Linear versus Monod representation of ammonia oxidation rates in oligotrophic recirculating aquaculture systems

Monod kinetics are widely used to model nitrifying biofilters. However, these kinetics are incapable of representing the collapse of volumetric TAN conversion rate (VTR) under high organic loadings. Failure to recognize the underlying heterotrophic interference can lead to calibration issues as a single Monod function is applied across contrasting levels of carbon loading. This, plus an historic bias towards the analysis of peak carrying capacities leave modelers poorly prepared to serve the needs of a mariculture industry demanding oligotrophic designs for broodstock maturation and larval/fingerling production. Consequently, data was generated by a Monte Carlo technique under the assumption of heterotrophic inhibition to nitrification. The data was used to compare the accuracy of calibration of the Monod relationship using the traditional Lineweaver-Burke and Eadie-Hofstee calibration methods against direct linear regression for low substrate (mesotrophic/oligotrophic) regimes. The results indicate that a simple linear relationship with a zero intercept, calibrated on data ranging from 0.1 to 0.5 g-TAN m−3, is most suitable for the representation of the mesotrophic/oligotrophic performance of nitrifying biofilters based on a comparison of SSE for both the Monte Carlo and field data analyzed herein. Additionally, the coefficient of variation was found to be between 7 and 8% for the parameter τ, which is the slope of the linear relationship between total ammonia nitrogen (TAN) and VTR while the CV for the Monod parameters ranged between 22 and 143% for VTRmax and between 29 and 137% for the apparent half-saturation constant showing the improved stability of the linear model to that of the Monod model.