Abstract: Trace metal elements (TME) play a crucial regulatory role in the synthesis of enzymes and metabolic activities of microorganisms within anaerobic digestion systems. The efficiency of TME in the three stages (hydrolysis and acidogenesis, acetogenesis, methanogenesis) of anaerobic digestion was comprehensively evaluated. It was concluded that TME not only promoted the hydrolysis efficiency of granular organic matter and enhanced the metabolic activity of acidogenic bacteria, but also significantly improved system stability and methane production by dynamically regulating the metabolic balance between acidogenic and methanogenic bacteria. Due to the typical characteristic of low-concentration promotion and high-concentration inhibition of TME in the digestive system, its dosing strategy was difficult to quantify. Therefore, the effects of key parameters such as substrate organic composition, inoculum sludge properties, temperature, organic loading rate, and TME addition methods on the requirement of TME dosing strategy were further investigated. The effective methods for changing the chemical form of TME and improving its bioavailability through chelating agents were further summarized. The potential of extracellular polymers (EPS) as natural green ligands was particularly explored, its rich functional groups could effectively regulate the bioavailability of TME through mechanisms such as electrostatics, complexation and adsorption, providing a new perspective for optimizing the TME dosing strategy.

Key words: trace metal elements, anaerobic digestion, bioavailability, chelating agents, extracellular polymeric substances