When constants are no longer constant: The case of inhibition in bioprocesses

Traditional models describing microorganisms growth assume that changes in the environmental conditions generate immediate responses in microbial population. However, reports are available indicating that in some cases this may not be the case. The existence of a delayed inhibition response (DIR) was studied, exposing a microalgae culture to inhibitory ammonia concentrations. Results revealed the existence of considerable delays in the microalgae response, even when imposing conditions promoting null microalgal activity. Even an extremely unfavourable condition such as 1500 mg L⁻¹ of total ammonia nitrogen and pH 9 takes about 24 h to induce complete inhibition. The existence of DIR may cause serious deviations between predictions given by traditional models and real behaviour of a culture. Inclusion of DIR in traditional models may be accomplished by assuming model parameters as variables depending on time. In other words, assuming that inhibition “constant” is no longer constant, but a function of exposure time.