Abstract
Anaerobic digestion is a promising technology for energy recovery from secondary sludge, yet the presence of humic substances in wastewater limits anaerobic digestion. In particular, humic substances make secondary sludge denser and more compact, reducing the availability of organic matter for biodegradation. Here we review the impact of humic substances on the anaerobic process, with emphasis on humic substances properties, effect on sludge structure and composition, effect on hydrolysis, acidolysis and methanogenesis, evolution of humic substrances, and strategies to counteract negative impacts. Strategies include removing humic substances, pretreatment of secondary sludge prior anaerobic digestion, and addition of metal salts, enzymes and organisms. We observed that humic substances with a high E4/E6 ratio, representing the absorbance determined at 465 nm and 665 nm, with a low carbon/nitrogen ratio, and with a low aromaticity are easier to digest anaerobically. The liquid–solid phases distribution of humic substances influences the efficiency of anaerobic digestion, and the repolymerisation of humic substances during anaerobic digestion reduces sludge degradability.
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Qilin Wang acknowledges Australian Research Council (ARC) Future Fellowship (FT200100264) and ARC Discovery Project (DP200100933).
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ZW: Conceptualization, Writing Original Draft, Critical Thinking. XL: Critical Thinking, Validation, Review & Editing. Muhammad Ahmar Siddiqui: Review & Editing. HL: Review & Editing. Ting Zhou: Review & Editing. LZ: Review & Editing. Siyu Huang: Review & Editing. Li Gao: Review & Editing. CSKL: Review & Editing. QW: Supervision, Review & Editing.
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Wang, Z., Li, X., Siddiqui, M.A. et al. Effect of humic substances on the anaerobic digestion of secondary sludge in wastewater treatment plants: a review. Environ Chem Lett 21, 3023–3040 (2023). https://doi.org/10.1007/s10311-023-01632-z
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DOI: https://doi.org/10.1007/s10311-023-01632-z