Experimental Study On Enhancing Anaerobic Digestion Efficiency Of Excess Sludge With Conductive Medium

As a by-product of digestion reaction,the output of excess sludge is increasing in recent years.Anaerobic digestion is one of the main technologies of sludge treatment.However,the current anaerobic digestion is time-consuming and inefficient.Some studies have shown that adding conductive medium,such as iron-based conductive media and carbon based conductive media,can improve the anaerobic digestion efficiency of sludge and promote methanogenesis by promoting direct interspecies electron transfer(DIET)of microorganisms.In this study,conductive media were added to the anaerobic digestion reactor to improve anaerobic digestion efficiency,and investigated the effect of its dosage and particle size variation on anaerobic digestion and the mechanism of promotion.Firstly,the experiment explored the influence of the dosage change of iron-based and carbon based conductive media on the anaerobic digestion process.2 g/L,5 g/L,10 g/L and15 g/L iron powder,magnetite,activated carbon and graphite were added to the anaerobic digestion reactor respectively,and the blank control group without medium was set.The dissolved chemical oxygen demand(SCOD)concentration,biogas output in the process of anaerobic digestion and the organic matter degradation rate and microbial population distribution after the process were analyzed.The optimal dosage of iron powder and magnetite were 10 g/L and 5 g/L,respectively,and the cumulative biogas production was12.4% and 11.9% higher than that of the blank group;the optimal dosage of activated carbon and graphite were both 10 g/L,and the cumulative biogas production was 11.2% and 10.6%higher than that of the blank group,respectively.In addition,with the dosage of the four kinds of conductive media increased to the best dosage,the effect of anaerobic digestion enhanced.However,as the dosage continued to increase,the promoting effect weakened.Secondly,the experiment explored the influence of the change of particle size of ironbased and carbon based conductive media on the anaerobic digestion process.On the basis of obtaining the optimal dosage of four conductive media in the first part,16 mesh,50 mesh,100 mesh,200 mesh and 400 mesh iron powder,magnetite,activated carbon and graphite were added to the anaerobic digestion reactor respectively,and a blank control group was set.Through the analysis of SCOD concentration,volatile fatty acids(VFAs),organic matter degradation rate,biogas production and methane content in the anaerobic digestion process,the results show that the iron powder group and magnetite group have better promotion effect on sludge anaerobic digestion with the decrease of particle size,and the total gas production and organic matter removal rate reach the maximum at 400 mesh,However,iron powder and magnetite have different ways to promote anaerobic digestion: magnetite mainly promotes the hydrolytic acidification stage and provides more hydrolytic acidification products;Iron powder can accelerate the consumption of acidification products and promote the methane production process.The addition of activated carbon can promote the hydrolysis acidification stage and methanogenesis stage,and the promotion effect is the best when the particle size is reduced to 100 mesh.The addition of graphite mainly promoted the methanogenic stage,and the promotion effect increased with the particle size reduced to 400 mesh.Finally,the mechanism of strengthening anaerobic digestion of excess sludge with conductive medium is preliminarily analyzed and discussed.By analyzing the conductivity of sludge supernatant and microbial population distribution after anaerobic digestion reaction,and setting the inert material group with corresponding dosage and particle size as the control,the influence of deducting the addition of conductive media as the carrier is analyzed.The results show that the addition of iron-based conductive media can significantly improve the conductivity of sludge system,while the carbon based conductive media has no obvious effect;Addition of conductive materials had little effect on microbial population categories,but had an effect on microbial population abundance.Adding magnetite and activated carbon can enrich specific hydrolytic acidification bacteria,while iron powder and graphite can effectively improve the abundance of electroactive methanogenic archaea.The comparative experiment of inert materials shows that microorganisms in reactors with conductive media were closely enriched on the surface of the conductive media,which facilitated direct interspecies electron transfer;In addition,the addition of inert media has little effect on the anaerobic digestion efficiency.Compared with the blank group,the change of gas production and organic matter degradation rate is less than 5%.It shows that the key factor to strengthen the anaerobic digestion process is not the carrier effect of adding conductive media,but the conductive characteristics of conductive media.To sum up,adding zero valent iron powder,magnetite,activated carbon and graphite can effectively strengthen the sludge anaerobic digestion process and improve the gas production efficiency.It has certain practical significance for exploring new ways of anaerobic digestion enhancement and improving the recovery and utilization efficiency of excess sludge.