In　this　study,　the　nitrification　performance　in　terms　of　ammonium　(NH4+-N),　nitrite　(NO2--N),　and　nitrifiers　activity　aftershock　oxidation　of　sodium　hypochlorite　(NaOCl),　chlorine　dioxide　(ClO2),　or　the　combination　of　both　(NaOCl/ClO2)　within　a　raw　water　distribution　system　(RWDS)　was　investigated.　The　biofilm　structure　was　evaluated　to　explore　its　role　in　nitrification　recovery.　Our　results　indicate　that　nitrification　could　be　recovered,　although　oxidation　initially　played　an　adverse　role　in　nitrification.　Compared　to　individual　NaOCl　or　ClO2,　the　combination　of　NaOCl/ClO2　had　a　less　negative　effect　on　NH4+-N　and　NO2--N　removal,　as　well　as　on　the　regrowth　of　nitrifiers.　The　improved　biofilm　regrowth　and　consequential　faster　nitrification　recovery　after　combined　oxidation　was　mainly　ascribed　to　the　faster　reaction　between　NaOCl/ClO2　and　surface　biofilm,　resulting　in　decreased　limitation　of　nutrient　transfer.　These　results　revealed　the　synergistic　effect　of　NaOCl/ClO2　on　nitrification　recovery,　and　that　the　biofilm　structure　plays　a　key　role　in　nitrification　recovery　within　RWDS.