The present invention relates to a biofilm nitrification - contact denitrification system, which provides advanced level of organic carbon, nitrogen and phosphorous removal from waste waters, and basically comprises at least one wastewater inlet through which the raw waste water is delivered to the system, at least one selector unit wherein the settleable organic substances in wastewater are mixed with the biomass, at least one intermediate settling tank wherein particulate organic substances are settled via biofloculation, at least one aerobic biofilm tank wherein nitrification process is carried out, at least one deoxygenation tank which enables to reduce high dissolved oxygen concentration originating from the aerobic biofilm tank , at least one contact denitrification tank wherein denitrification process is carried out, at least one aerobic tank which enables the nitrogen gas that is released as a result of denitrification process to be removed from the system and the residual ammonia nitrogen and/or dissolved organic substance in waste water to be oxidized, at least one final sedimentation tank which enables the treated water to be separated from the biomass, at least one treated water discharge which is located at the outlet of the final settling tank, at least one aeration/mixing unit which enables optimization of oxygen concentration in the aerobic tank and the aerobic biofilm tank, and homogenous distribution of oxygen, at least one waste sludge discharge through which waste sludge produced in the system is removed, at least one return sludge line which enables a part of the sludge settled in the system to return to the selector unit and another part of it recycled back to the deoxygenation tank .
