Results showed that urea, in addition to ammonia and nitrite, was
degraded although the substrate limited/dependent removal rate of urea (first order kinetic) was lower
than that of ammonia and nitrite. Degradation of urea could be described as first order kinetics below
2.5 mg N/l. Degradation of total ammonia nitrogen (TAN) and nitrite was substrate independent (zero
order kinetic) above 2 mg N/l and subsequently substrate dependent as substrate concentrations in the
bulk water declined. The transition zone from zero to first order degradation was elevated with increase
in long-term biofilter loading. For ammonia and nitrite, a significant increase in the zero order removal
rate constants related to long-term loading were observed up to a long-term feed loading of 207 g/d, corresponding
to 69 g feed/m2 filter media/d and an TAN + urea-N concentration of 2.70 mg N/l. Long-term
feed loading had no obvious effect on first order removal rate constants of any of the three nitrogenous
compounds. Degradation of urea resulted in generation of ammonia demonstrating that urea degradation
contributes to the ongoing nitrification activity in aquaculture biofilters. For all three types of spiking
(urea, ammonia and nitrite) accumulation of nitrate was observed in the moving bed biofilters, sustaining
that nitrification had occurred.