Water Gardening: bio/bog filter, 1 by

wrote:
Here's a photo of my 'bog' filter from last summer. I have a biofilter in my waterfall, too. So when the pump takes the water from the pond through the skimmer box, at the other end of the pond, it is routed to the waterfall. I have a splitter valve just before the falls and I divert part of the flow through the bog filter. I use one pump for the entire pond - a bit over 4000 gallons. In chosing my pump, I just calculated the total volume of water I needed to push, including the bog, at the head height of my waterfall. At the diverter valve, the hose runs underneath the streambed and puts the water into the bog filter from the bottom of the bog. I used a large ball valve for this, and I have an access to it from above so I can increase or decrease the flow of water through the bog at any time.
The bog is 20-24 inches deep. At the bottom of the bog I used short shallow crates put in upside down, then a plastic mesh over them to keep the gravel from falling through. Then I filled the bog with pea gravel. Bascially I wanted to build a very large under-gravel filter just like you would buy for an aquarium. The water enters the bog from the back/bottom and it perks up through the gravel and spills over into the pond. Just about anything grows and flourishes in there! Any cuttings I take, I stick in the bog filter and they root quickly and thickly. The gunnera got huge last year. I'm considering moving it this year because it grew too fast!
This system has worked extremely well for my pond. I made my own biofalls filter, too, and used the savio filter ribbon for the inside. I recently cleaned it out and there was almost no detritus at all in the biofalls filter. While I have had string algae (who hasn't?) I have never had any green water at all, even when I add water to the pond directly from the hose. (I let it go into the pond with a shower head attachment, so the chlorine evaporates quickly. Because the pond is very large, I have more leeway with this than many other folks.)