An unstructured model used to describe the correlation of substrate concentration with microbial growth kinetics. The model is based on enzymatic Michaelis–Menten kinetics:
where μMAX is the maximum specific growth rate of the microorganism, S is the substrate concentration, and Ks is the saturation constant and equivalent to the Michaelis–Menten constants. Ks varies with the type of substrate while the maximum specific growth rate is dependent on process conditions such as temperature, pH, and dissolved oxygen. When homogeneous conditions are achieved in the bioreactor and the microorganisms are freely suspended, a relatively low substrate is required to maintain maximal growth. At high substrate concentrations, substrates can inhibit growth for which the Monod kinetic model can be extended accordingly.