Construction And Regulation Of A Microbial Electrochemical System For "Ammonia Recovery-in Situ Protein Synthesis"

Due to the development of industry and agriculture,various human production activities have led to various types of nitrogen-related pollution that pose a major threat to the environment.Compared to conventional denitrification processes,ammonia recovery based on microbial electrochemical systems（BES）can generate electricity or energy while degrading organic pollutants,with high treatment efficiency and adaptability to wastewater.The current process of electrochemical recovery of ammonia nitrogen involves the’ex situ’transfer of the recovered ammonia nitrogen to other reactors for resource utilisation.However,the transfer process is cumbersome,involves losses and requires adjustment of the ammonia nitrogen concentration.Therefore,in this paper,a third chamber（i.e.a protein-producing chamber）is added to the two-chamber BES,and the ammonia nitrogen enters the third chamber to produce microbial protein through concentration gradients and electromigration,creating an"ammonia recovery-in situ protein synthesis"microbial electrochemical system.The system is capable of simultaneous wastewater treatment,ammonia nitrogen recovery,CO₂fixation and in-situ microbial protein production,and has significant environmental benefits.In this paper,the anode microorganisms were firstly domesticated to tolerate ammonia nitrogen,and the migration pattern of ammonia nitrogen was investigated from three factors:cathode liquid concentration,cathode liquid p H and voltage to verify the feasibility of the"ammonia nitrogen recovery-in-situ protein synthesis"microbial electrochemical system.Then,the conditions of applied voltage,ammonia nitrogen concentration,temperature and carbon nutrition were optimised to improve of the system performance.In order to improve the efficiency of ammonia nitrogen utilisation,the two-stage"in situ-ex situ"protein synthesis was further investigated.Finally,the ammonia transport pathway of the"ammonia recovery-in situ protein synthesis"microbial electrochemical system is enhanced by using different cathode materials and regulating the concentration of organic matter in the anode.（1）Firstly,the domestication of ammonia-nitrogen tolerant anode microorganisms was successfully achieved by gradually increasing the ammonia-nitrogen concentration.Then,the effects of cathode liquid concentration,cathode liquid p H and voltage on the ammonia nitrogen migration of the microbial electrochemical system were investigated at a gradient of ammonia nitrogen concentration.The results show that increasing ion concentration can be considered as a method to improve ammonia nitrogen recovery,but considering the inhibitory effect of high salt on the anode microorganisms,a mixture of 20 mmol K₂HPO₄and 20 mmol Na₂HPO₄can be selected as the cathode solution;further considering the long-term stability of the reactor operation and avoiding contamination or corrosion of the cation exchange membrane,the cathode chamber can be used with a neutral p H（i.e.p H=7）.The higher the voltage,the stronger the current effect and the greater the contribution to the ammonia-nitrogen migration process in the cathode chamber（in the range of 11-16%）;finally,the feasibility of the microbial electrochemical system"ammonia-nitrogen recovery-in situ protein synthesis"and the protein synthesis performance were evaluated.The product had a dry weight of 1.82 g/L and a crude protein content of 66.83%.（2）The effect of different operating conditions such as applied voltage,ammonia nitrogen concentration,temperature and carbon nutrient conditions on the microbial electrochemical system of"ammonia nitrogen recovery-in-situ protein production"was investigated,and it was determined that the best operating conditions of the system were 1.0 V applied voltage,1 g N/L ammonia nitrogen concentration and 20°C.Under this condition,the degradation rate of the anode organic matter reaches 100%,the average carbon fixation rate reached 1.82 L CO₂/L/d,the nitrogen fixation capacity was the strongest,the ammonia nitrogen fixed as microbial protein reached 35%,the amino acid yield reached 1536 mg/L,19 types of amino acids,which can be regarded as high-quality protein.Based on the optimal operating conditions,the two-stage protein synthesis of"in situ-ex situ"was further explored,which increased the efficiency of ammonia nitrogen utilization by 36.3%,the yield of microbial protein by 25.13%and the economic efficiency by 25.16%,The result is a value-added microbial electrochemical system for"ammonia recovery-in situ protein synthesis".Finally,the microbial community composition was analysed.The dominant groups at the genus level in the in situ protein synthesis stage were all Hydrogen oxidizing bacteria（HOB）:Corynebacterium（49.9%）and Alcaligenes（26.36%）,while the dominant groups at the genus level in the ectopic protein synthesis stage were:Paracoccus（20.82%）and Hydrogenophaga（15.70%）,all typical of HOB bacteria.（3）To investigate the effect of different cathode materials and regulated organic concentrations on the enhancement of the ammonia-nitrogen transport pathway in the microbial electrochemical system"ammonia-nitrogen recovery-in situ protein synthesis".Among the graphite,Pt/C and MOF cathode materials,the MOF cathode material system had the highest concentration of ammonia nitrogen in the cathode chamber at 258 mg N/L,with 27.86%of ammonia nitrogen electromigration and a maximum current density peak of14.16 A/m³.At this point,the percentage of ammonia nitrogen fixed as microbial protein was 34.53%,which was 14.32%higher compared to the graphite plate.The highest amino acid yield of the MOF system was 1614mg/L,which was 5.1%higher compared to the graphite plate.This may be due to the enhanced electromigration of ammonia nitrogen from the anode chamber to the cathode chamber,which increased the concentration of ammonia nitrogen flowing to the product chamber,thus further increasing the yield of HOB converted to protein and enhancing the level of nitrogen fixation by HOB.In the experiments to regulate the organic matter concentration of the anode microorganisms,the concentration of ammonia nitrogen in the cathode chamber of the graphite plate system was the highest,at 283 mg N/L,with the highest amount of ammonia nitrogen electromigrated,at 99.48 mg N/L,accounting for 35.15%of the ammonia nitrogen electromigration,while the current density peaked at 29.10 A/m³.Regulating the anode organic matter concentration could not significantly enhance the biological activity of HOB,but can have a catalytic effect on amino acid yields.For example,the graphite plate system showed an amino acid yield of 1552 mg/L,an increase of 1.04%compared to the unregulated system,while the MOF system showed the highest amino acid yield of 1720 mg/L,an increase of 6.57%compared to the unregulated system.The increase in the conversion of ammonia nitrogen after the regulation of organic matter concentration was due to the enrichment of denitrifying bacteria in the anode microbial community,such as Rhodococcus,Comamonas and Thauera,which can remove ammonia nitrogen through denitrification or ammonia assimilation.The dominant groups of the anode microbial community were Methanosarcina（14.60%）,uncultured＿f＿＿Anaerolineaceae（8.41%）,Rhodococcus（7.97%）,and Advenella（7.05%）.