Nitrifying biofilm acclimation to free ammonia in submerged biofilters. Start-up influence

Biofilms grown in two submerged nitrifying biofilters developed the capability to overcome inhibition caused by free ammonia during a 6 month period. The biofilters were started with different start-up protocols, fed with the same synthetic waste-water and further operated under the same conditions for 6 months. The threshold value of free ammonia concentration over which ammonia and nitrite oxidisers experienced inhibition was evaluated on biomass basis and named specific free ammonia concentration. The initial biofilter colonisation by nitrite oxidisers appeared to be influenced by the start-up conditions. A nitrite accumulation of 65% was observed for the filter started in batch mode while an accumulation of only 30% was obtained in the filter started in continuous flow mode. Six months after start-up, nitrite oxidisers showed increasing activities for the same specific free ammonia concentration, specially in the filter started in batch mode, which appeared to be a consequence of the capability of the nitrifying microorganisms to become acclimated to higher concentrations of free ammonia.