1. General Discussion
Biochemical oxygen demand (BOD) testing determines the relative oxygen requirements of wastewaters, effluents, and polluted waters. Its widest application is in measuring waste loadings to treatment plants and in evaluating a plant’s efficiency in removing BOD. The BOD test measures the molecular oxygen used during a specified incubation period to
• biochemically degrade organic material (carbonaceous demand),
• oxidize inorganic material (e.g., sulfides and ferrous iron), and
• oxidize reduced forms of nitrogen (nitrogenous demand) unless an inhibitor is added to prevent such reduction.
The seeding and dilution procedures provide an estimate of BOD at pH 6 to 8.
The methods below measure oxygen consumed in a 5-d period (5210 B), oxygen consumed after 60 to 90 d of incubation (5210 C), and during continuous oxygen uptake (5210 D). Other BOD methods published elsewhere may use shorter or longer incubation periods; tests to determine oxygen-uptake rates; and alternative seeding, dilution, and incubation conditions to mimic receiving-water conditions, thereby estimating the environmental effects of wastewaters and effluents.
The ultimate BOD (UBOD) test measures the oxygen required to totally degrade organic material (ultimate carbonaceous demand) and to oxidize reduced nitrogen compounds (ultimate nitrogenous demand). Ultimate BOD values and appropriate kinetic descriptions are needed in water-quality modeling studies [e.g., UBOD:BOD5 ratios for relating stream assimilative capacity to regulatory requirements; definition of river, estuary, or lake deoxygenation kinetics; and instream ultimate carbonaceous BOD (UCBOD) values for model calibration].
A number of factors (e.g., soluble versus particulate organics, settleable and floatable solids, oxidation of reduced iron and sulfur compounds, or lack of mixing) may affect the accuracy and precision of BOD measurements. Presently, there are no effective adjustments or corrections to compensate for these factors.
2. Carbonaceous Versus Nitrogenous BOD
Microorganisms can oxidize reduced forms of nitrogen, such as ammonia and organic nitrogen, thus exerting nitrogenous demand. Nitrogenous demand historically has been considered an interference in BOD testing; adding ammonia to dilution water contributes an external source of nitrogenous demand. The interference from nitrogenous demand can be prevented by an inhibitory chemical.1 If an inhibitory chemical is not used, the measured oxygen demand is the sum of carbonaceous and nitrogenous demands.
Measurements that include nitrogenous demand generally are not useful for assessing the oxygen demand associated with organic material. Nitrogenous demand can be estimated directly from ammonia nitrogen, and carbonaceous demand can be estimated by subtracting the theoretical equivalent of the nitrite and nitrate produced in uninhibited test results. However, this method is cumbersome and subject to considerable error. Chemical inhibition of nitrogenous demand provides a more direct, reliable measure of carbonaceous demand.
The amount of oxidation during a 5-d incubation of nitrogenous compounds depends on the concentration and type of microorganisms that can carry out this oxidation. Organisms capable of oxidizing nitrogenous compounds quite often are present in raw or settled primary sewage in adequate numbers to oxidize enough reduced nitrogen forms to contribute oxygen demand in the 5-d BOD test. Most biological treatment plant effluents contain enough nitrifying organisms to cause nitrification in BOD tests. Because nitrogenous compounds can be oxidized in such samples, nitrification inhibition (as directed in 5210 B.5e) is recommended for secondary-effluent samples, samples seeded with secondary effluent, and polluted water samples.