Study On The Methane Production Performance Of Anaerobic Co-digestion Of Sewage Sludge With Pig Manure

Anaerobic digestion is one of the most important technologies to achieve both reduction and stabilization of energy recovery.The anaerobic co-digestion of sewage sludge with pig mature could provide extra organic matters and balance the ratio of carbon and nitrogen,which could solve the problems of low organic matter content and low methane production in municipal sewage treatment plants,but also fully improve the utilization rate of existing biogas digester in sewage treatment plants and increase the biogas production of anaerobic digester.It can also provide centralized treatment solutions for small and medium-sized farmers to reduce the operation and maintenance cost of decentralized biogas fermentation facilities,and to achieve a win-win goal.However,the feasibility and stability of co-digestion of these two substrates have yet to be verified,and it is necessary to optimize the co-digestion process parameters.In this paper,the feasibility of codigesting sewage sludge and pig manure was verified by adopting sequential batch digestion and semi-continuous digestion,and various parameters were optimized to promote the methane yield from co-digestion.（1）Firstly,this study has investigated the influence of sludge and pig manure’s ratio and solid content（total solids,TS）on methane production in co-digestion.In total,five different ratios of pig manure to sewage sludge were mixed to achieve different solid contents,i.e.TS of 4%,5%,6%,8%and 10%.It was found that the methane accumulation yields firstly increased and then decreased with the increase of TS.The optimal mixing ratio was TS 5%with the VS ratio of pig manure and sludge 1.76,and the accumulated gas yield was 251.4 m L/g VS_added.The accumulated gas yield of TS4%,6%,8%and 10%were 211.9 m L/g VS,216.0 m L/g VS,178.0 m L/g VS and113.8 m L/g VS_added,respectively.（2）Secondly,this study has explored the influence of activated carbon amount on the co-digestion performance.By adding the activated carbon of 0 g/L,2 g/L,4 g/L,6g/L and 8 g/L,the practical methane yields were 2459.6 m L,2472.8 m L,3296.95 m L,3018.3 m L and 2900.2 m L,respectively.The maximum methane yield was obtained by adding 4 g/L of activated carbon into the digestion system,leading to the increment of 33.3%as compared to the blank group.（3）In view of the reutilization of digestate,four kinds of biochar（HC/HTC/HC-Fe/HTC-Fe）prepared from digestate were obtained by using different methods,i.e.pyrolysis（HC）,hydrothermal carbonization（HTC）,and iron-modified biochar（HC-Fe and HTC-Fe）,respectively.The influence of different biochar on the methane production was studied as compared to the blank group（CK）.It was found that four kinds of biochar could all shorten the delay period of methane production and increase the methane production.From the perspective of accumulated methane production,HTC performed best with the methane yield of 308.4 m L/g VS,followed by HTC,HC-Fe and HTC-Fe with methane yields of 305.7 m L/g VS,242.6 m L/g VS,216.4m L/g VS,respectively.Among them,the cumulative methane yield from HTC digester was increased by 48.2%as compared to the blank group.Three-dimensional fluorescence and infrared spectroscopy showed that the functional groups in HTC were more abundant than that of HC,which could effectively enhance the microbial cooperation,strengthen interspecific electron transfer,and promote organic matter hydrolysis and methane-producing transformation of substrates.（4）In order to explore the feasibility of the practical application on the co-digestion of sludge and pig manure and the ability of biochar to resist high organic load,a semi-continuous flow experiment was conducted.The organic loading rate（OLR）of 3.4 g VS/L/d（L-OLR）and 5.3 g VS/L/d（H-OLR）were adopted respectively.Under the condition of L-OLR,the average volumetric gas yield was 0.7 L/L/d and the highest gas yield was 0.9 L/L/d,which was 18.3%higher than the blank group（0.6L/L/d）.Under the condition of H-OLR,the average volumetric gas yield was 1.3 L/L/d and the highest gas yield of the biochar group was 1.4 L/L/d,which was 41.6%higher than that of the blank group.As for the content of methane,the group added biochar was increased by 9.7%under the condition of L-OLR and 5.4%under the condition of H-OLR.Therein,biochar can effectively promote the volume gas production rate and methane content in the co-digestion of sludge and pig manure,and the promotion effect is more obvious at the higher organic loading rate.