BOD Measurement

The determination of biochemical oxygen demand is always a topic of discussion with regard to the method and its applicability.

In addition to other recognized methods (EN 1899-1/EN 1899-2), Carl Stuart offers OXITOP respirometric solution. This procedure comes closest to representing the natural conditions of biological degradation. Interference with the sample solution is kept to a minimum. Basically, respirometric measurement using the OxiTop system is nothing more than a small-scale wastewater treatment plant, poured into a bottle and operated in the absence of air.

Together with a full range of thermal cabinets Carl Stuart offers the complete measuring station for routine BOD-5 analysis in the simplest and most accurate way possible. Additionally, we are also offering exclusive “Web-Customers only” promotion with 15% discount for all OXITOP products.

Biological background of all BODn determinations

The primary difference between BODn determination and other measurements such as pH, conductivity, oxygen, COD, nitrate, etc., is the fact that biological systems rather than chemical or physical characteristics are being examined. The biochemical oxygen demand results from the respiratory processes of microorganisms and microorganisms are alive!

Let us consider an extreme comparison: Bacteria require certain living conditions just as humans beings do. While we do not feel comfortable at the North Pole or when our food is withheld, bacteria similarly place specific demands on their environment. These can be quite variable and extreme because microorganisms are highly adaptable. The bacteria that can be expected to be present in a municipal water treatment plant require a pH range around the neutral point of pH 7 and a balanced supply of nutrients that is guaranteed by an adequate pollutant load including carbon, nitrogen and phosphorus. They react to temperature fluctuations with a reduced degradation. In light of this background, it is easy to understand why operators of biological water treatment plants make an effort to protect their .biology. from harmful foreign influences. However, these relationships not only affect the water treatment plant directly, but also have an influence on the BODn determination.

A BODn determination is only possible with an adapted biology that must not be damaged, inhibited or destroyed by the sample. It is essential that the microorganisms are compatible with the sampled water. For this reason, it is best to use microorganisms that .know. the sampled water, i.e. that have adapted to it.

Waters, or test substances, that contain inhibiting, disinfecting or even toxic agents interfere with the microbiology. Waters containing these substances do not have a BODn. Measurement results in these waters can only provide information on the toxicity of the substances used.

Respirometric BODn determination The determination of the BOD concerns the determination of the degradation of organic substances by microorganisms. The main application of respirometric BODn determination lies in the analysis of wastewater in wastewater treatment plants. Respirometric measurement in a bottle corresponds to the processes in a wastewater treatment plant, but on a greatly reduced scale. At the same time, the analysis can be used for various aqueous media, e.g. in flowing or standing surface waters and in natural as well as artificial waters. The measurement period can vary widely. For the classification and evaluation of the degradation performance of a wastewater treatment plant (with the exception of some Scandinavian countries), it is customary to specify the BOD5. In this case, the analysis time is 5 days. During this time, the measurement solution must be incubated at 20°C, i.e. the sample bottle is thermostatted to (20 ± 1)°C in an incubator for the entire measurement duration. Some Scandinavian countries specify the BOD7 value. In a seven-day incubation period, a measurement that is started on Tuesday is also completed on Tuesday; however, in a BOD5 measurement, it is completed on Sunday. And who wants to go into the wastewater treatment plant on a Sunday to read the measured values! In the days of the mercury manometer, the BOD7 measurement offered a distinct advantage as measurements could be made on practically any day. The use of the OxiTop system now also makes this possible for the BOD5 measurement because it automatically stores the measured values. The measured values, even those recorded on Sundays and holidays can be read just as well several days later. A further advantage of the OxiTop system is, moreover, the mercury-free pressure measurement. Many laws and regulations namely call for the avoidance of chemicals and substances that are injurious to health! Compared to the other recognized methods, this procedure comes closest to representing the natural conditions of biological degradation. Interference with the sample solution is kept to a minimum.

Basically, respirometric measurement using the OxiTop system is nothing more than a small-scale wastewater treatment plant, poured into a bottle and operated in the absence of air.
All of the oxygen required for consumption comes from the graduated measuring flask. This includes not only the dissolved oxygen but also oxygen from the gas phase (air above the measurement solution). The partial oxygen pressures, i.e. the amounts of oxygen in the aqueous phase and in the gaseous phase, are balanced. Constant vigorous stirring ensures a good exchange of gas between the two phases.

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