A Bog filter system is an excellent filtration system borrowed from Mother Nature. It is quite simple in its design and works very well. Bog filters work so well in fact, that they are being used for public natural swim ponds. Pond water is pumped from the skimmer or submerged pump to an upper separate pond, then slowly overflows down a stream, or over a waterfall, back into the lower pond. Bog filters work very well because of the large amount of filtration media, ( gravel, pea stone) that fills the upper bog pond. The gravel in the bog pond traps and holds organic matter that would normally settle at the bottom of the pond. This system uses mechanical and biological filtration. The pea stone, or gravel is a very efficient mechanical filtration media, and the bacteria and plants growing in the bog pond are the biological filtration. The gravel has a very large surface area that allows beneficial bacteria to colonize and it also traps and holds debris for the bacteria to feed on. Plants filter the water in this system with their roots and they do a great job of cleaning the water by absorbing nutrients.
Please check out these websites on Bog Pond Filters...
https://www.minnpost.com -Webber Pool