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

有机废物的厌氧消化包括两个微生物步骤：产酸和产甲烷。在产酸步骤中，产生挥发性脂肪酸，并导致pH值降低。重金属在这种条件下更易溶解。本研究采用改良的污泥处理工艺，结合产酸反应及固液分离装置，研究污泥中重金属的去除。实验在30 ℃下以分批或连续模式进行。使用具有pH和/或ORP控制的101 CSTR作为培养容器。结果表明，pH值稍低时，Cd、Ni、Zn的去除率可达50%以上，ORP升高时，As的去除率可达50%以上，Hg、Cr、Cu、Pb的去除率较低。去除效率的顺序似乎取决于重金属与硫化物的沉淀形成强度。曝气控制氧化还原电位（ORP）破坏了大量的有机物，导致可供后续产甲烷步骤使用的有机物数量减少，认为酸化副产物使重金属保持在比理论计算高得多的水平，这是由于络合物的形成。这一现象在废活性污泥中表现得比初沉污泥更为明显，因为重金属浓度与上清液中的高分子有机物浓度有很好的相关性，这种改良的污泥处理工艺比单独用于重金属去除的工艺具有经济优势。需要进一步研究化学方面，如复杂的形成和生物方面，如改善产酸步骤。

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.