Abstract: Biological denitrification of water using one or more bioreactor units containing bioreactor media, a prefilter assembly containing prefilter media, and a slow sand filter containing sand filter media. Each bioreactor unit requires certain water flow related values and the bioreactor media must have certain material or dimensional characteristics. An air scouring process is constrained to predetermined parameter values to remove excess biomass material from the bioreactor media. The prefilter assembly also includes a flow distribution device for providing uniform water flow throughout the cross-section of the prefilter media. In one embodiment, the prefilter assembly and the slow sand filter assembly are semi-portable in that they have a reduced size with limited dimensions. This enables the prefilter assembly and the slow sand filter assembly to be made at a remote location and transported to the site at which water is to be denitrified.