Thermophilic vs. mesophilic anaerobic digestion of waste activated sludge: Modelling and energy balance for its applicability at a full scale WWTP

Sewage sludge produced in WWTPs are currently digested in mesophilic anaerobic digestion (AD) processes with the aim of recovering heat and electricity. However, often, the low biodegradability of waste activated sludge (WAS) limits the complete thermal self-sustainability of the process. This study presents the results of AD tests carried out on WAS in semi-continuous reactors (44 L and 240 L) in mesophilic (38 °C) and thermophilic (55 °C) thermal regimes. The hydraulic retention time (HRT) was 20 days and the organic loading rate (OLR) of 1 kg VS/m3∙d in all tests. The tests returned a specific methane production (SMP) of 0.120 Nm3/kg VS added for the mesophilic process (240 L reactor) and SMPs of 0.188 and 0.176 Nm3/kg VS added for the tests carried out under the thermophilic regime in 44 L and 240 L, respectively. Experimental data were modelled with a first-order rate reaction, where B0, that is the SMP after an infinite HRT, and k, the hydrolysis constant, were the key parameters. B0 and k were found equal to 0.147 Nm3/kg VS and 0.08 d−1 respectively, for the mesophilic process, and to 0.218 Nm3/kg VS and 0.350 d−1 for the thermophilic process. For the thermophilic process, the model was calibrated with the data from the 44 L reactor and validated with those from the 240 L reactor. An error of only 1% resulted. Finally, it was demonstrated that a full-scale digestion scheme, where primary and secondary sludge were digested separately, in mesophilic and thermophilic conditions respectively, and the heat of the digestates was used to heat the cold sludge, allowed to carry out the process with a complete thermal self-sustainability already at a sludge TS content of 3%.