The biological or biochemical oxygen demand indicates the amount of oxygen that is needed for the biological degradation of organic substances in water.
Generally, the BOD5 is perceived as the BOD for controlling wastewater treatment plants (WWTP). However, there are nitrification inhibitors added to the BOD5 to suppress nitrogen degradation. Obviously, nitrification is an important part of the biological degradation of organics and thus is very important for the control of WWTPs. Consequently, the BOD5 is poorly suited for the control of wastewater treatment plants.
Toxicity is described as the direct harmful effect of a substance on organisms. These effects can already occur at low concentrations of toxic substances and are dependent on the incubation period and the dosage.