The traditional process for separation of free oil in wastewater is the use of a gravity separator designed per guidelines and recommendations of the American Petroleum Institute (API), publication #421. Flow distribution, surface loading rate (rise rate), tank geometry, effective surface oil removal are some of the important design considerations in Tenco Hydro's API gravity oil separator. Oil characteristics such as specific gravity, viscosity and temperature are applied to Stoke's Law to determine the design rise rate for capture of oil globules 150 microns or larger. Separator size is determined by influent flow rate and rise rate. Tank proportions and flow distribution are per API guidelines.

Options offered for removal of separated floating oils are roll skimmers and rotating pipe skimmers both with and without internal oil storage. Where site conditions allow, Tenco Skim Kleen automatic belt skimmers are sometimes used. For enhanced operation, mechanisms for surface skimming or settled solids removal are also available. Covers are also a common addition. As with other Tenco Hydro equipment, materials of construction can be chosen to suit project requirements.