Skimmer Efficiency Reduction on Freshwater

[hart24601]

Hey Randy, question bit outside the normal reef topics, but I was wondering if you knew or could guess the reduction of efficiency of a skimmer from SW to FW. I know in freshwater bubble size is larger so skimmers don't work as well, but I have heard about people running skimmers and FW and they do report skimmate.

I know for most FW setups waterchanges are cheap, so there isn't a need to spend money on equipment or running cost, but where water quality is poor, like here in Iowa I have to use an RO/DI drip in my FW tanks. I know there are other options, but it was just something I was thinking about, possibly with carbon dosing. Even with the drip nitrates tend to be higher than what I want - I am shooting for 0 for fish growout - and using the RO/DI there isn't enough other nutrients in the water to really use plants as a good export, they just don't grow fast enough.

 

[Randy Holmes-Farley Verified Community Expert]

I discuss that issue here:

What is Skimming? by Randy Holmes-Farley - Reefkeeping.com
http://reefkeeping.com/issues/2006-08/rhf/index.php

1. Why does skimming work better in saltwater than in freshwater?

There are two fundamental reasons that skimming is more effective in seawater than in freshwater. One is the reduced solubility of organics, especially hydrophobic ones. Because many organics are less soluble in saltwater than in fresh, 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."

A second reason for less efficient skimming of freshwater relates to bubble formation and coalescence. It turns out that air bubbled into seawater forms smaller bubbles than if the same device bubbled into freshwater.1-4 The possible reasons for this have been discussed in the scientific literature, but the exact reason is not universally agreed upon.

Despite the fact that skimmers usually produce larger bubbles in freshwater, and that organics are often more soluble in freshwater, it is not impossible to skim freshwater. Rivers from certain areas of the northeastern United States sometimes have foam on them, which comes from tree sap and other natural organics that enter the water. They have a low solubility in water, and are easily collected as foam in a natural skimming action.

 

[hart24601]

Dang.... and here I thought I had a question you wouldn't have already answered in some form!

How do you think carbon dosing and FW skimming would work?

Perhaps a bit off topic, but the other idea I was considering for nitrates with this system are 2 of the 8x4x4 marine pure blocks. I have a large sump, and there is a large area of very little flow (pretty much extra room for a power outage to hold water). Do you think the blocks in that spot would perform any meaningful denitrification, aka similar to a remote deep sand bed?