Advanced Respirometry

Non-invasive Tracking of Oxygen, Carbon Dioxide and pH as Indicator for Nitrification Activity

  • Fig. 1: Set-up for respirometric assays: A closed vessel is placed on a magnetic stirrer; all runs were conducted in 300 ml glass bottles with integrated sensor spot, which are read out through the vessel wall via optical fibers. The size of the sensor spots would allow to use even smaller vessels.Fig. 1: Set-up for respirometric assays: A closed vessel is placed on a magnetic stirrer; all runs were conducted in 300 ml glass bottles with integrated sensor spot, which are read out through the vessel wall via optical fibers. The size of the sensor spots would allow to use even smaller vessels.

Respirometry is an indirect way of measuring microbial activity, such as nitrification. The oxygen uptake is proportional to the substrate consumption - given by the stoichiometry of the process. Combining the oxygen uptake with the release (heterotrophic) or uptake (autotrophic) of carbon dioxide and pH changes extends the applicability of these respirometry assays. The measurement of combined processes becomes feasible as tracking pH and carbon dioxide allows allocation of the oxygen uptake to different processes on a pro-rata basis illustrated here for autotrophic nitrification.

Nitrification is a key process in the natural nitrogen cycle and is used in biological wastewater treatment. It describes two successive biochemical reactions which are performed by different groups of bacteria: The first step is the oxidation of NH4 + to NO2 - (see Eq. 1), the so-called nitritation, which is carried out by ammonium oxidizing bacteria (AOB). The conversion of each mole NH4 + requires 1.5 moles O2 and additionally releases two hydrogen ions which lowers the pH. For the further oxidation of NO2 - to NO3 - (see Eq. 2), the so-called nitratation, which is carried out by nitrite oxidizing bacteria (NOB), only 0.5 moles O2 per mole NO2 - are required and pH does not change.

NH4 + + 1.5 O2 →NO2 + H2O + 2 H+ (Eq. 2)
NO2 - + 0.5 O2 → NO3

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