With increased concerns over odor and gas emissions from livestock production facilities more efficient technologies of air pollution control are needed to mitigate the deleterious effects of air contaminants. Gas-phase biofilters for treating contaminant gases from poultry and livestock operations can be cost-effective and environmentally friendly. However, a biofilter's media is an important factor to be considered when in its design. The goal of this study was to assess three compost biofilter particle sizes (12.5 mm > R1 > 8.0 mm > R2> 4.75 mm > R3> 1.35 mm) for ammonia (NH3) removal efficiency and pressure drop using three different residence times (5 s, 10 s, and 20 s). The three compost particle size ranges were physically and chemically characterized, their capability to reduce NH3 was analyzed, and their resistance to airflow was determined. Accumulation of ammonium (NH4) and production of nitrite (NO2) were observed after running NH3 through the media continuously for nine days. The combination of the highest airflow rate (5 s residence time) and largest particle size range (R1) achieved higher NH3 removal efficiencies: 33.4 ± 5.7% and 35.8 ± 5.8%, respectively and the third highest pressure drop: 173.4 ± 16.4 Pa/m. The interaction between medium airflow rate (10 s residence time) and large particle size range (R1) yielded the highest NH3 removal efficiency: 36.7±6.2% and the third lowest pressure drop: 50.2 ±7.0 Pa/m. These results were found over a start-up period of nine days.