The first step in the Homestake biological treatment process is the oxidative breakdown
of cyanides and thiocyanate, and subsequent sorption and precipitation of free metals into
the biofilm. Cyanide and thiocyanate are degraded to a combination of ammonia,
carbonate, and sulfate.
The second step converts ammonia to nitrate through the conventional two-step nitrification process with nitrite as the intermediate.
Various Pseudomonas species are responsible for complete oxidation of cyanide, thiocyanate,
and ammonia. A schematic of the basic aerobic biotreatment process is shown in Fig. 1.
Biological treatment provides the most efficient and economical means of removing
nitrogen from wastewater in the form of ammonia or nitrate. Typically, a mixed population
of bacteria is alternately exposed to aerobic conditions for nitrification and then anoxic
conditions for denitrification. During nitrification, ammonia is first oxidized to the
intermediate nitrite slowly and then rapidly to nitrate.