In wastewater monitoring, often only the BOD from the degradation of carbon compounds is determined (carbon BOD); with the addition of allylthiourea (ATH), nitrification is inhibited in order to prevent oxygen consumption through the degradation of nitrogen compounds. In general water monitoring, the total BOD is often measured as the sum of the degradation of carbon and nitrogen compounds; this is referred to as uninhibited BOD. Both measurement methods lead to very different results: in surface waters, the uninhibited BOD can reach three times the value of the inhibited BOD in summer.

Usually, the BOD5 is used. This value is the amount of oxygen in mg/l that bacteria and all other microorganisms present in the water consume within five days at a temperature of 20 °C, from which one can deduce the amount of organic substances degraded in the process. In addition, the BOD2 and the BOD∞ are occasionally determined, which indicate the oxygen requirement within two days or until respiration ceases, hypothetically until all biodegradable organic substances have been degraded. As a rule of thumb, BOD5 is about 70% of the BOD∞.

Wastewater constituents

The ratio to the chemical oxygen demand provides information about the type of wastewater constituents:

If BOD5 = (50 … 100) % COD, the ingredients are easily biodegradable.
If BOD5 < 50 % COD, the ingredients are difficult to biodegrade and therefore remain in the environment for a long time, or they are toxic to microorganisms and therefore difficult to break down.
BOD5 = (12 … 25) % COD: this is the ratio that wastewater usually has after biological treatment.
The BOD5 is only intended to record the organic carbon compounds. The result can be distorted by the biotic oxidation of ammonium ions or ammonia, i.e. by nitrification. Any oxygen consumption that may occur through nitrification must therefore be suppressed by the addition of nitrification inhibitors (allylthiourea).

Furthermore, the significance of the analysis of the organic degradability of the wastewater constituents can be distorted by the wrong choice of microorganisms. Adaptation of the microorganisms is only possible to a limited extent within 5 days, which reduces metabolism and leads to an incorrect result in terms of degradability.