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Pa!!er 9 Model Development When a water reuse system is in a steady state (i.e., ammonia concentration is neither increasing nor decreasing), ammonia re- moval by the biofilter equals ammonia production by the fish: BA = FZ (1) where: B =fish biomass (kg); A =ammonia production rate of the fish at the feeding rate being practiced (mg/kg fish/minute); F = flowrate through the biofilter (I/minute), Z =the ammonia-nitro- gen concentration reductionn in the biofilter (mg/I). The ammonia concentration reduction in the biofilter is equal to the ammonia concentration in the biofilter influent (C) minus the ammonia concentration in the biofilter effluent (E): Z=C-E (2) When ammonia removal conforms to half-order kinetics, the am- monia concentration in the biofilter effluent is a function of the influent ammonia concentration and the ammonia removal in the biofilter: E = C (1 - R) (3) where R is equal to ammonia removal expressed as a decimal R - (C - E) (4) C (ln practice, it is best to use an average value of ammonia removal derived from a number of paired measurements of C and E). R subsumes a number of important factors, such as media surface area, mass transfer efficiency, and biofilm composition into one empirically derived number. To directly model the influence of these factors on biofilter performance would require that they be measured and their interaction clearly understood, neither of which is possible without complex and specialized studies. In the approach used here, the net effect of these factors is indirectly quantified by measuring the amount of ammonia that is removed from water that passes through the biofilter.