Effects Of Sludge-derived Biochar On Mesophilic Anaerobic Digestion Of Waste Activated Sludge And Microbial Community

Anaerobic digestion can realize the degradation of organic matter and energy recovery at the same time,which is considered as a promising treatment method for waste activated sludge（WAS）.In order to promote anaerobic digestion of WAS and obtain more biogas production,scholars enhance the anaerobic digestion process by adding biochar and other fortifiers.In this study,two kinds of sludge-derived biochar were prepared under different conditions,i.e.sludge hydrochar（HC）and sludge pyrochar（PC）,using WAS from sewage treatment plant as raw material.The effects of biochars on anaerobic digestion of WAS were investigated.Bacterial and archaea populations during anaerobic digestion of WAS,especially those involved in direct interspecific electron transfer（DIET）,have also been studied based on their abundance and diversity.The main results of the experiment are as follows:（1）Hydrothermal carbonization retains most of t he surface functional groups of the raw materials.Three-dimensional fluorescence spectra also showed that more humus were retained in hydrochar.When the pyrolysis temperature rose to 700℃,most of the functional groups on the surface of sludge-derived pyrochar were decomposed and destroyed.Compared with sludge-derived pyrochar,sludge-derived hydrochar has a higher specific surface area,and HC180 has the largest specific surface area and pore volume（58.01 m/g and 0.31 cm/g,respectively）.This may be the flocculating structure of the sludge particles,which will shrink strongly during dry pyrolysis and is not conducive to the formation of porous biochar.Under the hydrothermal carbonization condition,liquid molecules and tiny bubbles violently impinged on the biomass,which promoted the formation of the pore structure of the sludge hydrothermal carbon.（2）Compared with the sludge-derived pyrochar,the sludge-derived hydrochar can better promote the production of methane in the WAS anaerobic system.The enhanced effect of HC180 was most obvious,with a cumulative methane production of 132.04±4.4 m L/g VS_added,which increased 18.40%compared with the blank group(111.52±1.09 m L/g VS_added).Meanwhile,in the presence of pyrochars,only in PC300 group t he cumulative methane production increased slightly,while both PC500 and PC700negatively affected the final methane production.（3）The monitoring of soluble organic matter and short chain fatty acids（SCFA）in the reaction system showed that hydrochar c ould enhance the activity of hydrolytic acidizing bacteria,promote the hydrolysis of sludge organic particles,and increase the yield of SCFA（especially acetic acid,propionic acid）.However,pyrochar（especially PC500 and PC700）has a certain inhibitory effect on the release of soluble proteins and carbohydrate,and reduces the yield of SCFA.On the 4th day of anaerobic digestion,the SCFA concentration in the reactor added with hydrochar climbed to 312.73～349.60mg/L,much higher than that in the single sludge anaerobic reactor（163.98 mg/L）.（4）The addition of sludge biochar significantly changed the microbial community abundance in sludge anaerobic system.Archaea and bacteria in HCs group had smaller Simpson and Shannon indices compared with blank gr oup,indicating that hydrochar reduced the diversity of microbial communities（archaea and bacteria）,and the effect of pyrochar on the diversity of the flora of the anaerobic system was small.The heat map of colony composition combined with cluster analysis showed that the community structure of archaea and bacteria in Blank32d,PC500 and PC700 was close,but the gap with HCs group was large.It indicated that the addition of hydrochar had a greater impact on the microbial community of sludge anaerobic digestion sys tem.（5）Bacteria Candidatus Microthrix were enriched in HC240 and HC300（relative abundances of 3.3 and 3.4%,respectively）.Filamentous structure of Candidatus Microthrix is considered to be beneficial to the formation of the microbe-EPS aggregates,and there is a strong positive correlation between its abundance and the removal of organic matter.Therefore,hydrolytic acidification rates in HCs group are higher than that in Blank.Moreover,the addition of hydrochar significantly increased the abundance of Methanosaeta（69.5～70.7%）,Methanobacterium（12.8～15.2%）and Methanosaeta（69.5～6.00%）,which were thought to be involved in the DIET process.At the same time,hydrochar led to the enrichment of specific DIET bacteria,such as Genus Syntrophomonas（0.166～0.186%）and Family Peptococcaceae（0.035～0.086%）,which promoted SCFA consumption and ultimately enhanced the methane production.