Produced water typically enters the water-treatment system from either a two or three phase separator, a free water knockout, a gun barrel, a heater treater, or other primary separation unit process. It probably includes small amounts of free or dissolved hydrocarbons and solids that must be removed before the water can be re-used, injected or discharged. The level of removal (particularly for hydrocarbons) and disposal options are typically specified by state, province, or national regulations. This article discusses techniques for the removal of free and dissolved hydrocarbons. Intermittent flow * 3 Gas flotation units * 3.1 Dissolved gas units * 3.2 Dispersed gas units * 4 Deoiling hydrocyclones * 5 Centrifuges * 6 Walnut shell filters * 7 Removing dissolved hydrocarbons from water * 7.1 Hydrocarbon discharges * 7.2 Determining removal * 8 Nomenclature * 9 References * 10 Noteworthy papers in OnePetro * 11 Other noteworthy papers * 12 Online multimedia * 13 External links * 14 See also * 15 Page champions * 16 Category Produced water contains small concentrations (100 to 2000 mg/L) of dispersed hydrocarbons in the form of oil droplets. In applying these concepts, one must keep in mind the dispersion of large oil droplets to smaller ones and the coalescence of small droplets into larger ones, which takes place if energy is added to the system. The amount of energy added per unit time and the way in which it is added will determine whether dispersion or coalescence will take place. Stokes' law, shown in Eq. 1, is valid for the buoyant rise velocity of an oil droplet in a water-continuous phase. Several immediate conclusions can be drawn from this equation. The third conclusion requires further elaboration.