The oxidation ditch process has become increasingly important as a secondary treatment process for small communities, because of its ability to remove nitrogen by providing aerobic and anoxic areas in a ditch. Through surveys at the three oxidation ditches in service in Japan, it has been proved that removals of BOD and suspended solids could be consistently achieved irrespective of seasonal, daily and hourly fluctuations of inflow. It has also turned out that providing an anoxic area in the oxidation ditch is effective for removing nitrogen and recovering alkalinity as well as saving energy. The removal efficiency of total nitrogen is expected to be more than 70%.

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