Alleviation Of The Inhibition Of High Ammonia Nitrogen On Anaerobic Digestion Of Taihu Blue Algae By Bioelectrochemical System(BES)

Since 2007,the outbreak of Taihu blue algae has become a new normalcy,and how to recycle Taihu blue algae has also become a hot topic.Based on the high methane-producing potential of blue algae and the location advantage adjacent to Lake Taihu,we used Taihu blue algae as a substrate for anaerobic digestion to explore the effect of bioelectrochemical system（BES）on alleviating ammonia inhibition during anaerobic digestion.In this study,a new type of integrated BES-AD reactor was first designed,built and operated to evaluate the feasibility of removing ammonia nitrogen.Moreover,the optimal algae/sludge TS ratio and the inhibition threshold of ammonia for anaerobic digestion were explored in the fermentation bottle.Subsequently,the effect of different applied voltage,microbial inoculation amount and initial ammonia level on the performance of anaerobic digestion and biomethane production were studied in the BES-AD system.Finally,electrochemical test and Illumina high-throughput sequencing were used to analyze the electron transfer performance of the BES-AD system under different stress of ammonia and direct current,and the changes in the composition,abundance and community structure of anaerobic microorganisms.The main conclusions drawn were as follows:（1）Under the condition of 0.8 V applied and 1000 mg NH₄⁺-N/L ammonia filled in the working chamber,the anode potential was stable at about-0.57 V,and the current density could reach 31 A/m³.When 0.4,0.8 and 1.2 V were applied,the removal rates of ammonia nitrogen were 78.98%,83.56%and 88.42%,respectively.In addition,maintaining the voltage of 0.8 V,the removal rates of the 3000,5000 and 7000 mg NH₄⁺-N/L groups with initial ammonia concentration were 84.66%,86.62%and 88.32%,respectively.（2）Fermentation flask experiments showed that the ratio of algae/sludge TS of 3:1 was the optimal condition for anaerobic digestion.The methane yield under this condition was 137.70m L/g VS,and the average methane content was 54.75%.When the ammonia concentration reached 500 mg NH₄⁺-N/L,the anaerobic digestion process was moderately inhibited,and only57.13 m L/g VS methane production was obtained.The initial ammonia concentration of 1000mg NH₄⁺-N/L severely inhibited the biological methane production process,and the methane production was only 22.85 m L/g VS.（3）In the BES-AD system,when the algae/sludge TS ratio was 3:1 and the initial ammonia concentration was 1000 mg NH₄⁺-N/L,the methane production of the group without applied voltage was 1.97 times that of the group applied 0.4 V,which might be because the stronger electric field posed a threat to the survival of microorganisms.Then,under the condition of an external voltage of 0.4 V and an initial ammonia concentration of 1000 mg NH₄⁺-N/L,the methane production potential of the algae/sludge TS ratio 1:1group could reach 69.12 m L/g VS,which was 18.68 times that of the algal/sludge TS ratio 3:1 group（3.70 m L/g VS）,indicating that the increase of microbial inoculation was helpful to offset the inhibition of ammonia and direct current.Moreover,the methane yield of 3000 and 5000 mg NH₄⁺-N/L groups were 22.25and 10.93 m L/g VS,respectively,which were 5.66 and 3.36 times of the corresponding fermentation flask experiment,indicating that the BES-AD system could successfully alleviate the inhibition of ammonia.（4）The electrochemical test showed that the anaerobic sludge in the R2（1:1,1000 mg/L）reactor had the best electrochemical performance,and its peak current,closed area and double-layer capacitance(Q_{d I})were all the maximum,which were 4.17×10^-5 m A,3.85×10^-2 and7.56×10^-4 F.In addition,molecular biology technology showed that the high ammonia level made the methane production pathway in the four groups of reactors was mainly hydrotrophic methanation,and the microbial community in R2 were enriched in power generation,resisting to hostile circumstance,co-existence with Methanobacterium.Therefore,under the applied voltage,BES-AD with proper TS ratio might help to offset the suppression of ammonia and direct current,thereby maintaining the improvement of biomethanation performance.