A wide range of physical-chemical separation processes were studied to evaluate their effectiveness in purifying municipal and industrial waste waters. Laboratory investigations showed that adsorption by granular activated carbon is an effective method of removing organic contaminants but that other materials studied as possible low-cost adsorbents failed to produce consistently effective organic removals. Laboratory and pilot plant foaming of secondary effluent removed 30 to 40 percent of the organic contaminants and 70 to 80 percent of synthetic detergents. Single-pass electrodialysis reduced the concentration of salts by 40 to 50 percent. Initial work on distillation has established that volatile contaminants carry over into the distillate to produce a detectable odor, requiring further treatment by activated carbon. Laboratory experiments on reverse osmosis indicate considerable promise for this process, but much development work remains to be done on factors such as membrane life, flux rates, and biological effects on membranes. In studies on freezing, fine ice crystals are seen to agglomerate into large or massive chunks if the ice matrix is subjected to moderate compression. The agglomeration limits the ability to wash contaminants from the ice crystals. Solvent extraction, with lowmolecular-weight secondary and tertiary amines, has produced water with an organic content of less than 10 milligrams per liter and a total dissolved solids content of less than 150 milligrams per liter.