The results of the investigation demonstrated that chemical coagulation with salts of Fe(III) and Al(III) can complement and can substantially enhance the treatment efficiencies of existing wastewater treatment facilities. Effective removals of the hydrophilic organic colloids present in trickling filter effluents were achieved with the Fe(III) coagulant. Both laboratory and pilot-plant scale studies showed that consistent removals of COD of 70 to 90 percent could be obtained, despite variations in waste strength and composition. Bacterial cells were destabilized and removed from solution concurrent with coagulation of other colloidal matter. Approximately 89 percent of the viable bacteria initially present were removed at the optimum coagulant dose in the pilot studies. The continuous-flow studies showed removals of total nitrogen and total phosphate of 16 and 85 percent, respectively. Projected chemical costs are given.