Tolerance of anaerobic digestion sludge to heavy metals: COD removal, biogas production, and microbial variation

This study used six identical reactors to investigate the effects of heavy metals on anaerobic digestion sludge. The addition of 1–50 mg L−1 of Zn(II), Cd(II), Pb(II), Hg(II), and a mixture of four metals had a slight impact on COD removal. The removal efficiency decreased slightly to 92.9%, 92.0%, 92.7%, 91.8%, and 93.7% compared with 95.8% in control when 50 mg L−1 of heavy metals were added. The biogas production also varied slightly when exposed to 1–50 mg L−1 of heavy metals, with average production rates of 0.271, 0.244, 0.263, 0.269, 0.255, and 0.261 L d−1, respectively. Extracellular polymeric substance increased to protect microorganisms from the toxicity of heavy metals, and dehydrogenase activity increased from 0.42 to 1.20, 0.44, 0.51, 0.71, and 0.59 EU g−1 SS, respectively. The relative abundance of Methanothrix decreased from 91.2% to 88.5%, 87.6%, 85.7%, and 87.0% when Cd(II), Pb(II), Hg(II), and the mixture of heavy metals were added. Among the heavy metals, Zn(II) had the least impact on microbial activity but the most severe inhibition on biogas production, while the mixture of heavy metals showed relatively slight inhibition due to an antagonistic effect. Anaerobic digestion sludge exhibited good tolerance to heavy metals and could be a potential treatment option for heavy metal-containing wastewater. Anaerobic digestion has a promising application prospect for treating organic wastewater. Heavy metal pollution exists in many industrial wastewaters. However, few studies have comprehensively compared the effects of Zn(II), Cd(II), Pb(II), Hg(II), and the mixture of heavy metals on anaerobic digestion, especially in the treatment of relatively low-strength wastewater. The main objectives of this study were to investigate the effects of heavy metals on COD removal, biogas production, sludge properties, and microbial characteristics of anaerobic digestion sludge. The results have proved that the anaerobic digestion process can be used as a pretreatment unit for treating heavy metal-containing wastewater.