Hi. I met TimGod at our local ponding store, and he's the one who turned me on to A.P. I installed a bog filter according to the plans from that same pond store, and I think that's where Tim got his basic info, too. I'm sure he'll chime in, when he sees your post.
My bog worked well to filter and clean the pond. However, I ended up ripping it out, one short year later, mainly because it kept plugging up. I now realize the problems with the bog were due primarily to the info I got from that same pond store, to put salt in my pond, to the tune of 1 lb per 100 gallons - and NOW, a year later, (thanks again to A.P.) I found out that salt is a no-no for plants. No wonder all the plants in my bog died!
The other problem I had, I think is not really a problem at all, looking back. So much dirt and sludge would settle to the bottom of the bog that it clogged some of the holes in my PVC piping, causing water to be forced to odd sections of the bog. At places, the water gurgled up with such force, and was so high, that it actually went over the sides of the bog, out to the ground, and could have drained my pond. Quite a few times in the past year, I had to completely empty and wash all 600 lbs of gravel, to remove that excess sludge.
I guess the lesson here is to make sure the walls around your bog are high enough to contain any water that might splash up. I also think that, because the plants were not thriving due to the high salt content, their roots were not doing their job to eat nutrients and reduce the amount of sludge.
When I expanded my pond this spring, I ditched the bog filter in favor of a pressurized Tetra bio filter. I do, however, still have a very small (pre-form) mini-bog, which is mostly to house plants, not to filter the pond. I notice that water is gurgling up differently from when it was first installed, indicating that sludge is settling around the PVC holes again, but I'm not having any problems with it overflowing.
Oh, and be sure to install a back-flush / clear-out at the end of the PVC run. Just have the PVC make a "L" turn, upward, so that it sticks out of the gravel, and close it off with a removable cap.
