Abstract
Biological reactions often experience inhibition conditions from high concentration of substrate, reaction products or other external inhibitory compounds. The inhibitory compounds may affect the enzymatic system leading to different forms of competitive, non-competitive or uncompetitive reversible enzyme inhibition. In other situations, the concentration of inhibitory compound could result in enzyme poisoning leading to irreversible inhibition. There are several mathematical models to express reversible inhibition, however recovery/adaptation phenomenon is not well described by these models. Furthermore, the modelling approaches for unreversible inhibitions are not well developed. In this study, dynamic response of oxygen uptake rate from nitrite oxidising batch experiments were used to develop models for reversible and irreversible inhibitions. The OUR batch tests from nitrite oxidation experiments conducted for nitrite-N concentration range of 125-2,000 mg-N /L revealed that the inhibition effect of free nitrous acid (FNA) and free ammonia (FA) disappeared after several hours due to microbial adaptation from the shock loading. The OUR tests also indicated irreversible inhibition (poisoning) leading to permanent reduction in activity at higher doses of inhibitory compounds For the reversible inhibition a time-dependent switching function was defined to express the degree of the adaptation. The irreversible poisoning phenomenon was defined as an additional first-order type decay/death process that was initiated when the inhibitory concentration exceeded the threshold level. The modified model developed from the batch experiments data was able to reasonably reproduce the effluent nitrogenous concentration in the WERF benchmark dataset of over 250 days.
