The effect of heavy metals on methane production and hydrogen and carbon monoxide levels during batch anaerobic sludge digestion

Heavy metals can cause upset in the anaerobic digestion of municipal sludges. Using serum bottle assays, the impact of three transition heavy metals—copper, zinc and cadmium on methane production rate and trace hydrogen and carbon monoxide gas levels during the digestion of waste activated sludge was examined. Results indicate that normal substrate flow is interrupted at a point in the degradation sequence prior to the methanogenic steps by the pulse addition of heavy metals. Substrate pulsing experiments were used to help assess which groups of the anaerobic ecosystem were inhibited by heavy metal addition. Results indicate hydrogen monitoring holds promise to be an effective means to examine digester upset brought on as a consequence of heavy metal induced inhibition. Results also demonstrate the need to supplement pure and enrichment culture studies with assays that examine the response of the entire ecosystem of interest. A characteristic CO response was also observed and may be useful in detecting metal induced inhibition.