In-situ Determination of Nitrification Kinetics and Performance Characteristics for a Bubble-washed Bead Filter

Intensive recirculating aquaculture systems rely almost exclusively on some form of fixed film biofilter for nitrification.Currently there is no standardized way to determine and report biofilter performance to facilitate user selection among the numerous options. This type of information is critical for the end user, and also important for both the design engineer and the manufacturer. In an attempt to address this issue, a simple procedure for estimating nitrification reaction rate kinetics is described and applied to a bubble-washed bead filter. Reaction rate kinetics were determined through a series of batch reaction rate experiments with a commercially available 0.06-m3 (2.0-ft3) bubble-washed bead filter. Empirical mathematical models for the nitrification of ammonia-nitrogen to nitrate-nitrogen were developed. The kinetics of nitrification were found to fit a simple first-order reaction model, when the ammonia-nitrogen concentration was less than 1 mg NH4-N/L, and a zero-order reaction when the ammonia-nitrogen concentration was greater. The exact breakpoint between first and zero-order reaction kinetics was found to be a function of the flow rate. In addition, the first-order kinetic reaction rate constants were also a function of the flow rate, reflecting the influence of high nutrient gradients and associated higher nutrient gradient across the biofilm. No effect of flow rate was found for the zero-order reaction rate constants. Kinetic reaction rate parameters, maximum reaction rates, and half-saturation constants were determined for the Monod kinetics model as functions of hydraulic loading rate. Based on these results, an evaluation tool was proposed to help characterize bead filter performance based on reaction rate kinetics. A series of performance characteristic curves were developed to show maximum removal rates as a function of ammonia-nitrogen concentration and flow rates through the bubble-washed bead filter.