Controlling Foam in a Wastewater Evaporator

Some industrial wastewater has surfactants that cause foam build up when the water is agitated. Common sources for such wastewater are pressure washers or parts washers. A simple first step for foam control is to monitor the amount of soap used while cleaning by gradually stepping down the dosing rate until there is just enough surfactant to effectively clean.

wastewater foamThe design of the wastewater evaporator can also have an impact on foam build up. Evaporation is a result of boiling, which creates air bubbles and turbulence. If the boil rate is moderate and consistent across the entire surface of the water, or a flat boil, less foam will generate. Evaporator designs with an even heat source generate a flat boil. Some examples are systems using fire bricks or heat transfer oil between the burner tube and the water tank, as seen in RunDry Evaporators™. These systems run on the principle that evenly distributed heat creates an even boil and thus less foam.

If the boil is low in some spots on the surface and eruptive in other areas, or an eruptive boil, it will increase the foaming effect. Eruptive boil is caused by hotspots in the heat source. In electric evaporators, this happens when the heater is submerged in the water. In gas evaporators, this happens when the burner tube, sometimes called a serpentine tube, is submerged directly into the wastewater. In both cases, the water directly surrounding the heating source will get much hotter than the water 6 inches away, for instance. As a result, the boil directly above the heat source will be eruptive, causing more foam. There are some evaporator designs that have holes in the burner tube so the gas from the combustion process bubbles through the water on its way to the exhaust stack. This design is an effort to create more heat transfer by having the off gas bubble through the water. However, that process will create even more foaming.

When purchasing a wastewater evaporator, it’s best to be mindful of design characteristics that will best suit your purposes. If you are unsure whether a particular design will work for you, some manufacturers offer a pre-purchase bench test done with your wastewater, and some even for free. Take advantage of these tests and specifically ask about the foaming. There are scenarios in which minimal soap and a flat boil evaporator design do not prevent foam build up. In such a case, a chemical defoaming agent, or defoamer, can be added to the water to prevent foaming. Some wastewater manufacturers can recommend a defoamer, but you may have to try more than one to get the best results. Of the many available, some may work better than others depending on the source of the foam.

David N. Lyman, president of RunDry Evaporators™, is a mechanical engineer with 40 years of industrial experience.

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