In-Storage-Psychrophilic-Anaerobic-Digestion (ISPAD) process. Part I: Model Development and Calibration

Abstract

In-Storage-Psychrophilic-Anaerobic-Digestion (ISPAD) is a treatment system applicable to wastewaters stored for over 100 days, such as livestock wastes and municipal sludge. The ISPAD system differs from conventional reactors by being a sequentially fed batch process operating at a temperature fluctuating with ambient. The objective of this study was to develop a mathematical model to simulate the ISPAD process, verify the value of its microbial kinetics, and to simulate the pH evolution of its content along with its methane (CH₄) production. Furthermore, the values of the ISPAD microbial kinetics were compared to that of previous years to track for further acclimation to psychrophilic conditions. Simulation of ISPAD was achieved using the Simulink/Matlab software. The model was calibrated using laboratory data obtained from batch experiments using 7-year-old ISPAD inoculum, and glucose as substrate, and where glucose, VFAs and pH changes were monitored along with biogas production. The ISPAD model showed good agreement with the experimental data representing the system behaviour between 4 and 35 ºC. Although microbial activity at 4 °C was much slower than that at 18 and 35 ºC, it showed acclimation to low temperatures. Furthermore, comparison of microbial kinetic values over 3 years of field ISPAD monitoring demonstrated continued population acclimation, especially for the methanogens.