Modelling the anaerobic digestion behaviour of proteins in thermally treated sludge using a new biomass transfer kinetic model

In this study, a newly developed biomass transfer kinetic model was utilized for studying anaerobic digestion behaviour of proteins in sludge from two different thermally treatment processes [i.e., the thermal hydrolysis process (THP) and intermediate thermal hydrolysis processes (ITHP)]. Three different experimental runs comprising a high-level protein load (HLPL) (2000 < X < 3000 mg/L), intermediate-level protein load (ILPL) (1000 < X < 2000 mg/L), and low-level protein load (LLPL) (X < 1000 mg/L) were allowed to undergo anaerobic degradation for 312 h to monitor changes in total protein concentration. Analysis of protein degradation kinetics revealed that the first-order model was inappropriate for studying AD behaviour of protein in thermally treated sludge. However, the biomass transfer kinetic model provided an excellent fitting for the experimental data. In addition, it was observed that the anaerobically digested ITHP-treated sludge displayed higher values for the initial hydrolysis rate and overall biomass transfer coefficient at HLPL as compared to the anaerobically digested THP-treated sludge. Furthermore, step-wise kinetics was blamed on variability in the rates of utilizing highly soluble hydrozylate protein fractions present in the liquid phase in comparison to the slowly degradable proteins present in the liquid–solid sludge interface.