Cyanide are formed by many industriell processes. The chance of cyanide formation is highly in the iron metallurgy. In the blast furnace process, cyanide is formed in the high temperature area above the tuyeres. In the top gas scrubber the cyanides are transferred to the scrubber water. Because its adverse effect on the environment, cyanide must be removed from the waste water. Therefore many different processes are available. One of them is the biological treatment. The disadvantage of this process is that the process its not applicable on high cyanide concentration. In addition to the biological treatment, the INCO-process, the Degussa-process, the membrane process and the removal of cyanide with caros acid are also good alternative methods for the cyanide removal.