Algal Mat Fluid Dynamics

[Garf]

From regular inspection of the visual liquid flow over my scrubber in the last 12 months, I have noticed that when the screen is full, perhaps 80 or 90% of the flow goes Over the surface of the screen. This would suggest that the majority of the algae (when 15 or 20mm thick) is receiving very little relative flow (presumably the thickness of algae causes a restriction in flow). Anyone got any ideas of improving flow within the mat? I think the one sided screen cleanings help, but still think we are not taking full advantage of the volume of algae.

[Floyd R Turbo]

I have thought about this as well. This goes back to the 3D growth factor - when you grow in an enclosed box, and the algae gets thick enough, it will 'trap' water in a sort of suspended column of water that is constantly in motion, which is really the answer here.

But, if you are running an open screen, I think adding the saran wrap over the entire screen helps to "push" the water into the algae mat.

Then there is the though that even though a majority of the water is flowing over the top of the screen, this is really the only area that is growing most effectively anyways as it is exposed to the most light. The lower layers are shaded and not doing so much filtering, but as long as they are not dying, they are not detracting from the effectiveness of the screen. They are simply supporting the thicker mat's ability to grow even more thicker.

[Garf]

Do you reckon a heavier screen, Perspex for example, would move out with the algae growth. Ie not a fixed Perspex screen, but one that floats on the algae growth. Not explaining this well am I.

[Floyd R Turbo]

I'm following you. This would be an alternate to the flim-like saran wrap. a thin acrylic sheet, say 1/16" or less, that would have more rigidity and serve the same purpose - to "squeeze" the water flow so that it is closer to the screen?

I think it would almost have to be something with ends on it also, enclosing the screen by 4 sides, or else the water flow would eventually go out sideways possibly.

So maybe not 100% rigid but stiffer than saran wrap.

[Garf]

Yeah, that's it. Using the mass of the Perspex to force the flow through the mat. Well, that was the easy bit, how the hell do I try this without (like you said) squirting it all out the ends. Bare in mind, I'm not plastic fantastic, like you.

[Ace25]

Hmmm.. thought of this many years ago when I built my first ATS, which was double screens (at the time was recommended). I decided against the idea and instead split my overflow into 2 separate screens.

The idea.. an inverted V for the screen. Just a slight angle though, like take a dual layer screen and put a few 2" pegs at the bottom on the dual layered screen to spread the screens apart. As the algae grows, the water will still be forced to go through the algae, where as with completely vertical screens it can just sheet over the top of the algae, and in some cases, severely reduce the algaes efficiency.

I buried that idea in the back of my head so many years ago that it took reading this thread to pull it back out. Any thoughts on that idea, yeah or neigh?

[celtic_fox]

Tagging to follow the thread.

Also, I'd always planned to slope one sided waterfall and UAS screens by about 5% (reverse on the UAS of course). But as I'm just now transitioning from design to build on my first scrubber, I can still change that depending on where this discussion ends up.

[kotlec]

Whenever I got green hair growing on screen , it always starts from the very top. Gradually strands cover screen completely and become thicker at the same time (aka 3D). But "roots" always are in top inch of screen . Alien growth then is completely covered with green. I never observed any dye off though. Algae under green hairs remains yellowish (never whitening or darkening) and hard to clean.

There is my onesider. Usually much less growth. This was after snail was dead and I found it after returning from one week vacation.
http://algaescrubber.net/forums/attachment.php?attachmentid=4205&d=1366266540

[Garf]

Well, no expense made, as normal for me. Got some thin semi flexible plastic, like Floyd used on his UAS, maybe a bit thicker. Stuck it to the inside of my slot shield.

Mark 1

http://i1269.photobucket.com/albums/jj597/Garf1971/33c0a620c1a18e2118bd9622deafc03e_zpsf8eb1463.jpg

[Floyd R Turbo]

For reference, he is referring to the material I used at one point on my UAS to "trap" the bubbles, it was a piece of plastic from a report binder that I got at Wal-Mart for a couple bucks, the kind of binder that as a plastic "clamp" along one edge that you slide onto the report, then has a clear cover - I just pulled out the clear cover and trimmed it up.

[Ace25]

Seems putting a plastic cover directly on to the algae mat on an ATS, like the picture shows, would inhibit gas exchange? I understand what you are trying to do, I just don't think that is going to be the best route to achieve it.

[Garf]

Seems putting a plastic cover directly on to the algae mat on an ATS, like the picture shows, would inhibit gas exchange? I understand what you are trying to do, I just don't think that is going to be the best route to achieve it.

Im hoping the screen aeration will sort out the gas exchange problem. The squeezing action provided by the plastic sheet may actually push more aerated water deeper into the screen, and increase the effect of the bubbles on the screen. Obviously, failure is always an option with my tests ;)

[Garf]

12 hours in and it certainly looks like its squeezing the water through;

http://i1269.photobucket.com/albums/jj597/Garf1971/672e4af6f865dcf6548310234e6e74fd_zpsece00785.jpg

[Floyd R Turbo]

Garf, how wide is your screen and how much GPH are you running over it?

[Garf]

Garf, how wide is your screen and how much GPH are you running over it?

13 inch, was 2500 ltrs but not checked it recently. Mat is about 20 to 25mm thick on this side. Cleaning due on Monday.

This is before putting the sheet on;
http://i1269.photobucket.com/albums/jj597/Garf1971/1b2edffda37c26915e2bd977b00cf87c.jpg

[Floyd R Turbo]

Ok so 2500 LPH = 660 GPH, across 13" wide screen, you're at the 50 GPH/in flow rate. That's what I was going for. I am noticing that when you really increase the flow rate across the screen, the growth seems to jump dramatically. This has led me to conclude that the 35 GPH/in of width guideline is good for starting the screen up as the higher flow can prevent attachment, but when you get a base of growth on there, you can really crank up the flow and the algae will stay attached. It may even anchor better, because it needs to (it adapts)

[Garf]

Okay Dokey, cleaning day. Initial thoughts were, damn this algae is attached to the screen well (resorted to using a steak knife). Then weighed harvest, got 340g which is about 20% less than normal. Coincidence ?

9 day side (no plastic sheeting)
http://i1269.photobucket.com/albums/jj597/Garf1971/32fc5de289b67398455a917aea0114bb_zps0e9e5696.jpg

18 day side (with plastic sheet)
http://i1269.photobucket.com/albums/jj597/Garf1971/8d81a29d62645bfa2ee73f68fc8ad7e2_zpsa719967a.jpg

The plastic has only been on for 3 or 4 days so gonna do it again for a full 9 day cycle.

[iiluisii]

What about adding one extra pipe maybe half inch size on each side of the screen with a bunch of small holes pointing to the screen that constantly spray water towards the screen