Respirometry has been used in this work to assess the effect of S-0/X-0 ratio, knowing that this ratio is a very important operational parameter in the conduct of wastewater treatment plant. It affects its design and operation; part of the respirometry calculations, dissolved oxygen transfer coefficient K(L)a is calculate for each respixometric experiment, due to the fact that operational condition change. Under endogenous conditions OURend is calculated using the linear portion of the variation of DO concentration under no aeration conditions. Then the OURend is used in the endogenous phase at a no oxygen concretion variation to calculate the KLa. Samples of wastewater, with different S-0/X-0 ratios in the range between 0.01 and 0.23, containing sometimes ATU, a nitrification inhibitor, were injected in the respirometer with DO concentration variation recording. Exogenous OUR, were calculated from the DO data and they show that as S-0/X-0 ratio increased total OURt increased too as well as total oxidation time. However when, COD is present alone the removal kinetics is more rapid than when both substrates (nitrogen and carbonaceous) are present. It has been shown that the use of respirometry in biomass activation assessment is a sound approach because of the celerity, the reliability and the limited cost. Consequently it has been extended in to the assessment of biomass activation when it is exposed to antibiotics. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.