The purpose of this investigation was to study the aerobic digestion of trickling filter humus and the relationship between digestion and the subsequent filterability of the sludge. Five individual, ten liter, batch-type digestion studies were conducted at 20°C for detention periods ranging from 22 to 38 days. Various water quality parameters were monitored during the three digestion runs of trickling filter humus, one digestion run of a 70% primary and 30% trickling filter sludge (by volume), and one digestion run of waste activated sludge completed during this investigation. Results of the investigation showed that trickling filter humus responds to the aerobic digestion process in a manner similar to that of waste activated sludge. Significant total and volatile solids reductions along with consistent mixed liquor BOD₅ reductions were noted for digestion periods of 22 days. Using the concept of specific resistance, the filtration characteristics of the aerobically digested humus were determined to be poorer than those usually obtained for aerobically digested waste activated sludge. The degree of filterability was observed to deteriorate during periods of prolonged endogenous respiration. The primary-trickling filter sludge mixture was observed to adapt to the aerobic digestion process in a manner strikingly similar to that of the purely biological trickling filter humus. / Master of Science