Assessment of biomass activity by respirometric tests

In activated sludge systems, organic pollutants are degraded via aerobic biological processes. This degradation takes place in aeration tanks through the action of heterotrophic microorganisms, requiring large amounts of oxygen, which are accompanied by significant energy costs. The degradation rate of organic matter by a microbial culture is exclusively linked to the oxygen supply (and, consequently, consumption) rate. Therefore, the most effective method to assess the performance of biological processes in activated sludge plants is the Oxygen Uptake Rate (OUR), also known as the OUR test.

The Oxygen Uptake Rate (OUR) and the specific Oxygen Uptake Rate (sOUR), calculated by dividing the OUR by the biomass concentration, are also known as respiration rates. The OUR test is mainly used to assess the biomass activity in an activated sludge system and, indirectly, the quality of the feed (organic matter in the influent) and the effluent.

Along with the removal of organic pollutants, wastewater treatment plants using the activated sludge process aim at the extensive removal of nitrogenous components through biological nitrification – denitrification. Nitrification is a purely aerobic process in which specific groups of bacteria (nitrifying bacteria) oxidise the ammonia in the wastewater to nitrate ions (NO₃^–). On the other hand, during denitrification, the nitrate ions, being produced during the nitrifying stage, are eventually reduced to nitrogen gas through the action of other bacterial species, called denitrifiers, under oxygen-limited conditions. The activity of these sensitive groups of microorganisms, on which the nitrogen removal efficiency of a plant is entirely dependent, is assessed via – similar to OUR – tests, from which the Ammonia Utilization Rate (AUR) and the Nitrate Utilization Rate (NUR) are derived.

Performance of OUR, AUR and NUR tests can give the following very important information regarding:

Assessment of activated sludge activity

Evaluation of biomass health (incl. possible inhibition by toxicity)

Optimization of aeration in the biological reactor

Prediction and monitoring of population rearrangements in the microbial flora

Fast prediction and correction of activated sludge instabilities

Identification of root causes of problematic nitrogen removal