Biochemical oxygen demand (BOD) is the measurement of oxygen consumed by living microorganisms (primarily bacteria) while utilizing the organic matter present in a water or waste sample under conditions as close as possible to those that occur in nature. Microbial conversion of nitrate to nitrite will also contribute to BOD. Non-biodegradable (or refractory) organic compounds will not contribute to BOD.
In order to make BOD test quantitative the samples mus to be protected from the air to prevent reoxygenation as the dissolved oxygen concentration diminishes. Also, high concentration wastes must be diluted to ensure that dissolved oxygen will be present throughout the test duration. It is important that the environmental conditions during the test be suitable for the living ‘ organisms. Therefore nutrients are added to the dilution water. Nutrient blanks must be included in the test to ensure that the nutrient-enriched dilution water does not contribute BOD. For industrial wastes which may have low populations of microorganisms, seed microorganisms may be required.
The rate at which biologically medicated reactions proceed is primarily governed by the microbial population size and temperature. For this reason, BOD tests are carried out at a constant temperature of 20°C.
Complete biodegradation of the organic material in a water sample may take 14 to 20 days. However, 60 to 70% of the BOD in most domestic and industrial waste is expressed within 5 days. Therefore, in order to make BOD analyses less time consuming (1′. e. make it possible to complete the analyses within a 5 day work week), the BOD measurement was adopted and is the basis for many’ regulatory standards.
BOD can also be defined as the amount of oxygen required by bacteria while stabilizing decomposable organic matter under aerobic conditions. The quantity of oxygen required may be taken as a measure of its content of decomposable organic matter. The rate of BOD exertion is governed by the characteristics of sewage, its decomposable organic content, bacterial population, and temperature.