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Three reactor set-ups were investigated, namely nitrification UCBR prior to coupling, coupled UCBR-packed bed system for nitrogen removal, and de-coupled recovering UCBR. Batch studies were conducted on the biofilm particles taken from the UCBR. A mathematical model was developed to estimate the growth kinetics of the biofilm autotrophic and heterotrophic bacteria. Heterotrophs was noted to accumulate on the nitrifying biofilm due to the effect of residual COD from the packed bed column during the coupled reactor phase. The growth of heterotrophs had affected the substrate removal rates, biofilm morphology, and growth kinetics of nitrifying bacteria. For example, µ m of ammonium oxidizing bacteria decreased from 0.55 to 0.19 d -1 , while K s increased from 1.42 to 3.34 mg N/L. This study demonstrated that µ m and K s of the nitrify bacteria changed with the type of substrate. Hence, growth kinetics would not be the same for nitrifying bacteria that is exposed to residual carbon in a coupled UCBR-packed bed system and a single UCBR that is fed on organic-carbon free substrate.