Foam Fractionators or Protein Skimmers

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Foam Fractionators or Protein Skimmers

April 05, 2014 - 12:24

My ponds are finally coming out of winter, and the endless pursuit for better water quality this year has started!  I found this great article from Don Harrawood, Southwest Koi and Pond Association El Paso, TX.  It's great, detailed info on how to build one yourself, and I wanted to pass it along to all our readers.  There are lots of commercial versions available for sale (see one in use in the picture below) - but DIY is so much cheaper and more satisfying!

Foam Fractionators or Protein Skimmers:

Have you noticed foam on top of your pond?  Even though you have a good filter, is your water sort of a dark color?  These conditions are generally caused by excess dissolved organic solids, a condition that generally cannot be cleared by the filter alone.  Water changes will tend to clear the water; however this will most likely be a recurring condition after a short time.  One needs to find the cause for these conditions and a way to prevent them in the future.

Dissolved organic solids build up through the metabolism of koi and other aquatic organisms, depositing their products of digestion into solution into pond water. Protein levels within the water can also increase rapidly through the inappropriate use of high protein fish foods.  Compounds from uneaten fish food can leach into the water, leading to the formation of foam.  Other compounds causing foam include a variety of fats, fatty acids, carbohydrates, metals, detritus, phytoplankton, and trace elements.  Spawning activity can cause a foaming pond because of the release of large amounts of protein matter (in the form of eggs and sperm) into the water.  All these materials combined, causes an enormous quantity of different organic solids dissolved in the pond water.  These dissolved solids when subjected to water agitation, such as waterfalls, result in the formation of foam.

Foam fractionation or protein skimming is a process by which dissolved organic compounds are removed from a liquid by adsorbing them onto the surface of fine bubbles. The bubbles collect proteins and other dissolved substances, and carries them to the top of a device where the foam collects in a cup. Here the foam condenses to a liquid, which can then be easily drained from the system. The material that collects in the cup appears as a pale greenish-yellow liquid.  Constant removal of these compounds will help clear pond water and result in better overall water quality.

All foam fractionators have key features in common.  For one to function effectively, the following features must be presentA large amount of air/water interface must be generated.

  • A large amount of air/water interface must be generated.
  • Water containing dissolved organic solids must be allowed to flow through the air/water interface.
  • The bubbles must accumulate to form of foam.
  • The water in the foam must partially drain without the bubbles popping prematurely.
  • The drained foam must be separated from the bulk water and discarded.

Bubble size is the most important of these parameters and is controlled within the design of the protein skimmer.   An efficient air diffuser or venturi plays an important part in generating the bubbles that are as small as possible, ensuring maximum surface area for the adsorption of the organic compounds. Smaller bubbles also rise more slowly, allowing more contact time with the process water.

The 1” dia. x 12” pipe slides up and down inside the reamed out bushing.  You should make this kind of an interference fit so that it does not slide too freely or else the foam collector will not stay elevated to any fixed location.

The foam collector can be any size.  For example, use a gallon bucket, with a hole drilled in the bottom, and assembled with a 1” bulkhead attached to the bucket and the pipe with appropriate fittings.  A hose can similarly be added to the bucket to drain the foam.

This is a typical protein skimmer that can be built by anyone handy with using ABS or PVC pipe.  The cost of building this apparatus is less than $100. 

The water is pumped through the venturi, in which fine bubbles are introduced, and enters the skimmer body.  The input of air from the venturi creates a large volume of oxygen rich water which passes through the main column.  The foam and air then rises above the water surface and passes through the 1 inch stand pipe, collecting the foam in the chamber above, which requires either manual emptying or is fitted with a drain to waste.  The main water flow then empties back into the pond through the 3 inch return pipe.

When a foam fractionator is first installed, large quantities of foam and greenish-yellow liquid are first formed. Over time, as the DOC concentration drops, so does the rate at which the foam is formed and removed. When run continuously, once it has cleared the residual problem, it should keep the water free from excess dissolved organic solids.  After operation for a few days, a noticeable improvement of water clarity is generally realized.

Don Harrawood


!If you're not havin' FUN, you're not doin' it right