Pressure controlled gas flotation

A method of removing oil from an oily water stream by passing the oily water stream through at least two stages of treatment, wherein within each stage oil droplets are floated out of the oily water stream with the help of gas bubbles originally dissolved in a water phase, oil phase, or in both the water and oil phases, the method comprising the steps of:

• pressure controlling a release of a volume of dissolved gases contained within the oily water stream by reducing an operating pressure of a first stage flotation separator below that of the oily water stream in an immediate upstream unit;
• and
• pressure controlling a release of another volume of dissolved gases contained within the oily water stream by reducing an operating pressure of a second stage flotation separator below that of the first stage flotation separator;
• wherein the volume of dissolved gases being released from the oily water stream in each stage flotation separator is a controlled percent volume relative to a total volume of water treated in the stage flotation separator, and the method further comprises,
• prior to pressure controlling the release of the volume, increasing the operating pressure and amount of dissolved gases of the oily water stream prior to it entering the first stage flotation separator by providing the oily water stream to a pressure tank;
• adding induced gas bubbles to the first stage separator;
• and
• entraining the oil and released dissolved gases in a rising path of the induced gas bubbles in the first stage flotation separator.