I've done experiments with many different fabrics, in many different filters. I had heard about the quilt batting for years and I tried it but it didn't work the first time. But what really got me interested was a pond keeper who used pantyhose. They only made of couple of posts before being bullied off the forum so I never got a chance to get more details. But the poster did give enough details on how the pantyhose were placed in the filter that it got me to thinking there was more to this than just the fabric.
First I asked friends and neighbors to save old pantyhose for me...they were use to requests like that, although it still raised a few eyebrows. More experiments, more failures, just like with the quilt batting.
Much later, when designing a completely unrelated filter, a fabric cleared a pond. Eureka! I'd found the perfect fabric to clear a pond. So I set out to design a filter to hold the fabric which would be easy to build and clean. My pond was clear so I needed unclear. So I set up new ponds and threw in fertilizer to create green water. Set up my filter and readied myself to be amaze at my wondrous discovery. Failure, complete failure. I kept tweaking, making sure the setup was the same as the initial success. Failure after failure.
So I tried the filter in other people's ponds and bingo, cleared in a few days. But still sometimes failure. After awhile I could tell just by looking at the pond whether or not the filter would work. If you look close you can see particles, like white pepper. By tweaking I could get the filter to not work, then tweak it back and it would work, to learn some of the things that cause failure. Once I had a clue I was able to search for specific things and found a lot of info on industrial filters. So I felt like I had a fair idea what was going on.
Here's pantyhose and the fabric I used under a microscope. Both of these were used to filter pond water for a short time, so I could see individual particles. The fabric I was using is basically the stuff used in pantyhose.
You can see a few greenish specs here and there. A couple of the most clear are in the top left corner of the fine bag picture.
The big white areas are openings in the knit. Quilt batting has much, much larger openings. At normal size the fabric I used appears to be like a tee shirt material. What you'll notice is how small the green dots are compared to the huge openings. That's the first thing to learn about mechanical filters, it isn't the size of the openings. And making thicker and thicker mats has no effect on the openings, thicker doesn't catch more, at least not enough to make a difference. My working filter was a single layer, the thickness of tee shirt material.
However, when these filters work they are completely clogged, no or little water is able to pass through. A water molecule is about 0.000282 microns which means the fabric had collected some pretty small particles. As a comparison a single cell green water algae is about 2-10 microns, 10,000 times larger.
Most people think of filters like how water filters work...no particle larger than a certain size can pass. The downside to these filters is they clog fast, so they're only useful in water that is fairly clean to begin with.
Many filters don't work that way. Your furnace or AC filter for example let lots of particles through, but over time will clog. So it is catching smaller and smaller particles, but many also get by. These filters are used to reduce the volume of particles rather than eliminate every particle. That's what is desired in ponds.
Another way to think of how these work...Imagine a hole in the ground the size of a doorway. Back up a truck with a load of jumbled 2x4's and dump it into the hole. Some 2x4's will pass through, some will lay across the opening. You'll end up with a pile of jumbled 2x4s maybe 5' high. That opening stopped a lot of particles, even though every single 2x4 could have passed through the opening if oriented properly. The first 2x4s to not make it through created smaller openings, blocking more 2x4s. The opening is not actually the primary filter material, it's the 2x4s themselves making the filter matrix.
Now dump a load of marbles on the pile. Almost all will find openings between the 2x4s and pass through.
Now dump a load of grapefruit onto the pile. Most will not be able to pass and will get struck between the 2x4s. Now another load of marbles...most are now blocked. You could continue this using smaller and smaller objects until dust couldn't even pass, until even air couldn't pass.
This gives a few clues of why fabric filters work or don't work.
1. The material has to be good at making it easy for particles to get struck. Synthetic fabric thread has little hooks on it which seems to allow particles to stick to their surface. At least that's my theory since organic fabrics don't seem to work as well.
2. The water has to have the right mix of particles to create the matrix. New ponds generally don't have that. New green water algae is single cell. After awhile they clump together into colonies as protection from the sun's UV and get much larger, and different sizes. Other organic matter decays creating a good mix of different sizes and shapes. So in ponds less than say 6 months, probably not going to work unless a UV filter was used first and then maybe.
3. You can't kick the pile. From the 2x4 example...we trapped 2x4s, grapefruit and marbles. Now whack the pile with a hammer...a bunch of marbles probably fell through, maybe a few grapefruit. Now lay a running jackhammer on the pile, just for the vibration...slowly almost all the material is going to find it's way through the hole. So in a pond the speed and volume of water being pushed through the filter is as important as each other component. Has to be very slow and very calm.
So that is why quilt batting is both a great filter and also a worthless filter. If you happen to get lucky it could work. If you understand how these work you can increase your chance of getting it to work. For example. Placing the batting on a screen for support and then covering the batting with pea gravel does 2 things. It compresses the batting making it hard for particles to find a clear path. And the gravel spreads the flow of water a bit and reduces the turbulence at the batting. But, even that is no guarantee. And you have to be able to remove the filter for cleaning without disturbing the matrix and most of the crap falling right back into the pond.
One thing is for certain, if a fabric filter works it will clog in a couple of days. That's how you tell if you have the right conditions or you have to tweak. I've read where people leave them in place for months and then clean them, see a bunch of dirty water and say they're cleaning their pond. Not really. For the months in the pond the amount removed makes no real difference.
The examples I gave above is exactly how a swimming pool DE (diatomaceous earth) filters work. Inside is a fabric mesh, but a slurry (dirty water) of DE has to been poured into the intake while the filter is running to create the filter matrix. This is also what makes cleaning easy, the entire matrix is removed.
