N-compound to be examined. In case of spiking with nitrite, the
small circulating systems were operated for 3 h prior to spiking to
ensure that background levels of urea and ammonia were oxidized
tonitrite andnitrate concomitantly. Total volumes ofthe stock solutions
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 circulating
biofilter systems. The stock solutions of ammonia, nitrite and urea
were added in steps of 10, 10 and 5 ml, respectively, every 2 min to
the reservoirs in order to distribute the compound evenly within
the system