NOVELTY - Nitrogen-containing waste liquid is subjected to air aeration, where the air aeration amount (A) is adjusted so that dissolved oxygen concentration of liquid is set to a preset amount, the amount (A) is maintained constant and aeration is performed, pH of waste liquid is continuously measured from initiation of aeration, time required for changing pH of waste liquid from low to high value is measured, time for aeration is adjusted based on measured time, and aeration is terminated and anaerobic processing of waste liquid is performed.    USE - For processing nitrogen-containing waste liquid, such as methane fermentation waste liquid produced during methane fermentation of organic waste, such as organic refuse, food processing dreg, excrement and surplus sludge.    ADVANTAGE - The processing of nitrogen-containing waste liquid is efficiently, economically and quickly enabled for long period of time using microbe having high activity.    DETAILED DESCRIPTION - Nitrogen-containing waste liquid containing ammonia nitrogen is subjected to air aeration. The air aeration amount is adjusted so that dissolved oxygen concentration of waste liquid is set to predetermined amount, until waste liquid is passed for predetermined time from initiation of air aeration. The air aeration amount is maintained constant and aeration is performed. pH of waste liquid is continuously measured from initiation of air aeration. The time required for changing pH of waste liquid from low value to high value is measured, and time for air aeration is adjusted based on the measured time. The aeration is terminated and anaerobic processing of waste liquid is carried out. Aerobic processing and anaerobic processing are alternately performed. Ammonia nitrogen contained in waste liquid is removed as nitrogen gas.    DESCRIPTION OF DRAWING(S) - The figure shows the block diagram of processing apparatus used for processing nitrogen-containing waste liquid.    Waste liquid storage tank (1)    Feed pump (2)    Intermittent aerator (3)    Stirrer (5)    Aeration apparatus (6)