Unwanted foam can be eliminated by chemically with antifoamers or by physical meth-ods which include thermal temperature control or various mechanical methmeth-ods, such as sonic defoaming, liquid sprays or foam breakers (centrifugal basket and rotating disk).
In cases when antifoamers can be a contamination risk or can prevent refoaming, the mechanical methods are applied. Mechanical methods are also used with heat-sensitive materials that cannot tolerate thermal treatments.
5.3.1 Defoamers
Defoamers, or antifoaming agents, are used to reduce undesirable foaming. In some cases, the replacement of the surfactant with a poorer foamer can help to diminish the foam volume. If the replacement is not effective enough or the foaming is caused by another component that are not surfactants, the antifoaming agents can be added into the solution. Antifoaming agents work by transferring the surface active agents from the interfacial surfaces back into the liquid phase. For example soil particles and hydropho-bic silica can adsorb and transfer surfactants between phases. Hydrophohydropho-bic particles also tend to form lenses at the Plateau borders of the foam thus destabilizing the foam structure by enhancing dewetting.6
Another way to reduce foam is to remove the surfactant film on top to the liquid surface by displacing it with poorly foaming molecules. These molecules need to be rapidly diffusive, non-cohesive and not totally soluble in the solution. Substances, like tertiary acetylenic glycols, reduce the surface tension and the elasticity of the solution so that the bubbles break instantly back to the solution. Foaming can also be prevented by re-ducing surface viscosity. For example, tributyl phosphate intercalates into the surfactant layer and interferes the cohesive forces between the surfactant molecules. Thus, the sur-face viscosity lowers, and foam drainage accelerates leading to collapse.6
5.3.2 Physical methods
Thermal treatment of the foam comprises temperature changes. At high-temperature foam tend to collapse due to decreased viscosity of the surface, solvent evaporation and chemical degradation of the foaming agent. A low-temperature surface elasticity de-creases or freezes unstabilizing the foam structure. Heat collapse of the foam can be done using hot wire placed over the foam solution, wrapping a heat tape around a foam container, passing hot steam or water on top of the foam or by reducing the tempera-ture.144 Heat treatment can be applied, for example, in wastewater treatment plant (WWTP) to eliminate undesired foaming of the sludge digester.142
Liquid spraying uses strong liquid and sprayers to collapse foam. Impact, compression and shear forces destruct the foam structure and cause the collapse. Foam can be sprayed, for example, with pure water which in addition to mentioned physical effects also dilutes the foam solution and instabilizes the foam. The addition of antifoamers into the spray solution further enhances the collapse process. Liquid spraying is applied in many WWTPs for foam control.144
Mechanical foam beakers expose foam under rapid pressure change or shear, compres-sion or impact force. Whirling paddle, or rotating rod, is a foam breaker used mainly for dry foams. Rotating rod breaks the foam with impact and shear forces. The centrifugal basket can be used for large amounts of wet and stable foams. Centrifugal foam breaker is made of a metal bowl of mesh screen that spins inside an overturned bottle and breaks the foam with pressure change and shear forces. In orifice foam breaker the foam is drawn through an orifice using a vacuum that generates a pressure change breaking the foam.144 Different mechanical foam breakers pictured in Figure 14.
Figure 14. Mechanical foam breakers. a) bent rotating stirring rod, b) orifice, c) centrif-ugal basket, d) whirling paddle.144
Sound (< 20 kHz) and ultrasound (> 20 kHz) can be used for foam elimination. Sonic defoaming crates acoustic pressure, the resonance of the bubble and cavitation of the liquid film causing foam collapse. Sound wave reflection can be done from the surface or inside of the foam. Sound frequency, pressure and viscosity of the liquid have an effect on the efficiency of the foam destruction.145Ultrasounds enhance the foam col-lapse but affects the liquid drainage rate only at the top of the foam layer.146 Foam elim-ination by sound created by a loudspeaker revealed that sound caused detaching of small droplets form the surface of the foam layer and inside the foam a strong cavita-tion.147
6 SURFACTANT REMOVAL METHODS
Removal of surfactants from environmental water and wastewater can be done with different kind of removal methods that can be divided into degradation and separation methods (Figure 15). Degradation methods include biodegradation by microbes, photo-catalytic degradation and electrochemical oxidation. Separation can be achieved with adsorption or chemical precipitation/flocculation, where surfactant in the influent is transferred into a sludge, or with membrane technologies and foam fractionation, where influent is divided into two separate sections with different surfactant concentrations.
The removal efficiency of the methods can be confirmed by determining the surfactant concentration of the water sample (Chapter 3.) or by measuring the amount of organic material in the water sample using TOC, COD and BOD tests.
Figure 15. Surfactant removal methods.