Estimation of thermal energy released by thermophilic biota during sludge minimization in a fluidized bed reactor: Influence of anoxic conditions

Thermophilic biological fluidized bed reactors operating in aerobic/anoxic alternate conditions proved to be a feasible solution for sewage sludge minimization. However, to date, no data about energy released by thermophilic biota (ThBio) are available in literature. This work aims to estimate specific thermal energy released by ThBio highlighting the influence of daily anoxic conditions. A pilot-scale reactor (1 m3) was fed continuously with mesophilic sewage sludge and monitored for more than four months and a thermophysical model was applied to estimate thermal energy released by ThBio (kT,biota and kT,COD). Results suggested that the increase of daily anoxic time stimulated COD removal (92.7 ± 1.3 % vs. 81.3 ± 4.9 %, with 6 h and 0 h of daily anoxic time, respectively). The thermal energy released by ThBio was strictly dependent on anoxic conditions. In fact, increasing anoxic conditions from 0 h d−1 to 6 h d−1, kT,biota and kT,COD reduced from 1.8 ± 1.3 Mcal kg−1VS kg−1COD and 26.6 ± 13.7 Mcal kg−1COD to 0.5 ± 0.1 Mcal kg−1VS kg−1COD and 15.6 ± 4.2 Mcal kg−1COD, respectively. Although, biological mechanism responsible of this behaviour is not completely clear, this work can serve as reference for future studies about the optimization of conditions to maximize thermal energy release from ThBio during organic substance degradation alternate aerobic/anoxic, in view of subsequent energy recovery.