In biological air filters ammonia is removed due to the action of Ammonia Oxidizing Bacteria (AOB) resulting in nitrite accumulation exceeding 100 mM. Among filters treating exhaust air from pig facilities successful establishment of Nitrite Oxidizing Bacteria (NOB) sometimes occurs, resulting in accumulation of nitrate rather than nitrite and a significant decline in pH. As a consequence, ammonia is removed more efficiently, but heterotrophic oxidation of odorous compounds might be inhibited. To identify the controlling mechanisms of nitrite oxidation, full-scale biological air filters were investigated applying a broad in situ approach. Bacterial distribution and chemical gradients were examined at both a macro scale from filter inlet towards outlet and on a micro scale within the intact biofilm, applying Fluorescence in situ Hybridization (FISH), electrochemical microsensors, and chemical analysis. Furthermore, the effect of varying air load and water exchange was investigated. Absence of NOB in many filters was explained by the inhibitory effect of Free Ammonia (FA). When first established, NOB induced a self-perpetuating effect through oxidation of nitrite which allowed increased AOB activity resulting in a lowered pH and thus a decreased FA concentration, promoting further growth of NOB. Yet, in some cases a situation with a nitrate-to-nitrite ratio of 1 and moderate pH remained stable even under varying air load and water supply, suggesting that additional mechanisms were involved in controlling NOB establishment and activity.
12th International Symposium on Microbial Ecology, Isme 12: Microbial Diversity - Sustaining the Blue Planet, 2008