Impact of trace element supplementation on mesophilic anaerobic digestion of food waste using Fe-rich inoculum

Abstract

Trace elements (TEs) play an indispensable role in enhancing the stability of anaerobic digestion (AD) of food waste (FW). Significant research on AD of FW with TE supplementation has been conducted with low Fe content inoculum. However, the use of Fe-rich inoculum is inevitable due to chemical phosphorous removal from wastewater in North America. We conducted comprehensive mesophilic batch tests to investigate the effect of TEs (Fe, Ni, Co, Se, and Mo) on FW digestion inoculated with Fe-rich sludge (≥ 1000 mg Fe L⁻¹). This paper presents the impact of supplementing various concentrations of TEs on specific methanogenic activity (SMA), maximum specific methane production rate (SMPR_max), and apparent hydrolysis rate constant (K_h). The addition of TEs adversely impacted methanogenic activity by 20 to 58% in the SMA tests. The effects of individual and mixed supplementation of TEs on the SMPR_max and K_h during FW digestion were negligible; exceptions include Fe, Mo, and Co. Final soluble TE concentrations were 10–29% of the initial soluble TEs. The high Fe concentration in the inoculum reduces the bioavailable fraction of added TEs via coprecipitation. Contrary with many literature reports indicating the need to supplement TE to improve FW digestion efficiency, with Fe-rich sludges, FW digestion does not require TE supplementation.