Protein Skimming/Foam Fractionation Introduction
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 flow. 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 solids
- 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
The performance of the protein skimmer is dependent on many factors:
- 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 protein skimmer. 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 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, several 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. For further information on ozone.
Protein Skimmers Technical Information
The flow rate through a skimmer should not exceed the equivalent of the volume of the contact chamber being exchanged in less than one minute - ideally the resonance time of the water in the skimmer should be 1.5 minutes, and up to 2 minutes for high stocking density/feed rate applications. Recommended system turnover through the skimmer should be at least 1x total system volume per hour. For large public aquarium displays with low stocking densities, this is reduced to every 2 hours.
Ozone should be applied on an individual tank basis at the equivalent of 5–15 mg/hour per 100 litres of volume. In commercial applications, 0.05 mg ozone per litre of flow is sufficient to achieve a recommended redox of 300–400 mV. A much higher dose rate of 0.2–0.5 mg of ozone per litre of flow is required to achieve redox levels of 700–800 mV at which the water can be considered sterilised but unfit for livestock without further treatment.
All TMC Protein Skimmers (including those installed on complete filtration systems) come with internal and external rinsing fittings to keep the skimmer clean and scum-free at all times. Regular washing and rinsing inside the skimmer prevent the build-up of fatty deposits and keeps the skimmer working efficiently. An automatic battery timer valve can be installed on each of the internal and external washers to routinely and automatically rinse down the skimmer cup, reducing maual maintenance.