Evolutions Of Humic Acids During Slusge Thermal Hydrolysis And Their Effects On Anaerobic Digestion

As an important sludge treatment technology,anaerobic digestion（AD）is widely used in the recycling of sewage sludge.The total amount of sludge can be reduced,methane could be generated by AD.However,traditional AD has practical problems,such as limited rate of hydrolysis and low conversion rate of organic matter.Therefore,thermal hydrolysis of sludge has been widely used to improve the CH₄ production of sludge AD currently,and at the same time,it can provide favorable conditions for subsequent digestion and deep dewatering.However,it is worth noting that as the main organic component of municipal sludge,humic acids（HAs）account for 6-20%of volatile suspended solids（VSS）,and most of them exist inertly in the solid phase of sludge,thus the impact on sludge AD has been overlooked before.However,as Cambi technology has been widely used in the pretreatment of sludge AD,this inhibitory organic matter would be released into the liquid phase largely and have a negative effect on AD.Therefore,it is necessary to carry out research on the change law of HAs concentration and structural characteristics under different thermal hydrolysis intensities,establishing a quantitative or semi-quantitative relationship between content and intensity,and clarifying the threshold value of hydrolysis intensity at the critical point.The influence of HAs evolution on anaerobic digestion should be analyzed to observe the microbial communities as well.Finally,an efficient and environmentally friendly method would be built to eliminate the inhibition of HAs on AD.The main research contents and conclusions are as follows:（1）The release of HAs during sludge thermal hydrolysis was studied at medium and high temperatures,and it was found that the intensity of hydrolysis was significantly positively correlated with the content of soluble HAs.The release rate of soluble HAs first increased and then decreased.Under the condition of 180°C,the soluble concentration of HAs reached the maximum of 727 mg/L.In addition to changes in concentration,the structural characteristics of soluble HAs were also variable accompanied by hydrolysis intensity.The functional groups of HAs released at 160°C was mainly composed of unsaturated C≡C triple bonds,but as the temperature raised,the triple bond functional groups were cleaved to C-C single bond functional groups.The amount and types of functional groups were firstly decreased and then increased as well.The condensation degree and molecular weight of HAs also showed a dynamic process.The simple components of 200°C HAs functional groups decreased,such as aliphatic organic matter,on the contrary,both the aromatic condensation degree and molecular weight increased,due to the strengthen of hydrolysis intensity.The increase of thermal hydrolysis temperature prompted the stability of HAs structure,and deteriorated their biodegradability instead.（2）Thermal hydrolysis of sludge leaded to the evolution of HAs and inhibited AD.0.4g/L HAs inhibited the acidification rate by more than 50%.In addition to the influence of concentration,the structural characteristics of HAs also played an important role in the AD.In particular,E4/E6 and C/N ratios were significantly correlated with the inhibition of volatile fatty acids（VFAs）production.Interestingly,the effects of soluble HAs on hydrolysis stage and methanogenesis stage with simple substrates（glucose and sodium acetate）were quite different.It was found that HAs inhibited acid production but promoted CH₄ production,and the degree of impact was also closely related to the structural characteristics.From the perspective of enzyme activity,HAs inhibited the activity of extracellular hydrolase and promoted the activity of intracellular key enzymes for methanogenesis.In terms of sludge AD,low HAs concentration（<1.0g/L）boosted methane production,while high HAs concentration（≥2.0g/L）inhibited the accumulation of CH₄,owing to the fact that clustered HAs could flocculate substrate and combine hydrolytic enzymes,causing the reduction of hydrolase activities.However,the electron transfer promoted the activity of inner enzymes of methanogenesis.In summary,the concentration and structural characteristics of HAs would simultaneously affect the sludge AD.（3）The influence of soluble HAs on AD also included the effect on the succession of microbial community structure.The growth of the acidification bacteria and methanogens was induced by the addition of HAs.Results showed that Clostridia was the dominant bacteria in the acid-producing phase.The relative abundance of Clostridia was reduced with the increase of HAs content.What’s more,the dominant bacteria in the methanogenesis phase was Methanobacterium,followed by Methanosaeta.The increase of HAs concentration promoted the relative abundance of Methanobacterium,indicating that methanogenic archaea had adaptabilities to HAs.Except HAs concentration,the diverse structures were also the main factors.Especifically,the ratios of C/N,E4/E6 were significantly negatively correlated with the abundance of Clostridia,on the contrary,the ratio of C/H was significantly positively correlated with SHA-26 during acidification step.In the methanogenesis phase,the ratios of C/N,E4/E6 showed a significantly positive correlation with the relative abundance of Methanomassiliicoccales and euryarchaeota,but the correlation between C/H ratio and methanogenic archaea is not significant.（4）UV photocatalysis coupled with chemical flocculation could alter the structural characteristics of HAs and remove soluble HAs partially.The results showed that UV photocatalysis could change the structural characteristics of HAs,causing the rearrangement of functional groups.The compact cluster structure of HAs would be broken,and the degree of aromatic condensation,molecular size and electronic transition ability were all weakened after being photocatalyzed.Furthermore,UV photocatalysis destabilized the structures of macromolecular HAs,exposing more active binding sites.HA-Ca complexes would be formed through the adsorption and bridging of Ca²⁺,which significantly lowered the stability of HAs colloids.Part of HAs were mineralized into CO₂ after photocatalysis.Therefore,UV photocatalysis coupled chemical flocculation can not only change the structural characteristics of HAs,but also directly remove a portion of soluble HAs,alleviating the inhibition of HAs on VFAs production via sludge anaerobic fermentation.