Biodrying of dewatered secondary sludge: behavior of dynamic respiration index (DRI) and energy release under different operating conditions

BACKGROUND: Biological activity monitored through the dynamic respiration index (DRI) and the calculation of biological heat generation (Q_bio) through O₂ 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 gO₂ 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 O₂ 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 O₂ consumption is recommended.