This part of ISO 5815 specifies determination of the biochemical oxygen demand (BOD) of waters of undiluted samples. It is applicable to all waters having biochemical oxygen demands greater than or equal to 0,5 mg/l of oxygen (the limit of determination) and not exceeding 6 mg/l of oxygen. The results obtained are the product of a combination of biochemical and chemical reactions. They do not have the rigorous and unambiguous character of those resulting from, for example, a single, well-defined, chemical process. Nevertheless, they provide an indication from which the quality of waters can be estimated. The test can be influenced by the presence of various substances. Those which are toxic to microorganisms, for example bactericides, toxic metals or free chlorine, inhibit biochemical oxidation. The presence of algae or nitrifying microorganisms can produce artificially high results. In these situations a modification of the method may be necessary. Annex A describes alternative incubation periods. Annex B describes procedures for modification of the method by addition of seeding material, salts, inhibition of nitrification by allylthiourea (ATU) addition, neutralization, homogenization and/or filtration. These modifications may be found necessary for specific evaluations of the water quality of receiving waters. Annex C provides precision data.