Identification of factors contributing to degradation in autothermal thermophilic sludge digestion

A laboratory scale ATAD reactor (6 days SRT, 24 h feed cycle) was operated at quasi steady-state conditions with real waste activated sludge feed. Hydrolysis tests of waste activated sludge demonstrated that 10% of feed suspended matter is hydrolysed practically instantaneously and 20-30% of all fed suspended matter is dissolved after 60 minutes in tap water and in cell free reactor liqueur equally. Respirometric, VSS and COD concentration data served as basis for calibration of a simple VSS based kinetic model. The calibrated model provided good fit to two separate sets of measured data. This model was used to evaluate different operation strategies. Modification of cycle length does not affect overall VSS removal rate, while shorter cycle length or continuous operation helps avoid oxygen limited conditions. Further advantages of shorter feed cycles (reduced cooling effect, greater realizable load) support choosing continuous operation of the ATAD system if the main goal is VSS reduction. While reactor cascades increase efficiency, this advantage diminishes with increasing load. At high load rates increased construction costs are not justified by the expected improvement in efficiency.