Effect Of Air Mixing On Biogas Production And Microbial Metabolism

Mixing is an important measure to improve the efficiency of anaerobic digestion of organic material with high suspended solid.Compared with hydraulic mixing and mechanical mixing,biogas mixing has great development potential in terms of energy consumption,cost and practicability.However,biogas mixing is easy to be affected by biogas production,and requires complex safety facilities such as explosion-proof blower and safety valve.Aiming at the existing problems of biogas mixing,this paper puts forward the idea of using air instead of biogas to stir,that is,to inject limited air into the reactor for mixing,which is named“air mixing”.Firstly,the feasibility of air mixing was verified through the comparative experiment of anaerobic digestion for methane production from animal wastewater under different mixing modes?air mixing,biogas mixing,mechanical mixing and no mixing?.Then,the effects of different mixing modes on the substrate conversion rates in the hydrolysis,acidogenesis and methanogenic steps of anaerobic digestion process,microbial community structure and metabolism were studied to reveal the influence mechanism of micro-oxygen created by air mixing on methane production efficiency and the characteristics of microbial metabolism.Finally,the key parameters of air mixing,such as air mixing time,intensity and frequency,were determined by investigating the real-time changes of the degree of mixing,redox potential,CH₄ content and biogas production rate under different air mixing conditions,as well as the system recovery time after air mixing.The following main conclutions were drawn:?1?Compared with biogas mixing,mechanical mixing and no mixing,air mixing increased the volumetric CH₄ production rate by 6.4%,14.1%and 19.6%,respectively;air mixing increased the removal rate of hydrogen sulfide?H₂S?by 63.1%-70.4%and the soluble COD removal rate by 2.2%-9.5%.In addition,the air mixing improved the stability of anaerobic digetion system.?2?Gas mixing improved the mass transfer between the substrate and microorganisms.The homogeneous mixing time was shortened from 10 min of mechanical mixing to 1.5 min at the same input power.Meanwhile,a transient microaerobic environment was generated by the injection of air,which enabled both anaerobic and aerobic biological activities to occur within a single bioreactor.Air mixing improved the hydrolysis efficiency.The hydrolysis rates of particulate complex substrates were1.7-11.4%,7.7-23.8%and 21.0-29.1%higher than those of biogas mixing,mechanical mixing and no mixing,respectively;Air mixing promoted the generation and degradation of volatile fatty acids?VFAs?,especially the degradation rate of propionic acid increased by 93.0%.The metabolic rate of VFAs was simulated by ADM1 model,and the maximum specific uptake rate of propionic acid?km?pro?in air-mixed reactor was obtained 0.64 h^-1,which was higher than 0.54 h^-1 in no-mixed reactor.The relative contribution of the improved mass transfer and the microaerobic environment to methane production was 62.9%and 37.1%,respectively.?3?Air mixing optimized the syntrophic relationship among the microbial communities and an increase in the abundance of facultative bacteria and hydrogenotrophic methanogens was observed.Illumina MiSeq sequencing of the 16S rRNA gene showed that air mixing significantly altered the microbial abundances and influenced metabolic pathways.In particular,glucose-degrading bacteria?i.e.,Propionibacterium?,syntrophic propionate-oxidizing microorganism?i.e.,Smithella?,and oxygen tolerant methanogens?i.e.,Methanocorpusculum?were more abundant in the reactor with air mixing.Under air mixing,the dominant pathway for propionate oxidation switched from the methylmalonyl-CoA pathway to the Smithella pathway,which is much more thermodynamically favorable,and propionate oxidization was accelerated as a consequence.?4?The optimization of air mixing parameters was carried out with swine wastewater as raw material.The results showed that the air flow rate?mixing intensity?was the most important influencing factor of air mixing,and there is significant interaction between mixing intensity and mixing time.The optimal air mixing condition is that the air volume is 100 mL^-1,mixing intensity is 66.7mL L^-1 min^-1,mixing time is 1.5 min every time,and mixing frequency is 3 times per day.When air mixing time exceeded 3 min every time,total CH₄ production was less than that achieved under the no mixing condition due to a decrease in the effective CH₄ production time.