The biological or biochemical oxygen demand indicates the amount of oxygen which is needed for the biological degradation of organic substances in water.
Generally, the BOD5 is perceived as the BOD for controlling waste water treatment plants (WWTP). However there are nitrification inhibitors added to the BOD5 to surpress the nitrogen degradation. Obviously, the nitrification is an important part of the biological degration of organics and thus is very important for the control of WWTPs. Consequently, the BOD5 ist poorly suited for the control of waste water treatment plants.
Alternatively, the total BOD is well-suited, due to the determination of the nitrogen as well as the carbonaceous part of biological degradation.
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.
Some test methods that are available on the market can detect toxicity, but they do not however identify exactly which toxins are present. By using fish, daphnia, molluscs, algae or luminous bacteria, they simply test whether a water sample has a toxic effect.