Study On The Interaction And Mechanism Between Sludge Organic Matter And Ferric Phosphate During Anaerobic Digestion

Anaerobic digestion can not only realize sludge stabilization,harmlessness,reduction and resource utilization,but also promote the efficient recycling of nitrogen（N）and phosphorus（P）.In view of the fact that there are a large amount of organic matter and iron salts（including residual iron（Fe）flocculants and iron-phosphorus compounds（Fe Ps））in chemical P removal sludge using Fe salts,and the problems that the specific role of sludge organic matter in Fe Ps transformation is not distinct and the impact of Fe salts on anaerobic digestion performance is unclear,the interaction between sludge organic matter and iron-phosphate salts during the anaerobic digestion process was studied and the mechanism was further explored in this paper.Firstly,the abiotic interaction between major macromolecular organic matter in wasted activated sludge（WAS）and ferric phosphate（FePO₄）was studied,and the interaction mechanism between them was deeply discussed.Then,effect of macromolecular organic matter on the transformation of FePO₄ in sludge during the anaerobic fermentation process was investigated.Finally,due to the fact that there is a large amount of Fe in WAS,the influence mechanism of different Fe salts on the transformation of organic matter during the anaerobic digestion process was studied.The purpose is to provide a reliable theoretical basis for realizing resource utilization and P recovery in WAS at the same time during anaerobic digestion through the in-depth and systematic exploration of mechanisms.Firstly,the abiotic interaction between ferric phosphate and major macromolecular organics（protein,humic substance,polysaccharide（bovine serum albumin（BSA）,fulvic acid（FA）,glucose as the corresponding model organic compounds,respectively））in WAS were studied,and the effect of phosphate on protein and the reaction mechanism were discussed in particular.BSA,FA and glucose had little effect on P release from FePO_4。With the addition of FePO₄,the concentration of protein was greatly reduced accompanied by a remarkable increment of NH₄⁺-N concentration,while FA and glucose barely changed.PO₄^3-was the functional ion dominating the protein decomposition and NH₄⁺-N release.Experiments in sterilized sludge confirmed that phosphate can promote protein decomposition and NH₄⁺-N production.The analytical results of circular dichroism（CD）suggested that the reaction went through two stages:firstly,the process was relatively stable and phosphate showed a tendency to facilitate the transition from helices toβ-sheets;then peptide chains on the helices were broken and deamination of amino acid residues occurred.Due to the loss of hydrophilic groups,hydrophobicity of the residue was further enhanced,resulting in its co-precipitation with protein.The results are also applicable to casein that with similar structures to BSA.Then,effects of macromolecular organic matters,humic substance and protein（FA and BSA as the corresponding model compounds,respectively）,on the transformation of FePO₄ during anaerobic fermentation of WAS were investigated.Both FA and BSA promoted P release from FePO₄.While BSA had a stronger effect on promoting Fe（III）reduction,FA was more effective in inhibiting P re-precipitation with other metal ions,resulting in a higher P release efficiency with FA addition than that with the same dissolved organic carbon（DOC）dose of BSA.P release efficiencies increased by 26.6%and 19.7%with the addition of FA and BSA,respectively,after 8 days’fermentation at an equivalent of 500 mg DOC/L.Both FA and BSA promoted the formation of vivianite but decreased its crystallinity.The proportions of P in vivianite increased by 351.9%and 199.2%with the addition of FA and BSA（500 mg DOC/L）,respectively,after 8days’fermentation.Microbial analysis revealed that FA contributed to the enrichment of iron-reducing bacteria（IRB）such as Rhodobacter,Defluviimonas and Rubellimicrobium transporting electrons indirectly and increased the bioavailable Fe（III）via siderophores;BSA provided more electron donors,thereby enriched IRB like Geobacter and Clostridium＿sensu＿stricto＿12 that can transfer electrons directly to Fe（III）.Finally,effects of four different iron salts Fe Cl₃,Fe Cl₂,FePO₄ and Fe₃（PO₄）₂ on the transformation of organic matter in WAS during the anaerobic digestion process were studied with high Fe dose（110 mg Fe/g TSS）and low Fe dose（10 mg Fe/g TSS）.Considering the whole anaerobic digestion process,high dose of iron salts would prolong the lag-phase time of methanogenesis and decrease the maximal methane production rate and maximal methane yield potential,in which Fe Cl₂-H had the most obvious inhibition effect.Effects of iron chlorides（Fe-Cl）on each stage was significantly the stronger than that of iron-phosphate precipitates（Fe-PO₄）.The addition of Fe Cl₃ with high dose would inhibit the synthesis of coenzyme M（Co M）through pathway A,and the inhibition of methanogenesis in Fe Cl₃-H test was mainly related to the decrease of Co M content.In Fe Cl₃-H test,the relative high Na Cl and Fe contents both led to a significant increase in the relative abundance of methanoculleus,while the large enrichment of methanoculleus in the later stage was conducive to the efficient biosynthesis of Co M through pathway B,so as to relieve the inhibition of methanogenesis.The methane production of Fe Cl₂-H test was inhibited in the early stage and gradually recovered in the later stage,which was mainly related to the change of p H.Among the tests with low dose of Fe,Fe-Cl（especially Fe Cl₂）significantly promoted the increase of methane production,and Fe Cl₂promoted each stage of anaerobic digestion（hydrolysis,acidogenesis and methanogenesis）.Low dose of Fe salts had little effect on the community structures of methanogens,but the relative abundances of enzymes related to methanogenesis pathways increased significantly in Fe Cl₂ test.Fe Cl₂ with low dose increased the content of bioavailable Fe,and reduced the concentration gap between bioavailable Fe and Fe demand.It was conducive to the growth and activities of microorganisms,thus promoting the methane metabolism of microorganisms.The results of this paper are conducive to the deeply understandings for the transformations of organic matter and Fe Ps in Fe Ps-bearing sludge during anaerobic treatment process.And it can provide a reliable theoretical basis for the development of technologies that can simultaneously improve the anaerobic digestion efficiency and P recovery efficiency of Fe Ps-bearing sludge in the future so as to further recover inorganic P and realize sludge recycling,which has theoretical significance and practical value.