Enhancement Of Anaerobic Digestion Of Waste Activated Sludge Based On Zero-valent Iron/ Iron Oxide

Anaerobic digestion is considered to be an important development direction of sludge biological treatment technology in the future due to the characteristics of high resource recovery efficiency and low environmental impact.However,the complex flocculent structure and cell walls in the sludge often limit the release of sludge organic matter,which makes hydrolytic acidification a"rate-limiting"step for anaerobic digestion and methane production.In addition,the metabolic method of traditional interspecies hydrogen transfer（IHT）is slow and the metabolic balance is easily destroyed.How to break the"speed limit"step and strengthen the electron transfer between species has become the research focus of the new technology of sludge anaerobic digestion.Generally,a variety of pretreatment methods are used to promote the hydrolysis and acidification of waste activated sludge.However,pretreatment may introduce inhibitors that are not conducive to subsequent methane generation,and additional energy consumption and cost input need to be considered.Studies have shown that the addition of zero-valent iron（ZVI）into the anaerobic system can effectively reduce the impact of unfavorable factors and improve the efficiency of anaerobic digestion.In addition,iron oxides（such as magnetite,etc.）on the one hand can enhance the degradation of organic matter through dissimilar iron reduction（DIR）,and could be used to"replace"traditional pretreatment to enhance hydrolysis and acidification;on the other hand,they can participate in direct interspecific electron transfer（DIET）to enhance the efficiency of extracellular electron transfer,which opens up a new channel to strengthen anaerobic digestion.In response to the above-mentioned problems,this thesis first used calcium peroxide（Ca O₂）pretreatment combined with ZVI in the conventional anaerobic digester to explore the synergistic effect and mechanism.Secondly,this thesis used magnetite to replace the pretreatment step and combined ZVI to construct a new model of two-stage anaerobic fermentation,which explored the possibility of further reducing costs and maximizing methane production.Finally,iron-containing sludge was used to prepare iron-containing sludge-based biochar rich in iron oxides and functional groups,which is expected to effectively increase the extracellular electron transfer and improved anaerobic digestion.The main research contents and results are as follows:（1）CaO₂ pretreatment is proven to be an effective method to improve the efficiency of hydrolysis,but the subsequent anaerobic digestion process is affected by production residues（such as active free radicals and ammonia nitrogen）.In order to study the role of Ca O₂ pretreatment combined with ZVI in the anaerobic digestion of waste activated sludge,Ca O₂ pretreatment,ZVI,and their combination were applied to sludge anaerobic digester.The results showed that the reactors with Ca O₂ pretreatment and ZVI addition have the best performance.Compared with the digestion treatment with only Ca O₂ pretreatment and ZVI added,the cumulative methane increased by 9.7%and 13.8%,and the sludge reduction increased by 4.0%and 6.0%.Further mechanism analysis showed that ZVI can enhance the activity of the electron transport system（ETS）and coenzyme F₄₂₀,thereby reducing the inhibitory effect of residual active free radicals and ammonia nitrogen on methanogens.The analysis of the microbial community showed that ZVI can accumulate hydrogenotrophic methanogens with stronger ammonia tolerance.This study provides an effective method for pretreatment coupled with iron materials to enhance anaerobic digestion of waste activated sludge.（2）A new two-stage anaerobic digestion model was constructed using magnetite"replace"pretreatment step,combined with ZVI.Simply put,magnetite was added in the hydrolysis acidification phase,and ZVI was added in the methanogenesis phase.The results showed that compared with the reactor containing only magnetite and zero-valent iron,the methane production in the new model increased by 10.2%and 18.1%,and the COD removal increased by 7.9%and 10.9%,respectively.In the new digestion mode,magnetite could enhance the decomposition and acidification of organic matter through dissimilation iron reduction,providing more available substrates,while avoiding the competition between magnetite and methanogens for electrons.ZVI could stimulate enzyme activity and enrich hydrogenotrophic methanogens to promote methane production,which in turn strengthen the anaerobic digestion of waste activated sludge.（3）According to the above research results,iron-containing sludge-based biochar was prepared by pyrolysis of iron-containing sludge.In order to clarify the influence of temperature on the properties of biochar,the properties of iron-containing sludge-based biochar at different temperatures were characterized and applied to the anaerobic digestion of waste activated sludge.The results showed that biochar prepared at 600°C has richer functional groups and better electron transfer capacity,which can effectively improve the efficiency of microbial extracellular electron transfer,thereby improving the efficiency of anaerobic digestion.This study also provides a new idea for the treatment and reuse of iron-containing sludge.