Process And Performance Of Treatment Of Anaerobically Digested Effluent From Kitchen Waste Using An Algae-assisted Microbial Fuel Cell

Kitchen waste constitutes a large component(over 30%)of municipal solid wastes.Anaerobic digestion has been proved to be an effective technology to manage kitchen waste and also produce abundant energy resources like methane.However,anaerobically digested effluent from kitchen waste(ADE-KW)is still of severe environmental concern for its high content of organic matter and ammonium.Microbial fuel cell(MFC)as an emerging technology has received tremendous attention since it is capable of producing electricity from organic matter.ADE-KW possesses a high concentration of volatile fatty acids which are suitable substrate for electricity production in MFCs,but MFCs are incapable of removing ammonium from the ADE-KW.Algal cultivation is identified as an efficient way to assimilate large amounts of nutrients.Therefore,an algae-assisted MFC(AMFC),which consists of a combination of algal cultivation and a MFC,can not only save on energy for aeration and increase the energy recovery efficiency,but also simultaneously remove organics,nitrogen and phosphorus.Considering the high content of organic matter and ammonium in the ADE-KW,a stable AMFC process for ADE-KW treatment was developed first.The established system ensured the efficient removal of pollutants under the stable operation after optimizing the parameters.The results revealed the regulation mechanism of microalgae growth on power generation and pollutant removal,and analyzed the mechanism of organic removal and transformation behaviour of nitrogen.In order to achieve the simultaneous and efficient removal of carbon,nitrogen and phosphorus,and efficiently recover energy from ADE-KW,a new coupled process that integrating AMFC and PBR was established.To make the technology more practical,an AMFC stacks was developed by installing multiple anodic chambers into an algae raceway pool with the assistance of a capacitor circuit.This study is of great important to promote the AMFC into the large-scale application in the field of wastewater treatment and resources.The main results of the reseach were shown as follows:In the present study,a stable AMFC system was established first to recover bioenergy from ADE-KW.The ADE-KW,which also functioned as algal medium,has to be diluted to a proper concentration before being used as catholyte.The pollutant may inhibit algal growth when the concentration of ADE-KW was too high.On the other hand,the nutrients may be limited for algal growth if the concentration of ADE-KW was too low.The results suggested that the algae obtained the highest growth rate when the ADE-KW was diluted by a factor of 25.After that,the diameter ratio of cathodic chamber to anodic chamber(C/A ratio)was optimized under the optimal concentration of ADE-KW.Cation migration effect may disturb the algal growth environment when C/A ratio was low.However,increasing C/A ratio may decrease the illumination intensity in the culture.The AMFC system obtained the highest algal biomass and stable voltage output when C/A ratio was 2.Then,additional phosphorus was added into the catholyte to optimize the N/P ratio and promote algal growth.The results indicated that the highest algal growth rate and a stable AMFC system were obtained when extra P was 3 mg/L.The mechanism of organic removal and transformation behaviour of ammonium in the AMFC were investigated using the established system mentioned above.The organic removal rate in the anolyte of the AMFC was about 68.2%.The results of excitation-emission matrices(EEMs)demonstrated that soluble microbial byproduct-like material and aromatic proteins were the dominant organic species in the raw ADE-KW,and the two organic species were easily degraded in AMFC.Fulvic acid-like substances could also be degraded in the AFMC,whereas the removal rate was relatively lower compared with soluble microbial byproduct-like material and aromatic proteins.The ammonium removal efficiency in the anolyte reached about 71%due to the ammonium migration effect After diffusing into the catholyte,a part of the ammonium was assimilated by algae,and some ammonium was removed as electron acceptors at the cathode after being oxidized by photosynthetic oxygen.Consequently,the nitrogen removal efficiency in the whole AMFC was about 54%.Energy production from the AMFC system consisted chiefly of two parts,electricity and lipid energy.The total energy produced in the system was about 4.1 kWh/m3,with electricity of about 0.59 kWh/m3 and algal lipid of about 3.51 kWh/m3.After a preliminary estimation,the energy recovery efficiency of the AMFC system was 14.8%.The remaining pollutant(especially ammonium)in the catholyte needed to be further polished,though the AMFC system could run stably.Herein,a system that couples an AMFC with two photobioreactors(PBRs)was developed to remove pollutant and recover energy from the ADE-FW.The COD removal efficiencies in the anolyte of the coupled systems under continuous and batch operation were both around 75%.About 93.5%of the ammonium in the anolyte was removed in the continuous mode owing to ammonium migration effect.As mentioned above,ammonium in the catholyte was assimilated by algae or removed as electron acceptors after being oxidized.But large amounts of nitrogen oxides accumulated in the catholyte due to the high concentration of dissolved oxygen(DO)under continuous operation.Then,the catholyte was further polished in PBRs,and the nitrogen concentration in the effluent was less than 20 mg/L after being treated in the PBRs in the continuous mode.Under batch operation,large amounts of ammonium accumulated in the catholyte,and the effluent still contained over 70 mg/L nitrogen after being treated in the PBRs.The electricity produced by the system in continuous and batch modes were 0.84 kWh/m3 and 0.92 kWh/m3,respectively.DO concentration dominated the cathode potentials of the AMFC,whereas the substrate concentration was the main factor that influenced the anode potential and power generation when COD was below 3000 mg/L.More algal lipid energy was harvested in continuous mode,while more electricity was obtained in batch mode.Consequently,the total energy produced in the coupled systems in continuous and batch modes were both around 5.4 kWh/m3,corresponding to energy recovery efficiencies of 14.1%and 19.3%,respectively.An AMFC stack was developed to promote this technology into realworld applications.Herein,an AMFC stack,which consisted of multiple anodic chambers installed in an algal raceway pond,was established with the assistance of a prototypical capacitor circuit to avoid voltage reversal.The voltage output of an AMFC stack with capacitor circuits was about 0.6 V,which was about double the voltage produced by an individual AMFC,and 33%higher than the stack without capacitor.During long-term operation,the highest power density of the stack with capacitors reached 2.34 W/m3,which was 77%higher than that without capacitors(1.32 W/m3).The COD and ammonium removal rates of the stack with capacitors were both nearly 90%,which were superior to the stack without capacitors.The evolution of bacterial compositions at the anodes were analysed using Illumina high-throughput sequencing,and the results indicated that the variations in the bacterial compositions in the two systems showed similar tendencies during the experiment,whereas the bacterial community in the stack without capacitors changed more substantially compared to that of the system with capacitors.The variations in bacterial community composition following power density changes indicated that members of the Clostridia and Advenella were related to power generation.Bacteria affiliated to Bacteroidetes and Actinobacteria may contribute little to power generation since they were inhibited when power density was high.Overall,a stable and efficient AMFC system was established successfully,and the regulatory mechanisms of algal growth and substrate on electricity generation were investigated.The study presented a new method for the treatment of ammonium-rich wastewater via revealing the migration and transformation behaviour of nitrogen in the AMFC.In addition,a practical AMFC stack was proposed that combines multiple anodic chambers with an algal raceway pond,which is a concrete step to promote the use of the technology in real world applications.