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Paller lions on bacterial metabolic rate. Research in freshwater aquacul- ture systems has demonstrated that zero-order kinetics apply at am- monia-nitrogen concentrations > 2.0 mg/l, and half-order kinetics apply at lower concentrations (Bovendeur et al. 1990). While rela- tively high total ammonia concentrations are acceptable if hardy species are cultured and if ammonia is maintained in the ionized form by careful monitoring and adjustment of pH, it is usually de- sirable to maintain total ammonia concentrations below 2.0 mg/l. This complicates the sizing of biofilters, since ammonia removal will vary with the ammonia concentration when it is under 2.0 mg/l. A prerequisite for the continued development of water reuse in aquaculture is a method of quantifying the ammonia removal capac- ity of biofilters. While there are methods to size biofilters on the basis of fish loading and ammonia removal rates/mZ of media sur- face (Liao and Mayo 1972; Speece 1973; Nijhof and Bovendeur 1990), These procedures do not account for the changes in ammonia removal rate associated with half-order kinetics at tow ammonia concentrations. The objective of this paper is to present a general model that can be used to predict the carrying capacity of sub- merged biofilters at low (< 2.0 mg/l) ammonia concentrattons. MATERIALS AND METHODS The model presented herein is partly based on laboratory data collected from several small-scale biofiltration systems. Data essen- tial for the development of the model are presented in this paper; related data, emphasizing the performance and carrying capaaties of the experimental biofiltration systems, are reported in Paller and Lewis (1988). All biofilters we:re cylindrical, with 11.1 I of internal space occu- pied by biofilter nnedia, and were operated in a submerged upflow mode. The biofilters were filled with river gravel or granular car- bon, whic}~ served as supporting media for the growth of bacteria. The granular carbon particles were smaller and tighter and pos- sessed greater surface area per unit volume than the gravel (Paller and Lewis 1988). Biofilters employing granular carbon media were fluidized by increasing the hydraulic loading rate to 44 ml/cm2/sec- ond (Paller and Lewis 1988).