Basic investigation on the chemical forms of heavy metals in a sewage treatment plant

Abstract

Sewage sludge mainly consists of organic matter, and it is rich in nutrients. Therefore, sewage sludge is beneficial as an energy resource and as a raw material for fertilizer. However, heavy metals in it are the obstacle to utilization of sludge. This study was conducted in order to know the chemical forms of heavy metals and their behavior in a sewage treatment plant as a basic study for development of technology to reduce heavy metals in sludge. Chemical methods and biological methods, such as extraction using acid, bacteria leaching, are applied to reduce heavy metals. In using these methods, the efficiency depends on the chemical forms of heavy metals. From this point of view, factors, which affect the chemical forms of heavy metals, were investigated through the survey at some plants of different conditions. Besides, experiments on the anaerobic digestion process, in which the chemical forms of heavy metals were expected to change, were conducted. As the results of these studies, it was found that the chemical forms of heavy metals and their behavior vary according to the species of heavy metals. In comparison of zinc (Zn), copper (Cu) and nickel (Ni), Cu was found to be changed most easily into a stable chemical form such as carbonate and sulfide. Zn was the second to Cu in its tendency to be stabilized. As for Ni, the tendency was further less than Zn.