This work describes the time-variation of the biofilm thickness and the nitrification rate inside a two-tanks in series MBBR process, one reactor for COD removal (reactor C) and one for nitrogen removal (reactor N). Two plants were studied under different operating strategies, using the addition of a source of nitrogen, or an influent load increase. We have shown that the biofilm thickness increased with the load applied to reactor C, whereas they were almost constant in bioreactors N. On the contrary, nitrification rate increased with load in both bioreactor types. Recommendation for stakeholders were drawn with the necessity to apply a sufficiently high loading rate to bioreactor C to limit nitrification in this reactor, and solutions to promote nitrification in reactor N were given with the by-pass of bioreactor C to feed reactor N, or the supply of reactor N with the addition of alcali.