Review of the Effects of Trace Metal Concentrations on the Anaerobic Digestion of Organic Solid Waste

Anaerobic digestion is a promising technology for producing methane-rich biogas. Various wastes, including waste activated sludge, food waste, farm waste, agricultural waste, and wastewater, have been used as substrates to generate biogas, which can be transformed into electrical or thermal energy. However, process instability and low methane yield restrict the widespread application of this process. Several strategies have been employed to increase methane yield and energy production, including adding trace elements, such as iron, cobalt, molybdenum, and nickel. Trace elements are part of the cofactors of enzymes involved in methane synthesis and microbial growth. This review aims to analyze the effects of trace elements on methane yield and propose concentrations at which methane production increases, as well as potential inhibitory effects. The addition of trace metals to the anaerobic digestion process has been found to have positive effects, such as enhanced methane yields and productivities, volatile fatty acid (VFA) removal, and chemical oxygen demand removal. An analysis of previously published data from other researchers was adjusted to a variant of the Haldane equation, suggesting that ranges of mg of trace metals per gram of substrate added promote an increase in methane production (0.56–1.67 mg/g VS for iron, 0.01–0.1 mg/g VS for cobalt, 0.03–0.5 mg Mo/g VS for molybdenum, and 0.04–0.5 mg Mo/g VS for nickel). Furthermore, the literature review revealed that the efficiency of the process decreases with increasing metal dose beyond the proposed range. The frequency of trace elements addition directly influences VFA removal. Some trace elements, such as nickel, generate VFAs of higher molecular weight, modifying the process performance. Notably, the process efficiency decreases with higher doses of trace elements, although this promotes the VFA removal.