I haven't had time to fully read this thread, so I kinda jumped in and just picked that one thing to comment on, so there's that. I was just commenting generally, and on what I'm most knowledgeable on which is waterfall scrubbers. Now I see you are running a UAS, so my comment doesn't really directly apply, or at least not quite as well.
The part about it taking time to mature is still accurate. But in what I've seen posted over the years about UASs in general is that they are more hit or miss on the predictions about getting them going strong than waterfall scrubbers are.
As far as this:
they did invent an entire material just for growing algae.
It's quartz bonded in with epoxy, basically. It's technically not a "new material" but I get what you're saying - it's not roughed up canvas. My opinion is that was arrived upon because plastic canvas doesn't work well for upflow scrubbers - which is completely true. So the quartz material is more appropriate and does work better than plastic canvas.
Moreover, it's my opinion, based on results I and others have had, that the reason for canvas not being a good substrate underwater is because algae will, to an extent, adapt to it's environment. When algae is fully suspended underwater, it doesn't "need" to anchor strongly in order to survive. Compare this to an open-air algae scrubber (one not in an enclosure, or in a large one) - in such a case, for algae to stay attached in the presence of strong laminar flow, it will firmly anchor (over time) to the substrate (canvas) and will require strong scraping to remove it; some people have to use a knife or the corner of a plastic scraper.
Now, going into the middle of those 2 is the enclosed scrubber where you get an interesting result. As a disclaimer, I have to talk about my product here in order to explain this, so I don't mean to make this come off as just popping in here to toot my own horn. I made my growth chamber to specific dimensions and added a false bottom so that the chamber would encourage "3D growth". If you let it grow for a long enough period of time between harvests, the algae will form a "sandwich" of sorts, where the box/bottom and the mass of algae sort of supports itself. When this happens, the algae will basically weaken it's attachment to the screen (and this is where I guess) because it just doesn't require a strong attachment to a substrate to stay in place and thrive, so it diverts that energy to growth.
Now that all kinds makes it sound like algae is smart somehow...but it is one of the most primitive forms of life, so it goes to reason that it probably does a pretty good job adapting to it's environment.
Going back to the UAS though with the quartz, I believe the concept for that substrate follows along with my dissertation above - you have a microscopically porous surface where the algae forms a naturally stronger "foothold" so that it doesn't detach quite as easily in an environment where it doesn't require strong adhesion (underwater). This is why the quartz works better than the canvas, because plastic canvas isn't very porous, so algae that doesn't "need" a strong attachment ust won't stay very well attached to a relatively smoother surface.
It's a very good example of the saying "necessity is the mother of invention". It's just not something that is necessary in a waterfall scrubber, because the plastic canvas works just fine in that environment.
