Modeling of anaerobic digestion of slaughterhouse wastes after thermal treatment using ADM1

According to the European legislation, thermal treatment of category 2 slaughterhouse by-products at 140 °C, 4-5 bar for 20 min is obligatory for their hygienization prior to disposal. This process is known as "rendering". The product of the rendering process is rich in lipids and proteins making it an appropriate feedstock for biogas plants. The mathematical modeling of biogas production from slaughterhouse wastes after the rendering process has been studied adjusting the anaerobic digestion model (ADM1). For this purpose, two mesophilic (38-39 °C) continuous stirred tank reactors (CSTRs) have been operated in parallel under a hydraulic retention time of 21.5 ± 2.14 d, while the organic load was increased from 50 to 149.6 g COD L^-1. Recirculation of the mixed liquor suspended solids (MLSS) took place in one of the CSTRs, resulting in a different solids' concentration in it. The ADM1 was calibrated by estimating key kinetic parameters, such as the maximum specific consumption rate constant and the half-saturation constants of volatile fatty acids and verified. The degradation kinetics of this type of waste seemed to be faster, as a result of its emulsification through rendering, while the coefficient yields of the acidogens were lower than the default values of ADM1. The structure of the model was proven suitable for predicting the response of both bioreactors under small or medium step transitions, but not for abrupt impulse disturbances in the organic loading rate.