A comprehensive review of the impact of trace elements on anaerobic digestion for organic solid wastes

The deficiency of inherent trace elements (TE) limits the performance and product yields in anaerobic digestion systems for organic solid wastes (OSW). External TE supplementation is necessary to ensure the synthesis of enzymes and promote the growth and metabolism of anaerobic microorganisms. TE content of the system has exerted a certain influence on the hydrolysis efficiency of particle substrate, as well as the level of hydrolysis and acidification, and the growth of acidogenic bacteria. Due to the pivotal role of TE in the synthesis of metalloenzymes and regulation of methanogen metabolism, TE concentration exerts a significant positive or negative impact on the stage of methanogenesis. Comprehensive effects of TE on the methane yields, digestion performance, system stability, and ecological functions of methanogens were evaluated using some specialized substrates with different type of organic composition, nitrogen and sulfur contents as illustrative examples. The variation in organic characteristics of digestion substrate, inoculum type, temperature, organic loading rate, digester type, dosing strategies and synergistic effect are significant factors that contribute to the need for tailored TE dosing requirements in all OSW digestion systems. The estimation methods for TE requirements and dosages, considering substrate characteristics and various operational parameters, have been comprehensively reviewed. Additionally, the bioavailability of TE is mainly influenced by their bioavailable chemical speciation. Synthetic chelating agent such as citric acid (CA), ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA) and ethylenediamine-N, N-disuccinic acid ([S,S]-EDDS), have been extensively demonstrated to effectively interact with ligands, thereby enhancing the bioavailability of TE and subsequently reducing the dosage of TE in the digestion process. However, the non-biodegradable and persistent nature of synthetic chelating agents, coupled with their potential carcinogenicity, often limits their application in the environment. It's worthing that the natural microbial secretion, such as soluble metabolic products (SMPs) and extracellular polysaccharides substances (EPS), which encompass a variety of functional groups, negative charge, and numerous binding sites, demonstrate significant potential in complexing with TE through chelation and adsorption effects, thereby enhancing the bioavailability of TE.