Residence time is a crictical factor in determining the size of an open face biofilter. Four test biofilters were contrusted with media comprised of yard waste compost and brush chips to compare 8s. and 4s. residence time. Biofilter performance was monitored for ten months by measuring pressure drop, moisture content, odour threshold, hydrogen sulfide and ammonia concentrations. Odour reduction efficiency for the 8 and 4 second residence time biofilters is 91% and 87% respectively. Hydrogen sulfide reduction efficiency is 97% and 96%. Ammonia reduction efficiency id 82% and 74%. There is no significant difference between 8 and 4 seconds empty bed contact time for odour, hydrogen sulfide and ammonia removal. Based on current information, the design residence time for biofilters on swine buildings is 4 to 5 seconds.