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

Biofilms grown in two submerged nitrifying biofilters developed the capabil ity 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 bioma ss basis and named specific free ammonia concentration. The initial biofilter colonisation by nitrite oxidisers appeared to be infl uenced by the start-up conditions. A nitrite accumulation of 65% was observ ed for the filter started in batch mode while an accumulation of only 30% w as obtained in the filter started in continuous flow mode. Six months after start-up, nitrite oxidisers showed increasing activities for the same spec ific free ammonia concentration, specially in the filter started in batch m ode, which appeared to be a consequence of the capability of the nitrifying microorganisms to become acclimated to higher concentrations of free ammon ia. (C) 1999 Elsevier Science Ltd. All rights reserved.