Abstract
Anaerobic fermentation with biogas as an energy source is influenced by the presence of heavy metals. However, the availability of the heavy metals is dependent on the digestion temperature. In this study, the impacts of Cd on the characteristics of biogas, substrate biodegradation, and enzyme activity during anaerobic co-digestion were investigated under varying digestion temperatures. The results showed that 1 mg/L initial Cd concentration improved cumulative biogas yields by 404.96%, 16.93%, and 5.56% at 55 °C, 45 °C, and 35 °C, respectively. In contrast, at low temperatures (25 °C), the yield decreased by 0.77%. In the 55 °C group, Cd addition improved the activity of cellulase (p < 0.05) and coenzyme F420 (p < 0.01). The total chemical oxygen demand (COD) during the peak period and the transformation of hydrolytic organic components into volatile fatty acids (VFAs) influenced the CH4 and biogas yields. There were no significant differences in cellulase, dehydrogenase, and coenzyme F420 activities with or without Cd addition when the digestion temperature was 45 °C, 35 °C, and 25 °C. Therefore, thermophilic digestion is recommended for the efficient degradation of Cd-contaminated biowaste. Moreover, the impact of metals on the performance of anaerobic digestion should be considered together with temperature conditions in future research and practice.
Highlights
Lignocellulosic biomass wastes, which are being generated in large amounts annually, can be used as an energy resource by sustainable technologies, such as anaerobic digestion [1,2,3]
This study examined the combined effects of temperatures and Cd addition on anaerobic digestion
The addition of 1 mg/L initial Cd concentration into the anaerobic digestion improved the biogas yields of the 55 ◦ C group, followed by 45 ◦ C and 35 ◦ C groups, while it reduced the yield of the 25 ◦ C group
Summary
Lignocellulosic biomass wastes, which are being generated in large amounts annually, can be used as an energy resource by sustainable technologies, such as anaerobic digestion [1,2,3]. The digestion microorganism in the reactors requires a trace amount of certain metals for optimal growth and performance [4]. Only few studies showed that the biogas yields during anaerobic digestion of bioenergy crops were enhanced by Cd contamination at a certain concentration range [5,8,13,14]. Cd concentrations of 2.00 ± 0.44, 39.80 ± 1.25, and 6.37 ± 0.15 mg Cd/kg biomass in the shoot of canola, oat, and wheat improved the biogas yields by 59.37%, 79.23%, and 11.34% compared to the control group under thermophilic conditions (55.0 ± 1.0 ◦ C) [8]. A study on biogas production with maize contaminated with Cd as feedstocks (the final Cd concentration of residues achieved 5.34 mg/kg) found no inhibitory effects during the anaerobic digestion process (42 ◦ C) [7].
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