The first water sample was taken from the reservoir 2 min after the last addition of stock solution using a 25 ml syringe. Subsequent water samples were obtained at 15 min intervals after spiking with ammonia and nitrite, and at 10 min intervals after spiking with urea. Sampling in all cases continued for approximately 6 h (350 min). Nitrate-N concentrations were measured prior to closing the loop and after the final sampling in the ammonia and nitrite spiking trials, while it was measured every 20 min in the urea spiking 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 three N-compounds with three days interval. The nitrite spiking trial was initiated 3 days after the ammonium trial, while the urea trial started one month after the nitrite trial. Biofilters were operated in end-of-pipe configuration modes in between measuring periods.