Batch studies to determine the effects of pH on aerobic digestion of waste activated sludge were performed using a detention time of at least 20 days. Total and volatile solids reductions were not affected over a pH range from 3.5 to 9.5. Somewhat greater solids reductions were realized when pH was held constant rather than allowed to vary as occurs with normal aerobic digestion. High reductions in mixed liquor BOD were consistent among digesters over the pH range investigated, however, sludge digested 23 days at pH 9.5 developed a putrescible odor when aeration was discontinued for 24 hours. Cellular carbohydrate and cellular protein accumulated during digestion, but no distinct relation to pH 1 evel was obvious although cellular carbohydrate showed a slightly greater accumulation with lower pH. Striking improvement in the settleability, drainability and filterability of sludge digested at pH 3.5 occurred. This effect was attributed to the development of an - overwhelming protozoan population which caused observed improvement in the flocculation characteristics of the sludge. / Master of Science