Comments on "Some Observations on the Physical Realizability of Compartmental Models with Delays

A recent communication concluded that a linear differential-difference model presented by Bellman is structurally unrealizable. It is shown here that while the model is indeed unrealizable by a linear compartmental model, it is, in fact, structurally realizable by a linearized nonlinear compartmental model. The potential biological meaningfulness of this result is demonstrated for a simple model of a batch purifier process.