Gravity polyethylene oil separators for oils and hydrocarbons. The oil separators are designed to retain oils and hydrocarbons from yards, car parks and machine shops. They are made entirely of polyethylene and are of the vertical type and sized on the basis of square metres, ranging from 30 square metres to 2000 equivalent inhabitants. The de-oilers have a smooth or corrugated surface with a high thickness, aimed at suitably resisting backfilling operations. The oil separators have provisions for wastewater inlet and overflow, and inside the separators there is separation by gravitation of the largest particles of wastewater.