The Role Of Chitosan On Methane Production In Anaerobic Digestion And Hydrogen Production In Anaerobic Fermentation Of Waste Activated Sludge

Chitosan is one of the most commonly used flocculants in wastewater treatment plants,but its potential effect on anaerobic digestion of waste activated sludge remains unclear.The purpose of this study is to explore the effects and mechanisms of chitosan on methane production in anaerobic digestion and hydrogen production in anaerobic fermentation.Firstly,the effect of chitosan on methane and hydrogen production by sludge anaerobic fermentation was revealed through biochemical methane potential and biochemical hydrogen potential tests,and the kinetic analysis of methane and hydrogen production were analyzed with model fitting.Secondly,COD mass balance analysis was also carried out to investigate the distribution of carbon and hydrogen elements,and the contribution of chitosan degradation to the carbon source of anaerobic digestion was evaluated by theoretical calculation.Then,the effect of chitosan on each stage and the activity of key enzymes of anaerobic digestion were investigated by batch tests of model substrate degradation.Finally,long-term semi-continuous tests were carried out to explore the effects of chitosan on microbial community in anaerobic digestion,and the mechanism of chitosan in sludge anaerobic digestion was revealed from the aspects of flocculation,protonation and oxidative stress induction.The results showed that chitosan(5-30 g/kg TSS)could promote the methane production in sludge anaerobic digestion.The cumulative methane production increased from 215±1.52 mL/g VSS to 272±1.83 mL/g VSS with the addition of chitosan at 30 g/kg TSS,which was promoted by 26.5±0.0004%compared with control group.The positively charged amino group in chitosan can electrically neutralize the hydroxyl and carboxyl groups on the surface of sludge,and promote sludge agglomeration,thus limiting the release of organic matter.Chitosan also inhibits the hydrolysis stage by immobilization of hydrolase,and reduces its interference with electron transfer between acidogens and methanogens by flocculating humus,thus inhibiting the activity of acidulase enzyme and improving the activity of coenzyme F420 and methanogenic capacity.In addition,chitosan increased the relative abundance of methanogens.Further studies showed that chitosan(5-30 g/kg TSS)could inhibit hydrogen production by anaerobic fermentation of sludge.Chitosan with 30 g/kg TSS reduced the cumulative hydrogen production from 3.94 ± 0.12 mL/g VSS to 1.71±0.10 mL/g VSS,which was reduced by 56.7±1.22%.The inhibition of chitosan on hydrogen production is mainly due to the reactive oxygen species induced by chitosan,which leads to lipid peroxidation reaction between the increased reactive oxygen species and membrane lipid proteins and other biological macromolecules,resulting in the decreased activity of malondialdehyde accumulation electron transport system and abnormal cell physiological function,thus damaging enzymes related to sludge anaerobic fermentation.Chitosan can also increase the richness of microbial community and reduce the uniformity of microbial community and ultimately inhibiting hydrogen production.This study revealed the different effects of chitosan on acidic and neutral anaerobic digestion due to different degrees of protonation and the mechanism,which provided a new insight for clarifying the potential effects of chitosan on sludge anaerobic digestion.The mechanism of oxidative stress induced by chitosan complements the existing mechanism of flocculants,which is of great significance for further understanding the potential behavior of flocculants in sludge biological treatment.