Heavy metal removal by combining two phase anaerobic digestion and solids-liquid separation

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Anaerobic digestion of organic wastes consists of two microbiological steps ; acidogenesis and methanogenesis. During the acidogenesis step, volatile fatty acids are produced and the resulting pH decrease occurs. Heavy metals can be more solube under such conditions. In this research, heavy metal removal from sewage sludge was studied by applying the sludge treatment process modified to incorporate the acidogenesis reaction and a solids-liquid separation device. Experiments were performed at 30ﾟC in either batch or continuous mode. A 101 CSTR with pH and/or ORP control was used as a cultivation vessel. Heavy metals examined were Hg, As, Cd, Cr, Cu, Ni, Pb and Zn.The results showed that more than 50% of removal efficiency was achieved for Cd, Ni and Zn by a slight decrease of pH,and for As by an increase of ORP.Lower removal efficiency was obtained for Hg, Cr, Cu and Pb. The order of removal efficiency appears to depend on the precipitation formation strength of the heavy metal with sulfide. The ORP control by aeration destructed aerobically a significant amount of organics, resulting in less amount of organics available for the secceeding methanogenesis step.It is considered that by-products of acidogenesis keep heavy metals at a much higher level than thoretically calculated due to complex formation. This phenomenon was seen for waste activated sludge more clearly than for primary sludge, since heavy metal concentrations were correlated well with those of high molecular organic compounds in the supernatants.This modified sludge treatment process has an economical advantage over processes applied solely for heavy metal removal. Further investigations are needed on chemical aspects such as complex formation and biological aspects such as improving the acidogenesis step.