The test relies upon a microbial ecosystem with enzymes capable of oxidizing the available organic material. Some waste waters, such as those from biological secondary sewage treatment, will already contain a large population of microorganisms acclimated to the water being tested.  An appreciable portion of the waste may be utilized during the holding period prior to commencement of the test procedure.  On the other hand, organic wastes from industrial sources may require specialized enzymes.  Microbial populations from standard seed sources may take some time to produce those enzymes. A specialized seed culture may be appropriate to reflect conditions of an evolved ecosystem in the receiving waters.