A mobile, skid-mounted biofilter was designed and built to determine the kinetics of ammonia oxidation at a modern 2400-sow farrow-to-wean unit. The biofilter system consisted of a variable-speed blower, packed-bed humidifier, and two reactors (packing volume of 0.35 m3 per reactor) configured in parallel. Prescreened, composted yard waste was used since compost contains a large number of active microorganisms, is relatively inexpensive, and is readily available. Ammonia emissions (0 to 12 ppmv or 0 to 8.4 mg m-3) from the swine facility (and 0 to 25 ppmv or 0 to 17.4 mg m-3 in a simulated stream) were transported downward across the packing in the reactors, and spray nozzles at the top of each reactor were used to add moisture to the packing. Ammonia removal ranged from 25% to 95% depending on the residence time and inlet NH3 concentration. Using first-order kinetics, the measured ammonia degradation rate ranged from 0.06 to 0.80 mg NH3 m-3 s-1 for volumetric loading rates ranging from 0.05 to 0.25 mg NH3 m-3 s-1. Residence time distribution (RTD) analysis was also performed to determine the effect of scale-up on axial dispersion and deviation from plug flow. RTD analyses suggest that non-ideal reactor design equations may be required to predict reactor size for desired ammonia conversions. A reactor design method (i.e., sizing calculation) was developed based on the kinetics of ammonia oxidation. For example, assuming a volumetric flow rate of 13 m3 s-1, NH3 conversion of 95% (CNH3in = 25 ppmv), and a first-order rate constant of 0.08 s-1, a reactor volume of 487 m3 and residence time of 37.5 s is required. The size of the reactor will change depending on the characteristics of the swine facility. Pressure drops across the bed should be 50 Pa or less in order to utilize in-house fans. This will probably limit the height of the reactor to 1 m or less. The mass of compost required can be estimated from its bulk density. Moisture content must be maintained between 40% and 60% in the biofilter to maintain biological activity. Additional research is required to develop inexpensive methods of emission humidification, online moisture analysis, and water addition.