Regarding nitrite concentrations, unlike for TAN,no significant differences were observed between treatments, which averaged 0.42–0.58 mg/L, and were quite variable ( Table3 ). In the
same type of RAS, we have earlier shown that although TAN and CO2 removal capacity was higher than anticipated using common dimensioning rules, nitrite removal did not meet specifications because higher than 0.1 mg/L NO 2 –N was observed. In a study comparing fixed bed and
moving bed reactors Suhr and Pedersen (2010)found no differences between the bioreactor
systems in accumulation of NO2 –N. The finding that the experimental treatments did not affect
nitrite in the present study,suggest that in this set-up, nitrite removal was not limited by
alkalinity. The mean NO3 –N concentration exiting the MBBR ranged from 40 to 42 mg/L for the three treatments. No significant change could be distinguished either in NO3–N concentration across the MBBR, which indicates that equal TAN conversion to NO3–N occurred for the three treatments as was intended with the use of equal fish feed loading throughout the study.