Effects of the temperature range on the energy performance of mixed and unmixed digesters with submerged waste: An experimental and CFD simulation study

Abstract

This study evaluated the effects of temperature range and mixing on the thermal and energetic performance of a batch pilot-scale anaerobic digester with submerged waste (ADSW). This digester was operated under four configurations, namely unmixed mesophilic (UMAD), liquid recirculation-mixed mesophilic (LMAD), unmixed thermophilic (UTAD), and liquid recirculation-mixed thermophilic (LTAD). Anaerobic digestion (AD) experiments indicated that the mixed ADSW produces higher methane yields under the thermophilic temperature range (416.12 Nm3CH4/tVS) compared to the mesophilic range (362.23 Nm3CH4/tVS). However, when the ADSW is not mixed, lower methane yields are measured under the thermophilic range (184.82 Nm3CH4/tVS) compared to the mesophilic range (295.41 Nm3CH4/tVS). In an attempt to understand these opposing results, computational fluid dynamics (CFD) simulations were conducted. Findings showed that while a completely uniform temperature is achieved within the digester when mixing is performed, a temperature stratification occurs under unmixed conditions. This stratification, which is more severe in the thermophilic range, may be the reason behind the reported AD experiments findings. The energy balance calculations indicate that the ADSW achieves its best energy efficiency ratio under UMAD (1.72), followed respectively by LMAD (0.83), LTAD (0.66), and UTAD (0.29).