In a steady state, the OTR [g/m³/hr] of the aeration system (at a constant air volume flow) corresponds to the current respiration rate OUR of the microorganisms, whereby a corresponding dissolved oxygen value is established. If the respiration rate increases due to an increase in biological activity, the dissolved oxygen value decreases and vice versa.
Wastewater constituents and concentration can influence the efficiency of G/L oxygen transfer and therefore the OTR. Factors such as the presence of surfactants, dissolved solids, biomass concentration MLSS, SRT and temperature can influence the e.g. surface tension and viscosity of the liquid, which affects the mass transfer coefficient kLa and consequently the OTR. This is taken into account by the empirical alpha value (kLa wastewater/kLa pure water), i.e. OTR = alpha*kLa* (Cs-DO). In this sense, the OTR is not only determined by the oxygen consumption of the biology, but by the oxygen utilization of the aeration system under operating conditions with the amount of air supplied and the given wastewater constituents as well as the process characteristics (MLSS, SRT, sludge loading rate, etc.).
I agree that OTR = alpha*kLa* (Cs-DO). In this sense, the OTR is not only determined by the oxygen consumption of the biology, but by the oxygen utilization of the aeration system.
I suggest to account for oxygen consumption, the equation should be modified to OTR = alpha*kLa* (Cs-DO) - R, where R is the consumption rate of the microorganisms. In this case, the saturation concentration becomes a constant and can be determined by a test of the non-respiring system (i.e., where the microbes are filtered off or eradicated). I attach a pre-print concerning the gas-side oxygen depletion phenonmenon for your reference.
Dear Torsten
The current Standard Guidelines for In-Process Oxygen Transfer Testing has not included the effect of biochemical reactions for which I am suggesting a revision as added herewith.