N-compound to be examined. In case of spiking with nitrite, thesmall circulating systems were operated for 3 h prior to spiking toensure that background levels of urea and ammonia were oxidizedto nitrite and nitrate concomitantly. Total volumes of the stock solu-tions used for the separate spikings with ammonia, nitrite and urea were 50, 50 and 25 ml, respectively, yielding bulk concentrations of approximately 5.0, 5.0 and 2.5 mg/l, respectively in the circulatingbiofilter systems. The stock solutions of ammonia, nitrite and ureawere added in steps of 10, 10 and 5 ml, respectively, every 2 min tothe reservoirs in order to distribute the compound evenly withinthe system.The first water sample was taken from the reservoir 2 min afterthe last addition of stock solution using a 25 ml syringe. Subsequentwater samples were obtained at 15 min intervals after spikingwith ammonia and nitrite, and at 10 min intervals after spikingwith urea. Sampling in all cases continued for approximately 6 h(350 min). Nitrate-N concentrations were measured prior to clos-ing the loop and after the final sampling in the ammonia and nitritespiking trials, while it was measured every 20 min in the urea spik-ing trials. Water samples were sterile filtered (Filtropur S 0.2 m,Sarstedt, Germany) and stored in 15 ml tubes (Sarstedt, Germany)at 3◦C in the dark until further analyses in the laboratory.Spiking and sampling was repeated once for each of the threeN-compounds with three days interval. The nitrite spiking trialwas initiated 3 days after the ammonium trial, while the ureatrial started one month after the nitrite trial. Biofilters were oper-ated in end-of-pipe configuration modes in between measuring periods