Idea

Instead of using a flat, one or two sided screen, how about using a three dimensional shaped screen, like a cone, or a pyramid, or a hemisphere, with the water being piped onto it from a single point in the centre (or even pumped up through it from below).

Lighting could be from two or three directions.

You could squeeze a lot more surface area into a smaller space like that couldn't you?

Cleaning might be an issue though.

Is still one sided, and therefore much less efficient.

And much of the area would not be facing the light.

Flow would not be uniform

Would be too big, and hard to build.

And yes, hard to clean.

I guess you could make a square out of 4 screens and have 4 lights on the 4 sides and one light in the center of the square,,but I think that is a lot of work compared to having just one large flat scrubber