NOVELTY - The system has upflow reactor (1) performing anaerobic digestion during the introduction of influent raw water containing organic matters at high concentration and discharging methane gas and treated water. An upflow reactor (2) performs denitrification during introduction of influent raw water, effluent from reactor (1) and internally recycled influents from aeration and settling tanks (3,4) in reactor (2). The effluent of reactor (2) is aerated in aeration tank. The suspended solids are removed from the aerated effluent in settling tank for discharging final treated water.    USE - Treatment system for high concentration wastewater such as food waste leachate and livestock wastewater.    ADVANTAGE - The treatment of high concentration waste water containing large amount of nitrogen is possible and energy consumption is reduced by generating methane gas and using methane gas as fuel gas. The treatment system enhances stability of sludge and reduces toxicity of high concentration wastewater. The treatment system keeps multi-layer upflow, achieves ideal integration conditions of anaerobic/anoxic atmospheres, condenses sludge to high concentration, improves operation efficiency by reducing stay time of waste water in reactors, treats high concentration organic wastewater having various carbon/nitrogen (C/N) ratios and widely controls internal cycle ratio for resilient operation of the treatment system. The large amount of methane gas produced during the treatment process is not freely diffused to outside air for preventing air pollution and reducing green house gases. The treatment process is simplified and the operation expense is reduced by mixing pre-processed low concentration wastewater with influent raw water to reduce concentration of organic maters without using dilution water. The treatment system reduces amount of solids, nitrogen and phosphorus contained in treating water and obtains clean water.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of the wastewater treatment system.    Upflow reactors (1,2)    Aeration tank (3)    Settling tank (4)