Article Foam Fractionation

Most people don't realize that Foam fractionators can be use in your fresh water set up
Foam fractionators (also called protein skimmers) are another type of filter that you may want to employ on your pond. FFs operate by mixing fine air bubbles with the water. Pollutants such as proteins, dissolved organic compounds (DOCs) and fine particles cling to the surface of the bubbles. FFs are designed to collect the waste laden bubbles and remove them from the pond. FFs have almost become standard equipment on salt water aquariums. If you have ever noticed an oil slick like sheen on the surface of your pond or if you observe foam forming around your waterfall, then your pond could benefit from a FF.

FFs are more effective on salt water systems than on fresh. A FF will work better on your pond when you have salt present than when you don't. FFs will work with salt free water, but they just won't be quite as effective. You can buy commercially made FFs or you can build your own from common plumbing parts. FFs are generally constructed as tall vertical cylinders. There are two basic types of FFs to consider for your pond: co-current and counter current. In both types, foam is collected out of the top and removed from the pond.

In a counter current FF, air is injected at the bottom by an air pump and air stone and the air bubbles flow upward. Water is pumped into the top of the chamber and flows down. Since the air and water are traveling in opposite directions, the air has more time to collect pollutants and is more effective. A counter current FF requires very little water pump pressure, but it does require a separate air pump.

In a co-current FF, the intake water is pumped through a venturi where it draws in air. The air and water travel together in the same direction. In the final stage of a co-current FF, foam is collected at the top and water exits out the side. Since the air and water are traveling togehter, a co-current FF can use a zig-zag chamber design which allows the unit to be shorter. A co-current FF does not require a separate air pump, but it does require greater water pump pressure to overcome the resistance of the venturi. You could also construct a co-current FF using an air stone and air pump to inject bubbles instead of a venturi.

Points to consider in FF design are: retention time and bubble size. The longer the retention time (time that water is in contact with air bubbles), the better the unit will remove pollutants. The retention time is simply pump rate divided by chamber volume. A long narrow path has the same retention time as a short wide path. Small bubbles work better than large bubbles.

The diagram shows examples of both types of FFs. The dimensions can be altered considerably to suit your needs. The vertical pipes can be anywhere between 3" and 12" in diameter (larger is better since it will give more contact time). Height can be from 3' to as tall as you want (again bigger is better). You'll have to experiment with the height of the foam collection area. If it's too short, you'll get a lot of water. If it's too tall, you won't get the foam removed. For more FF ideas, take a look at the commercial FFs available at your local salt water aquarium store.


Rick