Protein skimming or foam
fractionation is the process by which dissolved and particulate
organic carbons are removed from a liquid, in this case the fish
culture water, by adsorbing them onto the surface of fine bubbles
rising in a closed contact column against a counter-current water
The bubbles burst and form stable foam at the top of the water
column and the accumulated organic wastes are discharged from the
column along with the foam produced.
The removed substances are called "surfactants" because they are
surface active or "charged"
Benefits of Protein Skimming include:-
- Removal of suspended
- Removal of proteins and
high molecular weight compounds.
- Increased water clarity
through removal of humic and phenolic compounds.
- Increased oxygenation of
the culture water.
- pH stabilisation through
removal of organic acids.
Performance is defendant on:
- Air to water ratio
- Air bubble diameter
- Column height
- Air/water contact time
- Use of ozone
Bubble size is the most
important of these and within the control of the design of the
An efficient air diffuser or venturi plays an important part in
generating a swarm of 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.
Generally it is difficult to generate a small enough bubble size
in freshwater to achieve effective foam production (except in
cases of very high organic loading) and therefore protein skimming
is mainly only recommended for seawater or brackish applications
where the salinity is more than 16 ppt. However, a number of
freshwater skimmers are available for use in ponds.
Protein skimming is further enhanced by the application of ozone
into the air mixture, which further oxidises and breaks down
complex organic compounds as well as providing a degree of
disinfection, depending on the quantity used.