An advanced wastewater treatment apparatus applying spent sulfidic caustic is provided to treat wastewater environmentally friendly by suppressing the generation of air pollutants. An advanced wastewater treatment apparatus applying spent sulfidic caustic is characterized in that: each of nitrification tanks has a carrier(16) filled therein to form a biofilm; front nitrification tanks are tanks separated in two stages; a volume ratio of denitrification tanks to the nitrification tanks is 0.5 to 0.7; the front nitrification tank is equipped with an internal return pipe(10) internally returning nitrified wastewater to the denitrification tank; and a settling tank(7) includes a sludge return pipe(11) for returning sludge to influent, and a sludge disuse pipe(12).
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