A fluidized bed reactor (14) for removing phosphorus and nitrogen from wastewater has a column comprising a number of sections. (15A, 15B, 15C, 15D) The diameter of the column changes stepwise between the sections. A flow velocity in excess of 100 cm/min is maintained in a lowermost one of the sections and lower flow velocities are maintained in subsequent sections. A struvite supersaturation ratio is controlled in part by recycling wastewater from an outlet of the column. Struvite pellets are removed periodically from the bottom of the column.