| Signing the Paris Climate Treaty has made low–carbon development upgraded to a political level.As a result,many countries have lauched a series of plans regarding reducing carbon emission.Carbon–neutral operation has now become an important direction for developing future wastewater treatment plants(WWTPs).Excess sludge contains potential organic energy,and anaerobic digestion(AD)is an important approach to converting organic substances into energy.However,the converting efficiency of AD for excess sludge is often not so high due to stable cell walls,and lignocellulosic materials,humic substances(HSs)in excess sludge.HSs are not only hard to be degraded during AD,but also inhibit hydrolysis of other organic matters in excess sludge by binding hydrolases with its carboxyl and phenolic hydroxyl.Owing to the complex structure of HSs,pretreatment might be difficult to destroy HSs,and thus the inhibition of them on hydrolysis can not be eliminated completely.Instead,the inhibition might be shielded by in–situ metal cations due to passivating these active functional groups.As the inhibiting degree of HSs on AD and the disinhibited strategies on HSs heavily depended on the content and structure of HSs,the variations of HSs(humic acid:HA and fulvic acid: FA)on the content and structure were analyzed and characterized during AD of excess sludge,in order to provide a basis for working out the disinhibited strategies on HSs.At the initial stage of the study,the variations of HSs on the content and structural characteristics were experimentally investigated along with AD of the ‘artificial' sludge,which was produced with the synthetic wastewater without HSs involved.The results indicate that HA and FA respectively were at 370,190 mg/L prior to AD and that HSs(the sum of HA and FA)accounted for 2.8% of TS in the sludge,they increased to 390 and 200mg/L respectively together with an increased HSs share of 3.5% in TS after AD.Meanwhile,the results inspected by UV–Vis and FTIR spectra revealed that aliphatic compounds decreased in the structural composition of HA and FA and even disappeared in FA after AD,whereas aromatic compounds increased in both HA and FA after AD.The results of intelligent potentiometric titration illustrated that both carboxyl and phenolic hydroxyl contents increased after AD.In addition,the sum of carboxyl and phenolic hydroxyl contents in both HA and FA were at 4.07~5.97 mmol/g during AD.At the further stage of the study,the variations of HSs on the content and structuralcharacteristics were also analyzed during AD with real excess sludge from a municipal WWTP in Beijing.The results indicated that both HA and FA in the liquid phase increased after AD,in which HA and FA were at 29 and 139 mg/L respectively prior to AD;after AD HA and FA respectively increased to 43 and 200 mg/L together with the increased shares of48.3% and 43.9%.However,both HA and FA contents in the solid phase gradually decreased during AD and were at 1,057 and 143 mg/L after AD respectively from 1,133 and 242 mg/L prior to AD,causing the decreased shares of 6.7% and 40.9%.Also,the HSs content showed to a declining trend during AD,declined from 1,543 to 1,443 mg/L together with the reduced share by 6.5%.Meanwhile,the results inspected by UV–Vis,FTIR spectra and intelligent potentiometric titration revealed that both HA and FA evolved towards improving aromaticity and humification and also increasing molecular weight and oxygen–containing functional groups.Moreover,results from two dimensional infrared spectra revealed that change order of material groups in the structural compositions of HA and FA followed the order: aliphatic group>carboxylic acids>polysaccharides–like group. |
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