Energy performance of an unmixed anaerobic digester with submerged solid waste: Effects of temperature distribution

This study examines the thermal behavior and energy performance of an Unmixed Mesophilic Digester with Submerged solid waste (UMDS) through anaerobic digestion (AD) experiments, computational fluid dynamics (CFD) simulations, and energy balance calculations. First, AD experiments were performed for different submerged waste positions. Results showed that a higher specific methane yield and VS removal rate (280 Nm3 CH4 tvs−1 and 65.42%, respectively) are achieved for the bottom waste placement case, compared to its top counterpart (110 Nm3 CH4 tvs−1 and 24.1%, respectively). Furthermore, temperature measurements revealed that a vertical temperature gradient occurs between the top and bottom portions of the UMDS. This temperature stratification was further investigated using a developed and validated numerical model. Simulation results indicated that the temperature non-uniformity's degree is considerably affected by the waste's position inside the UMDS. Moreover, the same CFD model showed that total temperature uniformity can be achieved inside the UMDS by using the double wall heating approach. Thereafter, the relationship between the temperature uniformity and the methane yield was discussed in an attempt to justify the differences reported in the AD experiments. Finally, energy balance calculations showed that the energy efficiency ratio of the UMDS reaches 1.08 (respectively 0.47) when the waste is placed in the bottom (respectively the top) of the digester. [Display omitted] •Mesophilic digestion was performed in an unmixed batch pilot-scale digester (UMDS).•A CFD model was developed and validated to simulate the thermal behavior of the UMDS.•Temperature distribution is affected by the submerged waste's position inside the UMDS.•Higher methane yield is achieved when the temperature is more uniformly distributed.•The UMDS has a better energy efficiency when waste is placed in the bottom placement.