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：酸化和产甲烷。在酸化过程中，会产生挥发性脂肪酸，从而导致pH值下降。在这样的条件下，重金属可以更好地溶解。本研究采用酸化反应和固液分离装置相结合的污泥处理工艺，对污泥中重金属的去除进行了研究。实验在30ﾟC下以间歇或连续模式进行。采用具有pH和/或ORP控制的101CSTR作为培养容器。结果表明，在pH略有降低的情况下，Cd、Ni、Zn的去除效率可达50%以上，提高OR值可使As的去除效率达到50%以上，而对汞、铬、铜、铅的去除效率较低。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。曝气控制的ORP以好氧方式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较少。人们认为，由于复杂的形成，酸化副产物使重金属保持在比理论计算高得多的水平。这一现象在废弃活性污泥中表现得比原始污泥更明显，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。与单独应用于重金属去除的工艺相比，这种改良的污泥处理工艺具有经济优势。需要在化学方面进行进一步的研究，如络合物的形成和生物学方面的研究，如改进酸化步骤。

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.