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Phone: 631-242-4249 | Toll-Free: 888-686-3454
Besides the chemical oxygen demand (COD), the biochemical oxygen demand after 5 days (BOD5) is the most important cumulative effect parameter for assessment of the degree of contamination of a water body.
The BOD is defined by the volume-related amount of oxygen which is consumed by micro-organisms to degrade the organic matter present in the water at 20°C by oxidation. In general, this determination is made after 5 days, why it is often referred to the BOD5 value.
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The BOD provides information on the biologically utilizable portion of organic contents in the sample solution. The determination is made by the measurement of the consumption of oxygen required for the oxidative degradation. The information is indicated in mg/L O2.
Alternatively, the carbon dioxide content can be measured instead of the oxygen content by so-called respirometric methods. Added sodium hydroxide reacts with the developing CO2 gas to form sodium carbonate, which creates a negative pressure.
The more organic nutrients are present in the sample, the higher is the metabolic activity of the bacteria and hence the resulting BOD value. A statement about the quality of a water sample and its biological activity can be made based on the BOD value.
The BOD is often evaluated in the context of the COD. If the difference between the two values is small, the sample consists mostly of readily biodegradable substances. If the difference is, however, very large, this can be attributed to a poor bio-degradability.
Nitrogen degradation also takes place, in addition to carbon degradation, in the sample solution. Nitrogen degradation, also known as nitrification, is the conversion of ammonia into nitrate. This happens also under oxygen consumption. In general, nitrification occurs later than carbon degradation. Nitrification inhibitors are used in order to exclude any disturbance on the BOD that might nevertheless occur, especially in lightly contaminated samples. The most commonly used compound is N-allylthiourea (ATH or NATH).
At high BOD concentrations, the sample must be treated with dilution water. Dilution water is unchlorinated tap water aerated for at least one hour in the dark. The oxygen concentration of the dilution water should be at least 8 mg/L.
Inoculating water, which is aerated inoculated dilution water, must be used for dilution, if the sample was frozen or does not contain enough micro-organisms. Inoculating water is obtained from microorganism-rich water such as sedimented municipal wastewater from the runoff of the primary treatment or from the mechanical feed. More information can be found in the instruction leaflet of the BOD accessories set (REF 916 925).
The BOD content is determined by measuring the oxygen concentration. Micro-organisms consume oxygen during the degradation of organic substances into carbon dioxide.
The simplified determination of the biochemical oxygen demand after 5 days (BOD5) is done with undiluted samples and without using a control in accordance with the German norm DIN EN 1899-2-H52. The oxygen-enriched, undiluted sample is incubated in a test tube for 5 days at 20 ± 1 °C in the dark. The determination of the dissolved oxygen after 5 days is based on the Winkler method. The incubation of the sample and the oxygen measurement (after 5 days) take place in the same test tube (REF 985 825). In the standard method, the BOD5 is determined according to the so-called dilution principle. The oxygen concentration is measured immediately after sample preparation and after five days of incubation in Winkler bottles (REF 985 822). This method is in accordance with the German norm DIN EN 25813-G21.
