The Biogas Generation From Food Waste By Anaerobic Digestion And Its Ex-situ Biological Upgrading Technology

The generation of food waste was increased year by year with the development of economy and society,however,food waste had the characteristics of high organic matter and easy degradation,which made it a good substrate for anaerobic digestion.The anaerobic digestion technology was one of the important methods for the organic solid waste treatment and it can produce an energy gas-biogas.However,the existence of a large amount of CO₂restricted the utilization of biogas.The biogas can reach the quality of biomethane if the CO₂was removed,which can become a new type of energy.In recent years,more and more attention had been paid to the biogas biological upgrading technology utilizing the metabolism of hydrogenotrophic methanogens.In this paper,the combination reaction system of food waste anaerobic digestion and ex-situ biogas biological upgrading was indicated.First,the optimal solid content and pH value on the anaerobic digestion process from food waste were studied,and then the combination of this experiment with biogas upgrading was indicated.The experiment showed that the CH₄ content can be improved through biological upgrading,but the effect was not obvious,therefore,the pH and injection time of the biogas upgrading phase were optimized,a joint experiment was investigated between the anaerobic digestion of food waste and the biogas biological upgrading under the optimal conditions.The experimental parameters in the reaction process were analyzed.Finally,the optimal injection gas ratio of biogas upgrading phase was studied and it was applied to the combined experiment of food waste anaerobic digestion in order to obtain the biomethane from biogas upgrading.The main results of this research were as follows:1.The experiment of food waste anaerobic digestion was carried out under different solid content and pH.The optimal solid content and pH were 15%and 7.6,respectively.The cumulative biogas production and CH₄ content were the maximum of 279 mL·gTS^-1 and55.2%under this condition,respectively.The degradation degree of organic acid and SCOD was also the best.The anaerobic digestion from food waste under the optimal condition and biogas biological upgrading were carried out in a combined experiment.The organic acid and SCOD in the anaerobic digestion phase were effectively degraded.The content of CH₄ and CO₂ was 52.7%and 23.3%in the biogas produced by anaerobic reaction.After upgrading process,CH₄ content increased by 36.4%and CO₂ content decreased by 42.5%,all the H₂that injected in reaction was consumed.At the same time,the reaction system was not inhibited by the exogenous H₂ to biogas upgrading phase.2.The optimal pH of 8 and the optimal injection time of 5 minutes were indicated by the optimization of pH and injection time in biogas upgrading phase,meanwhile,the CH₄ and CO₂ content were 59.1%and 10.98%,respectively.In the upgrading process,organic acid had the best degradation performance when pH was 8.The increase of injection time did not enhance the CH₄ generation,but resulted in the organic acid accumulation in the system.When the pH was 8 and the injection time was 5 minutes,the relative abundance of hydrogenotrophic methanogens was the maximum.A joint study was conducted on the anaerobic digestion of food waste and the ex-situ biogas biological upgrading under the optimal conditions.The content of CH₄ and CO₂ in the biogas produced by the anaerobic reaction was 53.83%and 12.27%.The content of CH₄ increased by 68%while CO₂decreased by 56.6%,and H₂ was completely consumed after upgrading.Compared with the control,the CH₄ content in the optimal experiment was effectively increased.No organic acid inhibition was occurred in the biogas upgrading phase.Comparing to the microbial composition at the beginning and end of the reaction,it was found that hydrogenotrophic methanogens was selectively enriched and became the dominant microorganism in the biogas upgrading reaction.3.The optimal injection gas ratio of H₂ and CO₂ was 5:1 in biogas upgrading phase.The CH₄ yield,CH₄ volume fraction and H₂ conversion rate were the maximum of 693.7mL·L^-1·d^-1,69.37%and 98.7%under this condition,respectively,and the acetic acid degradation degree was also the highest.In the upgrading process,the CH₄ yield and H₂conversion rate were not positively to the injection gas ratio,the increase of the injection gas ratio of H₂ and CO₂ led to the decrease of CH₄ yield and H₂ conversion rate.The continuous increase of injection gas ratio led to the failure of timely utilization of H₂,and the acidification of the reaction system.The combination of food waste anaerobic digestion and biogas upgrading under the optimal injection gas ratio showed that the content of CH₄ and CO₂ in the biogas produced by the anaerobic reaction was 54.7%and 24.1%.The content of CH₄ increased to 96.1%while CO₂ decreased to 1.8%,and the H₂ content was only 0.3%after upgrading process.The CH₄ content after upgrading was significantly increased and reached the quality of biomethane by the suitable injection gas ratio.The H₂ conversion rate in the upgrading phase was higher than 99%,and the CH₄ yield reached 960.73 mL·L^-1·d^-1without acid inhibition.At the end of the reaction,the number and diversity of microorganisms decreased in the upgrading system due to the enrichment of hydrogenotrophic methanogens.