An apparatus and method for treatment of water borne oxidized contaminants, using hydrogen as an electron donor for denitrification and reduction of other oxidized contaminants. Preliminary results reported here show that a biofilm of autotrophic denitrifiers accumulates rapidly in the wastewater setting, the MBfR can drive NO.sub.3.sup.- concentrations below 1 mgN/L, and the H.sub.2 pressure controls the NO.sub.3.sup.- flux.