| Microbial electrochemical systems(MESs)could accelerate the degradation of pollutants in wastewater by utilizing electrochemical active microorganisms as biocatalyst with simultaneously electricity generation.MES technology takes advantages in low wastewater treatment cost and synchronously energy recovery.It has been intensively studied in the field of electrode material development,configuration optimization,electron transfer mechanism and function extension.However,the application of MES is still limited in a scope of lab-scale reactors and the goal of scaling up MES for practical application in wastewater treatment is far from success.The main challenges lay in lacking proper configuration,stable cathode system and analysis of key factor in scaling up MES.Therefore,developing proper configuration,exploring the stable cathode system and cost-efficient separator were the essential objectives in this study.The final goals are to fabricate a pilot-scale MES at the scale of cubic meter and determine its operation control strategy in the actual operating condition of a municipal wastewater treatment plant.Employing MES units as a stack is a feasible way to construct a pilot-scale MES in this study.An easily-stackable air cathode module was designed and operated with synthetic wastewater in batch flow mode,producing a maximum power density of 518 m W/m2.In continuous flow mode,different HRTs were adjusted to investigate the system electricity generation performance.The maximum electrical energy of 1.93×10-3 k Wh was obtained at HRT of 3 d,with the substrate removal rate(SDR)of 0.226 kg COD/m3/d.A combined system of an MES and an intermittently aerated biological filter(MES-IABF)was designed and operated to investigate the possibilities of using MES-centered system for energy self-sufficient operation and deep treatment of wastewater.The combined MES-IABF used energy management system(EMS)to realize the recovery and reuse of generated electricity.Energy analysis indicated that the MES unit produced sufficient energy(0.27 k Wh/m3)to support the pumping system(0.014 k Wh m-3)and aeration system(0.22 k Wh/m3),and the energy loss was 0.28 k Wh/m3.This combined system obtained 91.7% COD removal and SDR of 0.28 kg COD/m3/d.The deterioration of cathode performance was the main reason of the decrease in power generation in long-term operation.A 100-liter MES stacked by 5 air cathode modules,fed with brewery wastewater,was designed.Tests were conducted under two different influent strengths(raw wastewater diluted 4 times,stage 1;raw wastewater,stage 2),and the COD removals were 84.7% and 87.6% with the corresponding SDRs of 0.23 kg COD/m3/d and 0.49 kg COD/m3/d.The TOC removals were both around 85% in the two stages,with the effluent concentrations of 33.4 ± 13.7 and 126.1 ± 14.5 mg/L.The stack system utilized EMS for successful energy self-sufficient operation.The average power generation decreased from 171 ± 8.4 to 150 ± 9.1 m W/m2 during the six month operation with the current decreasing from 0.059 ± 0.01 A/m2 to 0.55 ± 0.02 A/m2.The phenomenon of solution leakage through air cathode because of mechanical damage in cathode was observed.A decrease in power output of the modules along the length of flow was also observed.Due to the disadvantages in preparation and operation of air cathode,an aerated biocathode MES was constructed to test the performance of biocathode in domestic wastewater treatment.The aeration cost was reduced by optimizing the gas-water ratio(GWR),meanwhile,the effluent could meet the discharge standard in China.The maximum power output of the system is restricted by gas-water ratio following a Monod-like relationship.Within the tested gas-water ratio range from 0.6 to 42.9,the half-saturation constant(KQ)is 5.9 ± 0.9 with a theoretic maximum power density of 20.4 ± 1.0 W/m3.Energy balance analysis indicated an appropriate GWR regulation(from 2.3 to 28.6)for cathodic compartment is necessary to obtain positive energy output for the system.A maximum net electricity output is 9.09 × 10–3 k Wh/m3 with GWR of 17.1.Notably,the system achieved a COD removal of 98.3 ± 0.3%(effluent concentration of 6.0 ± 1.0 mg/L),a TN removal of 80.0 ± 0.9%,and NH4+-N removal of 99.6 ± 0.1%(0.2 ± 0.1 mg/L),with the effluent quality meeting the first grade A standard of discharge standard of pollutants for municipal wastewater treatment plant in China(GB 18918-2002).These results demonstrated that biocathode was not the limited factor of electricity generation of the system,while could help anode to further removal the pollutant,especially of NH4+-N.The pilot-scale MES was constructed based on current configuration and biocathode.The total volume of the system was 1.5 m3,and operated in the actual operating condition of a municipal wastewater treatment plant,fed with effluent of the primary clarifier.The system obtained a stable voltage and current of 0.30 ± 0.05 V and 1.0 ± 0.17 A/m3 after startup.The average removal of chemical oxygen demand(COD),NH4+-N,and TN were achieved at 91 ± 3%,91 ± 3% and 64.0 ± 2.0%,with the effluent concentrations of 25 ± 7 mg/L,3 ± 1 mg/L,and 13 ± 2 mg/L.The effluent quality met the first grade A standard of discharge standard of pollutants for municipal wastewater treatment plant in China(GB 18918-2002).The oxygen reduction performance of biocathode at low GWR was strengthened b y hydrophobic treating with PTFE.The system was operated with low energy cost by optimizing the GWR,and substrate was distributed equally and periodically throughout the reactor with alternating flow modes.The analysis of energy balance and investment of the pilot-scale system was conducted.The electricity generation of the MES was 1.93×10-3 k Wh/m3.The energy consumption was mainly due to the pumping(feeding)and aeration in biocathode.The electrical energy requirement for pumping was 6.8×10-5 k Wh/m3,which could be negligible when compared with that needed for aeration(3.63×10-2 k Wh/m3).Therefore,a negative energy balance of 0.034 k Wh/m3 was obtained.The total cost of this MES was estimated to be $1702.1.The raw material cost of carbon fiber brushes accounted for the main parts(52.5%),including carbon fiber($515.08)and Ti wire($378.8).The separators consisted of perforated plate and filter cotton accounted for 24.5% of the total cost,which reduced the investment significantly.Other investments including resistor,wire,PPR tubes,and valve totally only cost a small fraction($92.5,5.4%).The final investment could be reduced dramatically through industrial design and manufacturing. |
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