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Paller I9 gravel biofilter, predicted ammonia removal rates were reasonably accurate (within 9% of the actual rates) until the hydraulic loading rate was reduced to 12.5% of the original value; to the granular carbon biofilter, substantial (30%) differences between predicted and observed values occurred when the hydraulic loading rate was reduced to 25% of the original value. A change in hydraulic loading rate from 44 to 11 ml/cmz/minute affected the performance of the granular carbon biofilter more than the gravel biofilter, because this change caused the granular carbon bed to convert from a stable to a fluidized state. Fluidization caused changes in mass transfer and possibly in biofiim quantity and quality that were- not accommo- dated by the model. At this point, it is impossible to specify the range of hydraulic loading rates over which Equation 13 wtll gener- ate accurate predictions in all cases. However, data from the gravel biofilter suggest that reasonably accurate predictions may be possi- ble with stable (i.e., not fluidized) biofilter beds over atwo-fold to three-fold range of change in hydraulic loading rate. A more speculative use of Equations 13 and 14 is to extrapolate the results of small experimental biofilters to large production scale biofilters that employ the same type of media and design (i.e., flu- idized bed, fined bed, etc.). Although the data needed to verify this capability (i.e., comparative information from bench-scale and pro- duction-scale biofiltration systems) were unavailable, such extrapo- lations are theoretically possible, providing that the previously listed assumptions are met. If the ratio between biofilter volume and flow rate remains the same in both biofilters, Equation 12 simply expands the carrying capacity by an amount proportional to the change in biofilter volume. However, if the ratio between biofilter volume and flow rate differs between the two biofilters, Equation 13 also adjusts the carrying capacity estimate for the change in am- monia removal resulting from the change in retention time. Sensitivity Studies Sensitivity studies were conducted to determine how changes in the parameters of the mode! affect the model's output. These stud- ies began with a standard set of parameters: F = 811minute, C = 1.61 mg ammonia-nitrogen/l; A = 0.1232 mg ammonia-nitrogen/