Do you have any designs for larger scrubbers, preferably that can fit in a fairly small footprint?
You really don't want one big one. Having multiple small scrubbers is much better than one big scrubber. Bigger display tanks have more money in livestock, and need more reliable and consistant filtering. Unlike a skimmer which does not remove nutrients, if you have only one scrubber and it is not running for some reason, you will get a quick buildup of nutrinets: The glass will get dirty quickly, and film algae may appear on the rocks, sand, and backwall. So by having multiple smaller scrubber screens, pumps and lights, you always have one scrubber grown (and filtering) while waiting for the other one(s) to start growing again after being cleaned. And the more you can put the multiple scrubber parts in different "areas", the better, so that no matter what happens to one, you will always have the other(s). For example, put one scrubber on a different fuse/breaker than the tank; use a separate pump for each, or use a pump on one and the overflow on another; set one on the sump and another on the display, etc.
If your tank is new and is not yet stocked, you can always just start out with one scrubber and keep it that way until you're livestock and feeding are too much for it to handle. You will know it is time when you can measure any nitrate or phosphate at all. There really is no need for a scrubber larger than a 100 square inch; even if you have a 300 gallon tank, three 100's is far better than a single bigger scrubber. Only if your tank is big/fed enough to need more than three would you have any need for a "bigger" scrubber. Three scrubbers should be your target.
Lastly there is cleaning. When cleaning a screen, you need to be able to lay it flat on the bottom of a sink so you can scrape it. A 100 square inch screen fits nicely in most sinks. A larger screen might not.
"Small footprint" is another matter altogether. "Small" requires acrylic walls, and thus might not be easy to DIY. The SM100 I make is about as compact as you can get for a 100 inch screen, and it makes 3D growth within the acrylic walls, but it's not easy to make. Usually, however, you don't hear people needed "compact" when they have big tanks.
my biggest concern is on a narrow, long screen, how to light the whole thing properly.
Don't make a trough; they are far too inefficient. If you really want to make just "one", and put it in a trash can, lighting will be your challenge. You'll need to get up 300 real (not equivalent) watts into the trash can. What's I'd do is line the trash can with mylar (using spray adhesive), fit the screen, and put 6 bulbs on each side, each bulb bulb being a 42 watts CFL curly-Q spiral 2700K. This will definitely work, and can be your only filter, and keep N and P at zero with no waterchanges. But if a pump or fuse goes out and you the scrubber stops, nutrients are going to build up very quickly.
Can you explain the cons of using a double layer screen?
If your tank is good, then it's fine. But in general, double layer screens require stronger lighting, more frequent cleaning, and down-to-the-screen cleaning, in order to take advantage of the extra "attachment" material. Otherwise, the algae gets too thick too fast and shades the roots. Most people are cannot keep up with this; therefore a single screen is more forgiving. And indeed, the recommendations are for "most people".
We will see though.. I am sure you have seen quite a few attempts at an LED scrubber already... but hopefully I can be the first to make it work. :cheers:
