Hello guys I only found this thread because Duanes kindly gave me a link to it as I'm intrested in building my own SW Protein Skimmer. After reading it I see that you've been talking about Freshwater Skimmers, I also had questions about Freshwater Protein Skimmers and posted a tread on ************** asking people if they would work in freshwater.
Here's their responses below, some of it's quite scientific but if you read it all you get the idea:-
by nick
Skimmer functionality is based on the fact that complex organic molecules(proteins, but also fats, hormones, and many other wastes) are polarized molecules - they have one side which has more of a negative charge, and one side with more of a positive charge. Salt in a salt water tank is not bonded into a single neutral piece - when in solution, NaCl is a bunch of Na+ and Cl- ions floating around randomly, keeping the overall mix neutral in charge. There's one place they don't float randomly - the surface of an air bubble, where they line up. These charges attract one end of the complex organic molecules more than the other, sticking it to the bubble. The smaller the bubbles, the longer they float through the water, and the more stable they are as they get covered with organic "slime." This is why most air stones don't create a dense, stable skimmate, even in salt water tanks, though they may create a bit of bubble scum. For all this to work, you need a LOT of NaCL, so there are enough charges on these tiny bubbles that they can bind enough slimy wastes to make stable foam - hence the minimum salinity requirement. Some people have reported sucessful skimming in freshwater - but it isn't really skimming. Their water, for whatever reason(possibly using tons of slime coat products, or it being a pond in the sun full of slimy scum) happens to have something in it that forms slimy bubbles. But dissolved complex organics of all sorts are not being removed by this process, as they are in real skimming - only the slimy gunk in their water which is so dense it makes the water foam. This foam is simply that, foam, not real skimmate.
by covus
Mmh... as far as I know skimmers come from freshwater, specifically from treating polluted groundwater.
But - as Nick already explained well in detail - those based on flotation e.g. by gas bubbles can only remove specific molecules and will remove much more in high salinity waters like sea water. Nonetheless, freshwater skimmers, which have several differences in contrast to marine aquarium skimmers, can be useful especially in freshwater ponds (they are not too rare over here) even if they are of little use in a home aquarium. The stuff such pond skimmers produce... well... looking at it, you will be quite happy that this stuff is no longer in the pond.
by nick
Skimmers are technically "foam fractitioners" and were originally developed for sewage treatment, from my understanding of the technology - I hadn't heard they were used for treating ground water, but I don't know much about that. As Corvus and I mentioned, they do remove slimes and other "gunk" that naturally forms stable bubbles in freshwater, rather than needing an "assist" from salt to stick to the surface of a bubble - luckily, a typical freshwater tank doesn't have enough of this to actually create foam. I believe densely stocked goldfish tanks have been known to generate quite a bit of foam, even with a salt water skimmer design, but one should not think the presence of this stuff means the water is being cleaned in quite the way a skimmer can clean a salt water tank and that one can reduce normal maintenance on account of this.
Myaj - I'm pretty sure when a tank is very well biologically established, bacteria take care of most of the ammonia and other wastes that end up as nitrates, while skimmers remove wastes that are "slow breakdown", such as fats, complex proteins(rather than just waste created by protein metabolism, like ammonia) etc. Some of these eventually end up as nitrogenous waste without skimming, but many don't.
by riggers
Skimmers produce larger bubbles in freshwater than in saltwater, therefore the overall surface area for organic molecules to collect on is reduced. The organic molecules are also more soluble in freshwater, so it is harder to get them to the air-water interface and gather on the bubbles, of which there is less surface area anyway. You can use skimming in freshwater, it's just less efficient.
