Fans/Air Exposure/Gas Exchange... why?

Can someone give me a technical explanation, or a resource where I can read about why you would want the algae to have such good coupling to the air? There are several scrubber designs that would become advantageous if you could submerge your screen completely, and so I'm curious why you would not do this. Photosynthesis consumes CO2, I know, but is there enough CO2 in the air for the turbulent screen to really pick it up and bring it to the algae? I'd think just the opposite would be true, and any dissolved CO2 would off-gas into the environment and your water would be oxygenated in the process. Algae growth in your tank doesn't seem to be correlated to strong gas exchange - but only proximity to a light source.

Being able to simply build a thin chamber and fill it with water would remove a lot of the design challenges of trying to get even flow across your screen, prevent spray, use multiple screens with a single light source, and so on, so if someone can answer this it would be great.

I suspect that leaving the turf exposed to air occasionally doesn't have a lot to do with CO2 but more to do with reducing the number of competing algae that would be able to also grown there. Also while I see your point on the filled box you are missing the point that bright light is going to grow stuff everywhere. If you shine this bright light thru a window submerged on the inside it's going to grow stuff too, obscuring the turf's light.

The reasons screens out-grow other surfaces like rocks and glass is that the flow is much more rapid across the screen material, and also of course, because the light on the screen is (usually) stronger. Either factor alone is enough to out-compete, but together they are killer :) Last but not least is the weekly cleaning, which gives new algae a brand new chance of attaching to already-established algae that is left on the screen. Much easier than trying to attach to slick glass.

Flow not only delivers more N and P and CO2 to the algae, it also disrupts the "boundary layer" around the algae... the microscopic layer of non-moving water that "insulates" the algae from the moving water. Going dry (pulsed) also does this, as well (as kcress said) as it helps kill off green slime that would otherwise shade turf underneath. Of course, slime consumes N and P too.

Algae, however, does not remove CO2 from the air; it removes it from the water. Scrubbers can work just fine with no air/fan at all. It's just the attemps at high-performance (and yet unproven) scrubbers that use pulsing and fans to simulate the beach.

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Originally Posted by kcress

If you shine this bright light thru a window submerged on the inside it's going to grow stuff too, obscuring the turf's light.

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Fair enough, but if the window is smooth and clear the algae will not be able to gain as easy of a foothold there - especially if it's already established on the screen. I'd just wipe whatever algae off the window when I cleaned the screen. I also don't plan on growing turf, as mine will be a FW setup.

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Originally Posted by SantaMonica

The reasons screens out-grow other surfaces like rocks and glass is that the flow is much more rapid across the screen material,

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You've said this before, but my question was WHY? Is this just speculation, observation from experience, or scientific fact based on someone's research?

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Originally Posted by SantaMonica

and also of course, because the light on the screen is (usually) stronger. Either factor alone is enough to out-compete, but together they are killer :) Last but not least is the weekly cleaning, which gives new algae a brand new chance of attaching to already-established algae that is left on the screen. Much easier than trying to attach to slick glass.

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I would not be trying to get it to grow on glass, I'd still put a screen in the chamber for it to grow on, complete with weekly cleanings leaving some"rooted" algae behind. Having one established surface in the chamber for the algae to grow on should cause it to grow there preferentially to the window the light shines in through. As I said above though, anything that grows on the window can be wiped off with the weekly cleaning.

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Originally Posted by SantaMonica

Flow not only delivers more N and P and CO2 to the algae, it also disrupts the "boundary layer" around the algae... the microscopic layer of non-moving water that "insulates" the algae from the moving water. Going dry (pulsed) also does this, as well (as kcress said) as it helps kill off green slime that would otherwise shade turf underneath. Of course, slime consumes N and P too.

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We got into this a bit on the MFK discussion thread, and the long and short of it is I completely disagree. Boundary layers are not stationary, and if you have a short chamber the water is flowing through, with no well established stream lines, there will be no established boundary layer. The only disadvantage I see to a 1" thick chamber vs a 1/8" sheet of water is that you can't be sure that 100% of the water going through that chamber has a chance to fully interact with the algae surface. I think however that if your flow rate is reasonable enough, this will only be a minor performance factor.

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Originally Posted by SantaMonica

Algae, however, does not remove CO2 from the air; it removes it from the water. Scrubbers can work just fine with no air/fan at all. It's just the attemps at high-performance (and yet unproven) scrubbers that use pulsing and fans to simulate the beach.

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That's what I thought. I guess I'm just thinking one step even simpler than that. Has anyone tried submerged screens? I saw one person in the misc designs thread had put together an elaborate HOB design that looked like it used submerged screens, but it was a CAD model.

One other advantage I can think of is that the slower, less turbulent flow through the chamber would not tend to tear off the long, fragile strands of hair algae as much, and they could stay in the light, continuing to absorb pollutants and grow instead of making their way into the filter.

Well this is the great thing about DIY turf scrubbers. Build one! Let's see how your scheme works! It might work really well and would be easier to build.

When working with 'life forms' things often do not occur as expected. That means a plan may work better than expected or worse.
You won't know till you try it.

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Can someone give me a technical explanation, or a resource where I can read about why you would want the algae to have such good coupling to the air?

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Photosynthesis plods along when any of it's requirements are restricted. However, it really takes off when you provide good levels of light, oxygen, CO2, and nutrients. At the air-water interface in the turbulent water flow the algae can grab exactly what it needs and is not held back by a shortage in any one of these requirements- it's algae growth on steroids ;) . You could probably draw a good analogy between a waterfall ATS and plant hydroponic systems as both systems enhance growth by optimizing the nutrient and gas exchange requirements.

I can understand why you would want to build a closed system but you would be losing out on the rapid growth, unless you can come up with some alternative way of providing all the requirements the algae would probably grow just as fast in your tank as it would in the scrubber. The waterfall scrubber system has other benefits besides nutrient export. Since I installed mine my tank PH is much more stable and the Oxygen level in the water has improved to saturation point :D

Cheers,

Pat. Pending

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"The reasons screens out-grow other surfaces like rocks and glass is that the flow is much more rapid across the screen material" You've said this before, but my question was WHY? Is this just speculation, observation from experience, or scientific fact based on someone's research?

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Very much scientific, just do a google scholar search. I don't have link on hand, as I read many and did not have a reason to save them. You could also as the experts on the MD forum.

From experience I can also say that this is the case, I have my mesh 2" into the water at the bottom and there's hardly any growth on it at all, and the growth stops bang on the water line, and hardly anything below, and lots of vigorous growth above.
Similarly, in my tank, I used to get large amounts of algae growth on the glass support brace at the top of the tank where the light was highest, and where it only just fell below the water level when the powerhead was directed at it, with the most turbulent water and very shallow as the waves lapped over the edge of the brace.