Investigation of the impacts of thermal pretreatment on waste activated sludge and development of a pretreatment model

This study investigated the impacts of high pressure thermal hydrolysis (HPTH) pretreatment on the distribution of chemical oxygen demand (COD) species in waste activated sludge (WAS). In the first phase of the project, WAS from a synthetically-fed biological reactor (BR) was fed to an aerobic digester (AD). In the second phase, WAS from the BR was pretreated by HPTH at 150 degrees C and 3 bars for 30 min prior to being fed to the AD. A range of physical, biochemical and biological properties were regularly measured in each process stream in both phases. The COD of the BR WAS consisted of storage products (X-STO), active heterotrophs (X-H) and endogenous decay products (X-E). Pretreatment did not increase the extent to which the BR WAS was aerobically digested and hence it was concluded that the unbiodegradable COD fraction, i.e. X-E, was unchanged by pretreatment. However, pretreatment did increase the rate of degradation as it converted 36% of X-H to readily biodegradable COD (S-B) and the remaining X-H to slowly biodegradable COD (X-B). Furthermore, X-STO was fully converted to S-B by pretreatment. Although pretreatment did not change the VSS concentration in the downstream aerobic digester, it did decrease the ISS concentration by 46 +/- 11%. This reduced the total mass of solids produced by the digester by 21 +/- 8%. A COD-based HPTH pretreatment model was developed and calibrated. When this model was integrated into BioWin 3.1 (R), it was able to accurately simulate both the steady state performance of the overall system employed in this study as well as dynamic respirometry results. (C) 2013 Elsevier Ltd. All rights reserved.