To provide an apparatus and a method for treating high-concentration nitrogen-containing dirty waste water such as waste water from a livestock farmer and excreta by which denitrification and dephosphorization are performed effectively.

The apparatus for treating high-concentration nitrogen-containing dirty waste water such as waste water from the livestock farmer and excreta is composed of a screen tank 10, a flow rate adjustment tank 20, an aeration tank 70, a tank for concentrating generated sludge and an anaerobic treatment tank 50. The anaerobic treatment tank 50 is formed in the aeration tank 70 by arranging a partition wall 52 in the upper side part of the aeration tank 70 and allowing the lower part of the anaerobic treatment tank to communicate with the aeration tank 70. An agitator 80 which is rotated at a high or low speed and has multistage impellers 81 is arranged in the anaerobic treatment tank 50. An air jetting spout 110 is arranged in the lower part of the aeration tank 70. An oval pellet vessel 120 filled with an active microbe derivative is arranged above the air jetting spout. The inside of the aeration tank is kept in such an aerobic state that proper oxygen partial pressure is kept to oxidize and decompose a contaminant. Organic matter, nitrogen and phosphorus are removed in an anaerobic state.