Skimming Efficiency

[hillrc91]

Randy,

A couple of folks have told me that Salinity effects the efficiency of protein skimmers... Specifically, a salinity of 35ppt or higher increases foam production, while lower a lower salinity decreases it. Is this true, and if so, why?

Richard

[Randy Holmes-Farley]

I doubt that slight salinity differences impact skimming much, but the primary way that I believe salinity impacts skimming is that many organics are less soluble in salt water than fresh. And the solubility declines as the salinity rises. So organics are less "happy" in the water, but still equally happy at the air water interface, and so skimming works better in salty water.

I do not believe arguments that I sometimes hear from hobbyists about salts themselves impacting surface tension. I do not think that is particularly important for skimming.

I cover that a bit in this article (the first reef article that I wrote, years ago):

Protein Skimming: How It Works
http://web.archive.org/web/20030415033358/http://home.mweb.co.za/jv/jv79/reef/skimmers2.html

from it:

"1. Why does skimming work better in salt water than in fresh?

It is not that bubbles form more readily in salt water. In fact, in highly purified salt water (no organics at all) the surface tension is greater than in pure water, and bubbles will be harder to make. Any "evidence" that bubbles form more readily in salt water is due to organics present. remember, only 0.002 g of an amphipathic molecule would be sufficient to completely cover a large aquarium. Thus, only a tiny bit of contaminating organic can make salt water appear to form bubbles readily. One of the main reasons for better skimming in salt water is actually the reduced solubility of organics, especially hydrophobic ones. Since organics are typically less soluble in salt water, they are more easily squeezed out of it to an air/water interface, and collected as foam. This is the basis for the well known salting out effect of proteins. Quoting from a basic biochemistry text: "At sufficiently high ionic strength a protein may be almost completely precipitated from solution, an effect called salting-out." It is not impossible, however, to skim fresh water. Rivers from certain areas of the northeast US often have foam on them which comes from tree sap and other natural things that enter the water. They have a low solubility in water, and are easily collected as a foam. Other organics, with some solubility in fresh water, just have less attraction for the air water interface, and are consequently harder to skim. "