Introduction of lag phase of microbial growth upon changes, during modeling of the anaerobic digestion process

The aim of present study was to model the lag in anaerobic microbial growth, occurring due to changes in operational conditions or the organic loading rate and affecting non-steady state process dynamics. For this end, a simplified and dynamic mathematical model considering a two-steps bioconversion process was set up, in which glucose is converted to acetic acid by acidogenic bacteria (step one) and acetic acid is converted to methane by acetoclastic methanogens (step two). The model considers Monod growth kinetics, inhibition of acidogens by acetic acid and fixed-rate microbial death, and involves five mass balance equations. For modeling the growth lag, a substrate-dependent step response function is proposed. Present theoretical growth implementation provides a realistic representation of the microbial growth lag seen in experiments and as such offers a promising addition to anaerobic digestion models currently in use.