Degradation of urea, ammonia and nitrite in moving bed biofilters operated at different feed loadings

• Urea was degraded in moving bed biofilters and contributed to nitrification activity.
• Degradation of urea followed first-order kinetics below 2.5 mg N/l.
• Degradation of ammonia and nitrite followed zero-order kinetics above 2 mg N/l and first-order kinetics below 1 mg N/l.
• Zero order removal rates of ammonia and nitrite increased with long-term biofilter loading up to a certain loading level.
• First order removal rate constants of all three N-compounds were not affected by long-term biofilter loading.

This study investigated how removal rates of urea, ammonia, and nitrite in laboratory scale moving bed biofilters were affected by long-term feed loading. To generate different loadings, five identical freshwater flow-through systems (100 l/h) with rainbow trout (Oncorhynchus mykiss) were fed increasing fixed rations of a commercial diet. The filtered effluent from each system was lead through a moving bed biofilter installed end-of-pipe. After an acclimatization period of four months, the moving bed biofilters were spiked separately with urea, ammonia and nitrite in batch mode in three successive trials to investigate degradation kinetics. 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.