AN EVALUATION OF SAMPLING TIME TO ASSESS NITRIFYING BIOFILM DEVELOPMENT: ACTIVITY vs. PHYLOGENETIC COMPOSITION

In start-up studies of NTFs, the time to achieve a constant rate of ammonia-removal per media area has varied from 20 (Wik 1996) to 100 days (Thorn et al. 1996). For monitoring purposes, a more accurate determination of the time required is needed. This will allow for the selection of a sampling time that results in biofilm growth which is representative of the mature biofilm within the NTF. The length of time required for a representative biofilm to develop on a clean sampling surface was determined by (1) evaluating bench-scale nitrate plus nitrite generation rates for biofilm samples collected from a full-scale NTF and by (2) quantifying the ammonia-oxidizing populations through the application of Fluorescent in Situ Hybridization (FISH). Three separate determinations were performed. Biofilms between 14 and 70 days old were evaluated. The biofilms were considered to be representative of the mature biofilm within the NTF when there were no significant differences between sampling events for bench-scale nitrate plus nitrite generation rates for biofilms of increasing age. The time required for a biofilm to develop (on a clean sampling surface) that was representative of the ammonia-oxidizing capacity of the mature biofilm within the NTF was approximately 28 days. Under consistent operating conditions, 21 day old biofilms collected from sample Port 2 achieved 96.3% and 93.9% of the nitrate plus nitrite generation rates (g-N/m2*day) of 28 day old biofilms. Therefore, a 21 day sampling period should result in biofilm samples that are representative of the ammonia-oxidizing capacity of the mature biofilm within the NTF. Information on biofilm community structure obtained through Fluorescent in Situ Hybridization supports this conclusion. Moreover, changes in ammonia-oxidation capacity with time appear to be the result of shifts in the percent of ammonia-oxidizing bacteria present, suggesting that nitrifying biofilms are more dynamic than previously thought and that they are capable of responding to changes in environmental conditions in as little as 7 days.