BACKGROUND Biological activity monitored through the dynamic respiration index (DRI) and the calculation of biological heat generation (Q(bio)) through O-2 consumption has been reported only scarcely in the case of biodrying. Therefore, the present work studies the DRI and Q(bio) for a dewatered secondary sewage sludge biodrying process under different operating conditions: airflow rate (AFR) and initial moisture content (IMC). RESULTS It was found that AFR has an important effect on oxygen consumption, with higher DRI (<= 6 gO(2) kgVS(-1) h(-1)) at higher AFR for different studied IMCs. IMC also affects the DRI, with higher values of DRI at higher IMC. Determination of the Q(bio) indicated that its calculation through O-2 consumption provided better explanations of the water removal of the process. AFR also affects water removal and volatile solids (VS) degradation, with greater water removal (c. 30%) and lower VS degradation (c. 3.6%) at lower AFR. CONCLUSIONS AFR strongly affects the DRI, with higher DRI at low AFR. The calculation of Q(bio) through O-2 consumption is recommended. (c) 2019 Society of Chemical Industry