Mechanism Study Of Biomass Carbon Material On Anaerobic Digestion Of Waste Activated Sludge

Waste activated sludge（WAS）is the inevitable byproduct of biological wastewater treatment,and the rapid growth of WAS have restricted the sustainable development of sewage treatment industry in China.Anaerobic digestion（AD）is a reliable and sustainable technology for WAS treatment,while the efficiency of AD is often limited by its inherent drawbacks.Recently,great attentions have been aroused to improve AD by adding carbon-based materials（e.g.,activated carbon,biochar,hydrochar）,which have provided a good opportunity for the development of anaerobic digestion treatment technology.However,owning to their intricate physicochemical characteristics,the influences of biochar materials on AD remained controversial.Besides,the underlying correlations of the methanogenic process with biochar addition are still largely speculative,making it yet impossible to pre-select and design suitable biochar material for AD enhancement.Based on the above considerations,algal biochar（ABC）and activated carbon（AC）were chosen as the model biochar materials in this study.The main objectives included（1）to explore the effects and the related mechanism of different biochar materials on the AD under different operating conditions;（2）to verify the critical mechanism of biochar materials through comparing different anaerobic digestions with ABC and AC;（3）to explain the enhancing mechanism by biochar material through analyzing the dynamic changes of microbial community,metabolic function and electron transfer activity mediated by biochar material;（4）to investigate the key factors of biochar material that related to the methanogenic enhancement during AD;（5）to verify the feasibility of biochar modification methods during AD improvement.Finally,a new strategy based on engineered biochar was proposed for the improvement of AD.Firstly,the effects of AC on sludge AD at ambient（16-24 ^oC）,mesophilic（35 ^oC）and thermophilic（55 ^oC）temperature were investigated.After AC addition,although the methane yields of raw sludge were reduced by 6.5%-36.9%,the lag phases of methanogenesis were shortened by 19.3%-30.6%and the reductions of methane yields were declined to only 5.9%-8.1%simultaneously for the pretreated sludge.Inhibitory substances like phenols that generated by thermal pretreatment were reduced by AC,which was responsible for the methanogenic acceleration.Meanwhile,the methane reduction due to the non-selective adsorption by AC could be mitigated by sludge pretreatment and the elevated temperature.Using anaerobic digested sludge as inoculum,the effects of ABC on methanogenesis at different inoculation ratios were investigated.Results showed that ABC enhanced methane productions at the lower inoculation ratios（4%and 1%,v/v）,but not at the higher ratio（10%,v/v）.Mechanism analysis demonstrated methanogenic improvements at the lower inoculation ratios were not owing to the increase of initial organic loading rates.Otherwise,adding algal biochar at the lower inoculation ratios were more favorable for the enrichment of Methanosarcina than the higher ratio,which might be benefit for methanogenesis through directed interspecies electron transfer.For improvement of AD,microbial enrichments at different inoculation ratios would be more important than merely biochar addition.The addition of AC inhibited methane yield of sludge hydrolysate due to the predominance of non-selective adsorption.Whereas the shortened lag phase time（38.2%）,the improved methane production rate（16.6%）,and the increased methane yield（7.6%）were achieved by ABC.ABC enhanced methanogenesis was attributed to its redox property other than p H buffering capacity.Results showed that contents of phenolic and lactonic groups were4.8 and 2.3 times higher than that of AC,respectively,which endowed ABC with the stronger redox capacity,especially the electron-donating capacity.Therefore,surface groups like phenolic and lactonic groups would be valuable for AD improvement.Furthermore,the related characteristics and potential mechanisms of two surface modified biochars on methanogenesis were explored.Results showed the accumulative methane yield of H₂O₂-modified biochar group was 197.8±4.4 m L/g COD,which increased by 58.7%compared to the blank.Moreover,both methane yield and methane production rate were further enhanced by H₂O₂-modified biochar compared with ABC.While,methanogenesis ceased after the addition of HNO₃-modified biochar.Compared with ABC,specific functional groups such as phenolic and lactonic groups on H₂O₂-modified biochar were increased by 1.2 and 5.1 times,respectively.Especially the electron-donating capacity was increased by 64.9%after H₂O₂ modification,thereby leading to the enhanced methanogenesis.However,both acidogenesis and methanogenesis were inhibited by the HNO₃-modified biochar,which could be due to the generation and/or dissolution of inhibitory compounds and the p H disturbance after HNO₃ modification.Finally,ABC was modified by nitrogen/sulfur-doping and quinone-immobilization methods.Results showed that methane yield was increased by 12.6%and lag phase was shortened by 57.9%by S-doped biochar.The S-dopping process significantly improved the redox capacity of ABC,and the addition of sulfur-doped biochar also provided an opportunity for syntrophic relationship between sulfate reducing bacteria and methaonogenic archaea.The nitrogen-doped biochar inhibited methane yield by 7.7%,while the lag phase reduction was not observed comparing to ABC,which was probably due to the enhanced non-selective adsorption effects and the formation of nitrogen-containing inhibitors during the nitrogen-doping process.Methanogenesis was completely inhibited by the quinone-immobilized biochar.However,it was concentrated acid treatment during modification that caused the inhibition during anaerobic digestion.The effects and related mechanism of biochar material on sludge AD were explored in this study,and a strategy based on engineered biochar was proposed for the improvement of AD.The findings of this study were helpful to understand the physical,chemical and microbial ecological mechanisms during AD with different biochar materials,which also provided theoretical and technical support for the development of anaerobic digestion treatment.