Ammonia removal in bio trickling air filters takes place through dissolution in a biofilm with ammonia oxidizing bacteria (AOB) which partially oxidize ammonia to nitrite. Products of protonation (NH 4 +) and oxidation (NO 2 -) wash out via filter water drainage. AOB activity is limited by availability of their substrate NH 3 (not NH 4 +) and concentration of the oxidation product HNO 2 (not NO 2 -). Mechanisms controlling AOB activity in a bio trickling filter remains only partially understood. Recent clarification on separated inhibition kinetics for activity and growth suggest that when growth is fully inhibited by HNO 2, activity remains only 40-50% inhibited. This must have specific implications for AOB growth and activity patterns in bio filters with high ammonia load. The recent findings on AOB inhibition kinetics in general and specific studies of filter microbiology indicate that: 1 The AOB biomass should stabilize in relation to the average load of NH 3. If NH 3 load decreases, inhibition will increase and biomass will decrease. If NH 3 load increase, inhibition will decrease and biomass increase. 2 AOB activity will respond immediately to changes in NH 3 load, at constant biomass. 3 Outlet air NH 3 concentration is approximately in equilibrium with filter water and is therefore governed by its [NH 4 +] and pH, which again is controlled by filter water conductivity and AOB activity respectively. Effects of experimentally varied NH 3 loads on AOB growth and activity were investigated. Ammonia removal, filter water pH and conductivity, were measured on line during manipulations of ammonia load in a full scale biotrickling filter constructed for removal of NH 3 and odor from ventilation air at a pig farm. On line recordings were supplemented by point measurements of odor removal. Effects of changes in NH 3 load were studied over short periods (hours) and long periods (weeks). The proposed auto regulation of biomass and activity was in principle confirmed by observations on filter response to experimental changes in NH 3 and outlet NH 3 concentrations, but gain and loss of biomass happens surprisingly slow over weeks due to low growth rates and even lower decay rates. Implications of the function of the AOB community in bio trickling filter is that biomass adapt over weeks to the average load of NH 3, and will adjust activity to most of the short term (hours) variation typically seen in ventilation air rates from livestock production houses. The presence of high concentrations of organic gasses (e.q. in the same range as NH 3) may result in less predictable response to NH 3 load variations but the general mechanisms remains.
