dominant substrate of MFC-CW.A MFC with single cathode and four anodes was installed in MFC-CW.All of electrodes were in2D structure.The cathode of MFC-CW was made of stainless-steel mesh(SSM)which was attached to carbon felt.Then cathode was curved into inverted U shape and kept quarter of area under water level of CW.The space occupied by cathode was called cathode section.Aeration spargers were situated under cathode for promoting contact of air and cathode under water level.A water circulation system was installed in cathode section which was above water level.The water close to water level was pumped to the top of the cathode.Thereafter,water was flowed back to water level through cathode by gravity.All of anodes were made of SSM only.Each of anodes was embedded inside CW in different depth.The MFC-CW was operated under continuous up-flow feeding mode.The aim of this period of study was investigating the roles of cathode working condition and connection modes of MFC on performance of MFC-CW.There were totally four cathode working conditions in this study.They were neither aeration nor circulation;aeration only;circulation only;and simultaneous aeration and circulation.There were two MFC connection modes.The first was parallel connection which means that each anode was connected to cathode with an external resistor individually.The second was combined connection in which all anodes were connected to cathode with one external resistor.Therefore,the operational conditions of MFC-CW were determined by cathode working conditions and MFC connection modes.The results of this study were mainly reported in two aspects which were wastewater treatment and electricity generation.The COD concentration in influent was 500 mg/L.When MFC-CW was operated without aeration and circulation,more than 55%of COD was removed by CW.As either aeration or circulation was applied,more than 20%of COD was further removed.Moreover,more COD was removed under aeration only than circulation only.COD removal efficiency was higher than 83%once aeration and circulation were simultaneously applied.The COD efficiencies mentioned above in MFC-CW were achieved with open circuit MFC.MFC was available as external circuit was transferred from open circuit to close circuit.Under the same cathode working condition,more COD was removed in MFC-CW with parallel connection than combined connection.Furthermore,the highest enhancement of COD removal efficiency(8.7%)by MFC in MFC-CW was achieved under recirculation only in cathode working condition and parallel MFC connection mode.The absolute highest COD removal efficiency of MFC-CW was 91.7%under simultaneous aeration and circulation in cathode working condition and parallel MFC connection mode.NH₄⁺-N concentration in influent was 30 mg/L.The NH₄⁺-N removal efficiency was significantly affected by cathode working conditions and MFC connection modes.The highest NH₄⁺-N removal efficiency(97.3%)was reached under simultaneous aeration and circulation in cathode condition and parallel MFC connection mode.On the other hand,external circuit status(open circuit or close circuit)and cathode working condition played important roles on nitrification and denitrification processes in MFC-CW.In the aspect of electricity generation,cathode working condition was determined on whether reasonable production of electricity could be generated.As neither aeration nor circulation was applied in MFC-CW,potentials of cathode and all anodes were kept around-453.96 m V.This was the direct reason causing voltage output in very low level.Thus,this could be regarded as no effective MFC was established in MFC-CW.The potential of cathode was significantly raised when aeration or circulation was applied.Furthermore,as either aeration or circulation was applied in MFC-CW,cathode potential(58.64 m V)under circulation only was higher than aeration only(48.16m V).The highest cathode potential(144.33 m V)was reached when aeration and circulation were simultaneously applied.Oxygen diffusion into cathode section of MFC-CW was enhanced by both aeration and circulation.Therefore,anode situated closer to cathode was impacted by oxygen diffusion more than other anodes.Redox potential wasn't constant along the depth of MFC-CW.Lower redox potential was reached in deeper space of MFC-CW.In parallel MFC connection mode,all of anodes were kept in different potential individually without affecting each other.In combined MFC connection mode,all of anodes were kept in same and relatively high potential.Under same cathode working condition in MFC-CW,overall power output of MFC with parallel connection mode was higher than combined connection mode.Based on the results of this stage of study,ensuring the significant contact between oxygen and water in cathode section was one of the basic requirements for establishing an available MFC in MFC-CW.On the other hand,parallel connection mode was recommended in MFC-CW with multiple anodes MFC for higher power output.In order to achieve reasonable cathode reaction in MFC-CW,it was very important to identify the spatial relationship between water level and cathode.If there was no additional operation applied,both too low and too high of water level in cathode section were unfavorable for cathode performance.Protons could not reach cathode due to too low water level which resulted in a dead cathode.If water level was too high,oxygen diffusion was inhibited.Additionally,overgrowth of biofilm due to too high water level contributed to biofouling on cathode which drastically inhibited cathode performance as well.For the purpose of solving these issues,necessary intensified operations were required.Actually,the operations were applied for enhancing oxygen diffusion from the air into water body around cathode.The most popular operations were aeration and recirculation.Recirculation was reported better than aeration in terms of energy consumption in CW studies.Therefore,it was promising to apply recirculation in MFC-CW as well.Up to now,there were mainly two kinds of recirculation modes in CWs.The first was conventional recirculation mode in which water level was fixed while recirculated water was moving.The other was tidal flow recirculation mode.In tidal flow recirculation mode,water level was cyclic variable while recirculated water was moving.Because oxygen diffusion was enhanced by variable water level,the study in Chapter 4 was investigated the feasibility of tidal flow recirculation mode in cathode section and the roles of other factors in MFC-CW.MMFC-CW with total volume capacity of less than 1.56 L was established in Chapter 4.Tidal flow recirculation mode was applied in cathode chamber.For the purpose of energy saving strategy for achieving tidal flow,a siphon tube was installed in cathode chamber.The siphon tube was used for determining the highest water level and spontaneous water discharging.One terminal of siphon tube was connected to the bottom of cathode chamber.The top of siphon tube was as high as cathode chamber.MFC with one cathode and two anodes was installed in MMFC-CW.There was no separator between anode and cathode chambers.Cathode was very close to top and but didn't touch it.Up-flow continuous feeding mode was applied in MMFC-CW.There were two kinds of tidal flow recirculation modes applied in MMFC-CW.They could be classified by the range of water level variation.The first was called FSR mode where water level variation range was covered the whole cathode chamber.The second mode was named SSR in which water level only varied in the top half of cathode chamber while bottom half of cathode chamber was always immersed.The similarity of the two modes was that recirculated water was entered from the top of cathode chamber and then discharged from the bottom of cathode chamber.In order to investigate the performance differences between tidal flow recirculation and conventional recirculation,two kind of conventional recirculation modes(LLR and HLR)were applied as well.Water levels were fixed at the bottom and top of cathode chamber in HLR and LLR modes,respectively.In addition to water recirculation mode,the role of natural light illumination on performance of MMFC-CW was investigated as well.Illuminated cathode was established with clear cathode chamber in which light went inside cathode chamber.In contrast,dark cathode was established by wrapping cathode chamber with dark cloth so that light couldn't go into cathode chamber.MMFC-CW was operated by different modes in sequence which was 1^st FSR?LLR?HLR?2^nd FSR?SSR?DSSR?DSSRO.Dark cathode was applied in SSR mode(DSSR)and SSR mode with open circuit MFC(DSSRO).The COD concentration of influent was 300 mg/L.In the aspect of wastewater treatment performance of MMFC-CW,COD removal efficiencies under FSR and SSR modes(>88.63%)were higher than in LLR and HLR modes(<87.27%).Furthermore,COD removal efficiency of LLR(81.87%)was lower than in HLR(87.26%).As FSR was applied after the two conventional modes(2^nd FSR),COD removal efficiency(92.37%)was even higher than in 1^st FSR mode(91.53%).Then,slightly less COD was removed in SSR mode than 2^nd FSR(91.49%).Further reduced COD removal efficiency(88.64%)in DSSR mode suggested that light contributed to COD digestion.COD concentration of effluent was raised by 15 mg/L when MFC was switched from close circuit to open circuit(DSSRO).This suggested that most of COD was removed by effects of CW in MMFC-CW operated with DSSR mode.Oxygen diffusion into cathode chamber was enhanced by tidal flow recirculation mode(the highest DO concentration in cathode chamber was5.07 mg/L),which was greater than that in conventional recirculation mode(the highest DO concentration in cathode chamber was 4.73 mg/L).Water variation range in SSR mode was shorter than in FSR mode.Thus,DO concentration in cathode chamber in SSR mode(4.88 mg/L)was lower than in 2^nd FSR mode(5.07 mg/L).It suggested that water variation range determined the oxygen diffusion capacity.On the other hand,DO concentration in cathode chamber was reduced if cathode was wrapped.This suggested that light was contributed to oxygen diffusion as well.Concentrations of both NH₄⁺-N and TN of influent were 18 mg/L.MMFC-CW showed a robust NH₄⁺-N removal efficiency(>98.8%in each mode)and NH₄⁺-N was completely removed in 1^st FSR and LLR modes.TN removal efficiency was 96.3%in 1^st FSR mode.In all of modes applied after 1^st FSR,TN removal efficiencies were no higher than 81%.Particularly,TN removal efficiency was reduced further as MFC was switched from conventional recirculation mode(HLR)to tidal flow recirculation mode(2^nd FSR).NO₃^--N removal efficiency was the key factor in determining TN removal efficiency.Although tidal flow recirculation mode was applied once again after conventional mode was applied,nitrogen removal efficiency wasn't raised reversely to as high as tidal flow recirculation which was applied first time.TN removal efficiency in 2^nd FSR mode(72.10%)was lower than in SSR mode(74.72%)which was applied just after 2^nd FSR mode.On the other hand,TN removal efficiency in DSSR(75.16%)mode was higher than in SSR mode.As DSSRO was applied after DSSR,TN removal efficiency was further reduced(73.03%).All of these TN removal efficiency variations suggested that TN removal was affected by factors which included water level variation range,illumination,circuit of MFC.Dewatered alum sludge(DAS)from drinking water treatment plant was the main substrate of CWs in this study.Robust phosphorus absorption capacity of DAS resulted in phosphorus concentrations in effluent being lower than the lowest detective limitation.Algae growth was identified from the first recirculation mode applied in MMFC-CW.The ranges of water immersion determined the growth space of algae.In FSR mode,algae were grown in the whole cathode chamber.Algae growth was significantly inhibited in LLR mode as cathode was exposed to the air.In HLR mode,algae were grown in the whole cathode chamber as same as in FSR mode.The growth space of algae in SSR was same as HLR.But algae were grown better in the space of water immersion rather than where the water level was variable.Illumination and MFC circuit didn't affected algae growth significantly.The average voltages output between cathode and anodes in 1^st FSR mode were 497.83 m V(top anode)and 358.90 m V,respectively.Higher voltage output between cathode and top anode was due to the low energy loss between them.On the other hand,there was a cyclic variation in voltage outputs in FSR.Voltage outputs were varying while water level was moving.Each voltage variation cycle was last for around10 mins in 1^st FSR mode.It divided in two periods which were filling(last for 9.5 mins)and discharging(last for 0.5 mins)periods.Cathode potential was kept unchanged in each cycle.Potentials of all anodes were decreasing gradually as water level was rising(filling period).In discharging period,anode potentials were raising rapidly while water level was decreasing until discharging was ended.As water level was rising again,anode potentials were raising correspondingly.Voltage outputs and electrode potentials were remained unchanged in two conventional recirculation modes(LLR and HLR).When LLR was applied after 1^st FSR in MMFC-CW,cathode potential was unchanged but anode potentials were raised significantly.All of anode potentials were lower than-33.11 m V in 1^st FSR mode and higher than 10.78 m V in LLR mode.Thus,voltage outputs in LLR were lower than in 1^st FSR mode.HLR was applied after LLR mode.Cathode potential in HLR mode was the same as in LLR mode.On the other hand,all anode potentials in HLR mode were lower than LLR mode which resulted in higher voltage outputs than LLR.However,voltage outputs in HLR mode were still lower than in 1^st FSR mode.The 2^nd FSR mode was applied after HLR mode.Although cathode potential was raised to some extent,all anode potentials were raised drastically(>51.17 m V).Therefore,voltage outputs in 2^nd FSR mode were quite lower than in HLR mode.SSR mode was applied after 2^nd FSR mode.Cathode potential in SSR mode was higher than 2^nd FSR mode.All anode potentials were decreased especially for top anode which was-3.97 m V.Thus,voltage output in SSR mode was higher than 2^nd FSR mode.Additionally,compared to 2^nd FSR mode,voltage outputs in SSR mode were more stabilized.This was because no significant voltage variations were occurred as water level was changing.Cathode chamber illumination was stopped by wrapping cathode chamber with cloth.Thereafter,there were no significant changes in voltage outputs and electrode potentials.The work in Chapter 5 was focused on scaling up of MFC-CW.It was necessary to investigate scaling up of MFC-CW in order to apply MFC-CW in full scale CWs.Actually,in MFC-CW,cathode type was air-cathode and the substrate section of CW was the anode chamber of MFC.Thus,establishing multiple electrodes(especially multiple anodes)MFC in large scale MFC was the reasonable strategy for MFC-CW scaling up.In order to identify the key issues in MFC-CW scaling up,bigger lab-scale MFC-CW(LMFC-CW)than MMFC-CW in Chapter 4 was assembled.MFCs with single cathode and multiple anodes were installed in both LMFC-CW and MMFC-CW.Liquid volume in anode chamber of LMFC-CW was ten times larger than anode chamber in MMFC-CW.One more anode was used in LMFC-CW than in MMFC-CW.Liquid volume of cathode chamber in LMFC-CW was close to cathode chamber in MMFC-CW.The height of cathode chamber in LMFC-CW was half of cathode chamber height in MMFC-CW.The volume ratios of cathode/anode in both LMFC-CW and MMFC-CW were the same.Compared to MMFC-CW,LMFC-CW showed the typical features of scaling up MFC-CW which were bigger anode chamber,more anodes,lower space percentage occupied by cathode in system.The concentrations of COD,NH₄⁺-N,and TN in influent were 300 mg/L,18 mg/L,and 18 mg/L,respectively,for both LMFC-CW and MMFC-CW.FSR and SSR modes were applied in cathode chambers in both LMFC-CW and MMFC-CW.The height of cathode chamber in LMFC-CW was lower than MMFC-CW.Therefore,DO concentrations of cathode chamber in all working conditions(<2.53 mg/L)in LMFC-CW were significantly lower than MMFC-CW(>4.87 mg/L).On the other hand,in all working conditions,COD removal efficiencies in LMFC-CW(<89.60%)were lower than MMFC-CW(94.18%).In LMFC-CW,the highest COD removal efficiency(89.59%)and the highest DO concentration in cathode chamber(2.52 mg/L)was reached in FSR mode.COD removal efficiency and DO concentration in cathode chamber were lower than FSR in SSR mode.As MFC was switched to open circuit in SSR mode,COD removal efficiency and DO concentration in cathode chamber were further decreased.In the aspect of nitrogen removal,in all working conditions,NH₄⁺-N removal efficiencies in LMFC-CW(<91.54%)were lower than in MMFC-CW(>99.55%).However,the highest TN removal efficiency in LMFC-CW(75.75%)was higher than MMFC-CW(74.15%).These suggested that,as MFC-CW was scaled up,nitrification process was limited but denitrification process was promoted.It was caused by insufficient oxygen diffusion into cathode chamber in LMFC-CW.Bigger anode chamber of LMFC-CW than in MMFC-CW resulted in more substrate of CW(DAS)being filled.Thus,phosphorus absorption capacity in LMFC-CW was higher than MMFC-CW.Phosphorus concentrations of effluent under all working conditions in LMFC-CW were lower than the lowest detective limitation.In the aspect of electricity generation,in FSR mode,voltage output between cathode and top anode in LMFC-CW(210.52 m V)was lower than in MMFC-CW(298.25 m V).This was because higher cathode potential(MMFC-CW:347.61m V;LMFC-CW:292.84 m V)and lower top anode potential(MMFC-CW:51.18 m V;LMFC-CW:82.26 m V)were obtained in MMFC-CW than in LMFC-CW.Voltage output between cathode and bottom anode in LMFC-CW(519.03 m V)was significantly higher than in MMFC-CW(270.25m V).This was because potential of bottom anode in LMFC-CW(-221.13 m V)was much lower than MMFC-CW(79.11 m V).As FSR mode was switched to SSR mode in both of MFC-CWs,cathode potentials in MMFC-CW and LMFC-CW were increased(to 393.42 m V)and decreased(to247.61 m V),respectively.Although top anode potentials in both of MFC-CWs were dropped,top anode potential in LMFC-CW was dropped much more than MMFC-CW.Thus,in SSR mode,voltage output between cathode and top anode in LMFC-CW was much higher than MMFC-CW.Additionally,due to one more anode in LMFC-CW than in MMFC-CW,total power output of LMFC-CW(>0.62 m W)was higher than MMFC-CW(<0.25 m W).Voltage outputs and electrode potentials of LMFC-CW in both FSR and SSR modes were stabilized.These suggested that more stable power output was obtained in bigger MFC-CW.Overall,the work in this study investigated the role of cathode configuration and operations on the performance of MFC-CW.Effective contact between oxygen and air on cathode was one of the key factors in establishing MFC in CW.In order to achieve the effective contact,aeration and water recirculation could be applied below and above water level,respectively.If applying water recirculation for enhancing oxygen diffusion,tidal flow recirculation mode maintained cathodic reaction while affecting anode chamber slightly.The tidal flow recirculation mode was much better than conventional modes.As MFC-CW was scaled up,larger volume of anode chamber reduced impact of cathode chamber to anode if it was far away from cathode chamber.Furthermore,voltage output in scaled up MFC-CW were more stabilized.These conclusions were valuable for MFC-CW development.