Semi-chemical pulp mills discharge large quantities or waste materials into the streams of the nation. If the flow of the stream is such that the waste does not become sufficiently diluted, biological reactions occur which deplete the dissolved oxygen in the stream, sometimes to the point where it will no longer support the higher forms of aquatic life. The purpose of this investigation was to study the treatment of these wastes by aerobic digestion prior to the discharge of the wastes into the stream. The waste studied was obtained. from the pulp mill of the National Container Corporation at Big Island, Virginia. The investigation was carried out by aeration of the waste in the presence of microorganisms. The results of this investigation show that microorganisms from soil are capable of digesting the waste, as 5-day BOD reductions as high as 74.5 per cent were obtained in 6-day aerations with nitrogen added as a nutrient. A limit of 5-day BOD reductions of about 30 per cent was found for aerations without added nutrients. The addition of sulfur, phosphorus, potassium, calcium, iron, or magnesium did not aid in digestion of the waste. A digestion temperature of 37° C gave no better 5-da, BOD reduction than 25° C, but the reductions were lowered about one-third when the temperature was lowered to 15° C. Adjustment of the initial pH did not affect the digestion. Reductions in 5-day BOD averaging 61.4 per cent were obtained with aeration of wastes of from 4,000 to 8,000 parts per million total solids, but with waste of 13,300 parts per million total solids the 5-day BOD reduction dropped to 29.5 per cent, and no reduction was accomplished with waste of 90,900 parts per million total solids. Semi-batch pilot plant aerations of three days duration with recycled aerated waste as inoculum gave a 5-day BOD reduction or 0.0109 pounds per cubic foot, day. A full scale treatment plant to consume 13,500 pounds per day of 5-day BOD was calculated to require an aeration tank of 9,250,000 gallons capacity, with an air rate of 6,440,000 cubic feet per hour, and 1445 pounds of ammonium nitrate per day as a nutrient. / M.S.