Effects And Mechanisms Of Polyferric Chloride And Polyethyleneimine On Methane Production In Anaerobic Digestion Of Waste Activated Sludge

Flocculants are widely used in sewage purification and sludge conditioning,but their potential effects on anaerobic digestion and their mechanisms remain unclear.In response to this problem,this paper selects the inorganic polymer flocculant polyferric chloride（PFC）and the organic polymer flocculant polyethyleneimine（PEI）as the research objects.Firstly,the biochemical methane potential test combined with model analysis were used to investigate their effect on the biochemical methane potential of sludge.Then,model substrate degradation tests and long-term semi-continuous tests were used to explore the effect mechanism of these two flocculants on methane production from sludge anaerobic digestion.The results showed that PFC inhibited methane production from anaerobic digestion of sludge.When the PFC concentration reached 40 g/kg TSS,the methane production decreased from 195±2.10 L/kg VSS to 156±1.50 L/kg VSS,a decrease of20.0±0.09%.Unlike PFC,the effect of PEI on methane production was related to its concentration.Low concentrations of PEI（i.e.,3 and 6 g/kg TSS）had no significant effect on methane production,while high concentrations of PEI（i.e.,12 and 24 g/kg TSS）significantly suppressed methane production.PEI at 12 and 24 g/kg TSS reduced cumulative methane production to 154±4.00 L/kg VSS and 141±4.52 L/kg VSS,a reduction of 7.56±2.40%and 15.7±2.71%compared to 167±5.13 L/kg VSS in the control,respectively.Mechanism studies showed that PFC can release hydroxyl polymers and Fe³⁺.Hydroxyl polymers agglomerated sludge flocs,which impeded the release of dissolved organic matter,thereby reducing the substrate available for anaerobic digestion.Fe³⁺induced dissimilatory iron reduction process competed with CH₃·S·CoM for electrons,which further reduced the production of methane.At the same time,PFC enriched iron-reducing bacteria and reduced the abundance of methanogens,resulting in microbial communities that are not conducive to methane production.PEI also agglomerated sludge flocs,preventing microorganisms from acquiring substrates.Sludge agglomeration also hindered interspecies hydrogen transfer,coupled with the decrease in the concentration of cytochrome C,which restricted the electron transfer between syntrophs and methanogens.In addition,PEI utilizes amine groups to adsorb CO₂,which hindered the hydrogenotrophic methanogenesis.PEI promoted the decomposition of acetate by enriching Synergistaceae sp.,thereby hindering the acetyltrophic methanogenesis.In this paper,the effects of inorganic polymer flocculants PFC and organic polymer flocculants PEI on the methane production of sludge anaerobic digestion and the relationship and difference of their mechanisms were revealed,which will provide an in-depth understanding of the potential effects of flocculants in sludge anaerobic digestion.The theoretical basis is of great significance for the selection of flocculants in wastewater treatment plants and the stable operation of sludge anaerobic digestion systems.