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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 hydrophobic silica can adsorb and transfer surfactants between phases. Hydrophobic 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 reducing surface viscosity. For example, tributyl phosphate intercalates into the surfactant
layer and interferes the cohesive forces between the surfactant molecules. Thus, the surface 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 decreases 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 temperature.144 Heat treatment can be applied, for example, in wastewater treatment plant
(WWTP) to eliminate undesired foaming of the sludge digester. 142
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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, compression 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) centrifugal basket, d) whirling paddle.144
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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.145 Ultrasounds enhance the foam collapse but affects the liquid drainage rate only at the top of the foam layer. 146 Foam elimination 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 cavitation.147