Process for treating wastewater

A wastewater treatment method that has: a step 1 for supplying organic wastewater to a first aerobic tank and an anaerobic tank; a step 2 for performing nitrification and denitrification with bacteria carried on a carrier in the first aerobic tank and discharging primary treated water; a step 3 for performing denitrification of the organic wastewater and the primary treated water with bacteria in an activated sludge in the anaerobic tank and discharging secondary treated water; a step 4 for performing nitrification of the secondary treated water with bacteria in the activated sludge in a second aerobic tank and discharging tertiary treated water; and a step 5 for separating the sludge from the tertiary treated water in a solid-liquid separation device and discharging the water as final treated water and for returning the sludge to the anaerobic tank. By virtue of this method, the load on the later stage anaerobic tank can be reduced, the anaerobic tank can be made more compact than conventionally, and an exce