A pilot plant with capacity to treat up to 8 m(3)/day of supernate from settled flushed swine wastes was monitored for 12 months. The main system is composed of two upflow aerated biofilters connected in series. The aerated biofilters, operated under warm weather conditions (average temperature of 27 degrees C), were able to remove about 88% of biochemical oxygen demand (BOD), 75% of chemical oxygen demand (COD), and 82% of total suspended solids (SS) with loading of 5.7 kg COD/m(3)/day of biofilter media. The total Kjeldahl nitrogen (TKN), total ammonia nitrogen (NH3-N), and total nitrogen (Total-N) reductions averaged 84%, 94% and 61%, respectively, during warm weather, with a significant portion of the NH3-N being converted to nitrite plus nitrate nitrogen (NO2 + NO3-N). At higher organic loading (over 9 kg COD/m(3)/day) during September, the biofilters had only slightly lower percentage removal rates. Operation at lower temperatures (average of 10 degrees C) resulted in lower performances. The GOD, TKN, NH3-N, and Total-N removal averaged 56%, 49%, 52%, and 29%, respectively, in December through March. The COD mass removal rate was linear with loading rate over the range of approximately 2-12 kg COD/m(3)/day of filter. A mass balance average for the 12 months indicated that about 30% of the influent volume, 35% of Total-N and 60% of total phosphorus (Total-P) are removed with the biofilter backwash. Management and utilization of the backwash are important factors in implementing this type of system on farms. The unaccounted-for nitrogen was about 24% and could have been lost as ammonia volatilization or possibly through denitrification within the biofilm.