Nitrate in the groundwater can cause a number of health and environmental issues. Therefore, a novel denitrifying reactor was investigated regarding rapid granulation of biomass and reactor start-up. The effects of the wastewater characteristics, reactor operating conditions, and reactor geometry on microbial granulation were studied. It was possible to achieve granulation in just 15days of reactor start-up. In 15days, the settling velocity of the granules was almost tenfold higher than that of seed sludge. The reactor was able to handle a nitrate loading rate of 460g NO3-Nm(-3)d(-1) in just 30days of reactor start-up, with a removal efficiency of almost 100%. Based on the experimental observations, a hypothesis for the cause of the rapid granulation is proposed.