The purpose of this paper is to present the hydrodynamics and mass transfer of a novel rectangular circulating bed reactor as a function of operating conditions and reactor configuration. Within the range of gas velocities studied (0.01 < U-gr < 0.04 m s(-1)), solid hold-ups up to 40% v/v did not affect reactor hydrodynamics (liquid circulation velocity of 0.3-0.4 m s(-1) and k(L)a in the range 50-300 h(-1)). Moreover, solid input enhances the local gas hold-up in the downcomer as well as liquid velocity and mass transfer in some operating conditions. After biofilm development, k(L)a does not change significantly compared with values measured in water. Improved hydrodynamics and mass transfer of the CBR guarantee high nitrification rates and operation stability in three different applications for tertiary (up to 2 kg N m(-3) d(-1)) and secondary nitrification (up to 0.6 kg N m(-3) d(-1)), as well as in combined denitrification/nitrification treatment (up to 1 kg N m(-3) d(-1)). The good performance of the CBR is highlighted by the simplicity of the reactor design which does not require any complex technical devices. Thus, this innovative and more compact technology will be an attractive solution for intensive nitrogen and carbon removal for both new compact works and existing plant upgrading.