Fully Submerged screens

I think it's a combination of things.

Laminar flow or turbulence is primary - without this you can't break the boundary layer where the nutrient exchange takes place. The boundary layer is what forms as you get microscopically close the algae on a cellular level - the flow comes to a stop as you get closer to this. A thin sheet of water is what breaks the layer on a waterfall scrubber. Air bubbles in a UAS create the turbulent environment. In a horizontal, surge, or slanted scrubber, it's the thin layer or the surge motion that works (a static very low slope horizontal scrubber with more than say 1/4" of water will not perform well either).

Placing a substrate in a high flow sump area is not enough. Sometimes placing one on a baffle works, but lighting becomes difficult. @srusso postulated that you could place a substrate between 2 baffles in a sump, but they would have to be pretty close together I would think.

In the 3D growth chamber concept such as the SM100 or my units, or others like it (where the growth chamber area is narrow, usually less than about 2.5" from pane to pane) the growth will extend outward and "pile up" and suspend the algae, the water level will rise and it appears like a submerged screen. One could look at this and say that a submerged screen works, but that only seems to be true in that limited case. I believe the reason is that the screen started as a waterfall, so the screen matures and grows thick initially as a true waterfall (no water submersion) and then as growth progresses, the 3D growth takes off. But this doesn't happen with a regular submerged screen because the screen doesn't ever reach a maturity level - algae doesn't anchor on to the screen in the first place. The growth cycle of the screen has to start as a waterfall and progress into the 3D stage.

I think that leaving a screen in the 3D stage for too long actually causes the 'roots' or 'anchor algae' to 'ease up' on it's foothold. I've seen this when I leave a screen grow for more than say 18 days - when I clean it, not much is left in the holes. This was also very much true almost all the time of my UAS test unit, which was essentially an L2 with no bottom drain hole. It took well over a year for algae to anchor to the screen, and it never did reach the maturity level of any waterfall screen I have had (where the algae filled the holes). This led me to the conclusion that the plastic canvas knitting screen was not the correct subsrtate for a UAS.

 

I think that it gets to a point of diminishing returns, where you have water essentially bypassing the screen, at least in an open screen scenario. Excess flow would go right over the top and not touch any algae.

With an enclosed system there is less chance of that, especially when you get to the 3D phase.

Water chemistry may affect overall growth too.

That all being said, the amount of algae you can actually grow I think depends largely on the system you are running the scrubber on. Theoretically if you have an unlimited and perfectly balanced nutrient source to feed to the scrubber, you could jump the flow way up and get more and more production.

You may find you get very good production so you increase the flow to get more, and actually see a reversal. See @Jason's thread on RC here for an example of that, go back a few months and then forward where me shows pics after he increased the flow

 

Not off the top of my head...sorry! I don't even know if I have any. I probably could dig up a post-cleaning UAS screen though

 

Look at the pic of the screen on the cutting board in post #8 here

Turbo's L2 Waterfall vs. L2 UAS

stained green was the best I ever really got, you can see how much algae grew but the screen look nothing like what a waterfall scrubber looks like with the same amount of growth