The Transformation Of Humic Acid In Sludge Dry Anaerobic Fermentation And Its Effect On Methane Production

Sludge dry anaerobic fermentation technology is one of the effective technologies for sludge treatment and disposal.Biogas yield,methane content,fermentation cycle and organic matter utilization rate are important indicators for measuring the effect of dry anaerobic fermentation.Humic acid(HA for short)is the major component of organic matter in sludge.Its mainly contains a amount of carboxyl,carbonyl,hydroxyl,ketone and sulfhydryl groups.It has complex redox characteristics and has impact on methane production during sludge fermentation process.In this study,the influence mechanism of HA on the methane production of sludge dry fermentation was explored.The municipal sludge was used as fermentation substrate and dry anaerobic fermentation experiments with different humic acid content were constructed.The gas chromatograph,excitation-emission matrix and Fourier transform-infrared spectroscopy were used to detect the methane content of biogas and volatile fatty acid(VFA)concentration of fermentation liquid,and characterize the chemical structure of HA.The structural transformation of HA in the fermentation process was analyzed,and the effect of HA on the process of acid production and methane production in sludge dry anaerobic fermentation was studied,which provided a theoretical basis for improving sludge dry anaerobic fermentation treatment technology.The main conclusions are obtained as follow:(1)HA generally inhibited the methane production of sludge dry anaerobic fermentation.When the HA content is 10%,the cumulative methane production is slightly higher than that of the blank group.But when the HA content was 20%,the degree of inhibition was significant.Pearson correlation analysis showed that the addition of HA inhibited the gas production of dry anaerobic fermentation at the initial stage and the end stage of fermentation,and promoted the gas production in the middle stage of fermentation.Used the Gompertz equation to fit the cumulative methane yield showed that when the HA content was 10%,the maximum methane production rate was increased.When the HA content was 5% and 10%,the peak daily yield of methane in anaerobic fermentation was increased,but when the HA content increased to 15% and 20%,and the peak daily yield of methane decreased.(2)The content of HA in the mixture of sludge and kitchen waste accounts for about 1%-10% of the content of dissolved organic matter.The Humic acids is the main organic component in the dry anaerobic fermentation process.The Humic acids is produced by lignin degradation products and microbial metabolites,and the fulvic acids and the humic acids are mutually converted.The change of chemical structure characteristics of HA during anaerobic fermentation is mainly due to the increase of aromatic carbon content,and the relative reduction of aliphatic carbon,carboxyl and hydroxyl carbon content,and the degree of polymerization of HA structure was increased.The degree of aromatization of HA is significantly increased in the late stage of fermentation.The nitrogen-containing compounds are degraded,and phenols,esters,alcohols and polysaccharide compounds are produced during fermentation.(3)HA promoted the acidification reaction stage of sludge dry anaerobic fermentation,and the addition of HA increased the concentration of VFA in the fermentation system.Acetic acid is the main fermentation acidification product.The propionic acid,butyric acid and valeric acid can be degraded to acetic acid under the action of acetogens,which promotes the concentration of acetic acid.Pearson correlation results showed that HA promoted hydrolysis and acidification mainly in the initial stage of fermentation,and inhibited VFA production in the late stage of fermentation.The effect of HA on the concentration of VFA is consistent with the change in the effect of methane production.HA affects the methane production of dry anaerobic fermentation through the VFA methanogenic pathway.