Abstract: Process for the microbiological nitrification and denitrification of wastewater utilizes wastewater and endogenous carbon as carbon sources. After removal of settleable solids in the wastewater stream in a settling tank, wastewater is discharged into a fixed-bed, down-flow, aerobic, free draining nitrification field, containing suitable media to facilitate the growth of microorganisms capable of converting influent nitrogen, in the form of ammonia, to nitrite and then nitrate, by passing the wastewater through the nitrification field. Microbiological activity is facilitated by means of an adjustable recycle stream which adds dissolved oxygen to the aerobic nitrification field and provides sufficient dosing of the media to insure the wetting necessary to support continuous microbiological activity.