A method of separating an amount of an organic substance from water which is contaminated with the organic substance, said organic substance being at least one of a perfluoroalkyl or a polyfluoroalkyl substance (PFAS), the method comprising the steps of: - admitting said contaminated water into a chamber via an inlet thereinto; - introducing a flow of gas into a lowermost region of the chamber, wherein the introduced gas induces an upward flow of water in the chamber, and produces a froth layer which rises above an interface with the water in an upper portion of the chamber, the froth layer including a concentrated amount of the substance when compared with its concentration in the contaminated water first admitted to the chamber; - collecting a sufficient amount of said froth layer and, after allowing it to collapse back into a liquid form, passing said liquid to a second chamber via an inlet thereinto; - introducing a flow of gas into a lowermost region of the second chamber, wherein the introduced gas induces an upward flow of water in said chamber, and produces a froth layer which rises above an interface with the water in an upper portion of the second chamber, the froth layer including a further concentrated amount of the substance; and wherein - in said second chamber, regulating: (i) depth of the froth layer above the interface, controlling a physical parameter of the froth layer by using a froth layer depth regulation device for maintaining a froth layer depth above the interface, the device being arranged to be moveably positionable within the chamber in response to movement of the location of the interface or at a fixed location within the chamber, and the location of the interface is responsive to at least one of the flow of the introduced gas, and an inlet flow of the water, and (ii) depth of water in the chamber, by at least one of the group comprising: controlling a physical parameter of the flow of introduced gas; and controlling an inlet flow of additional water; said regulation being responsive to movement of the location of the interface such that the water content of the froth layer near the uppermost region of the second chamber is controlled, to influence the concentration of the substance therein, and wherein for at least one of the first or the second chambers, the upward flow of gas and the production of the froth layer occurs in a batchwise operational manner. - admitting said contaminated water into a chamber via an inlet thereinto; - introducing a flow of gas into a lowermost region of the chamber, wherein the introduced gas induces an upward flow of water in the chamber, and produces a froth layer which rises above an interface with the water in an upper portion of the chamber, the froth layer including a concentrated amount of the substance when compared with its concentration in the contaminated water first admitted to the chamber; - collecting a sufficient amount of said froth layer and, after allowing it to collapse back into a liquid form, passing said liquid to a second chamber via an inlet thereinto; - introducing a flow of gas into a lowermost region of the second chamber, wherein the introduced gas induces an upward flow of water in said chamber, and produces a froth layer which rises above an interface with the water in an upper portion of the second chamber, the froth layer including a further concentrated amount of the substance; and wherein - in said second chamber, regulating: (i) depth of the froth layer above the interface, controlling a physical parameter of the froth layer by using a froth layer depth regulation device for maintaining a froth layer depth above the interface, the device being arranged to be moveably positionable within the chamber in response to movement of the location of the interface or at a fixed location within the chamber, and the location of the interface is responsive to at least one of the flow of the introduced gas, and an inlet flow of the water, and (ii) depth of water in the chamber, by at least one of the group comprising: controlling a physical parameter of the flow of introduced gas; and controlling an inlet flow of additional water; (i) depth of the froth layer above the interface, controlling a physical parameter of the froth layer by using a froth layer depth regulation device for maintaining a froth layer depth above the interface, the device being arranged to be moveably positionable within the chamber in response to movement of the location of the interface or at a fixed location within the chamber, and the location of the interface is responsive to at least one of the flow of the introduced gas, and an inlet flow of the water, and (ii) depth of water in the chamber, by at least one of the group comprising: controlling a physical parameter of the flow of introduced gas; and controlling an inlet flow of additional water; said regulation being responsive to movement of the location of the interface such that the water content of the froth layer near the uppermost region of the second chamber is controlled, to influence the concentration of the substance therein, and wherein for at least one of the first or the second chambers, the upward flow of gas and the production of the froth layer occurs in a batchwise operational manner.