Help me design the bio filter for my pond

[Jonna]

OK, since the great oil spill, I have a chance to redo the filtration on the outside pond.  The problem with the way I had it before is that I had to split the output from the pump (or use 2 pumps) in order to run water up through the filter plus run it about 16' down to the falls.  Falls is kind of a misnomer here as the water just trickles down over some rocks and back into the pond, not much elevation and not much force. 

I'm thinking of putting the pump on the bottom of the filter, putting egg crate above it, putting some filter media on top of the egg crate and having the water come in from the top, fall down to the pump on the bottom and then get pumped to the other end of the pond. 

Pluses: 

- I don't have to split the outflow of the pump ** big plus**

- I can get to the filter media on top to clean and change it easily. 

Minuses:

- the water will not really be sucked through the filter media completely which makes me think that debris will be floating on top and I'll need to skim it.  I want to keep the water level high in the pond and that will keep it high in the filter too. 

- it will be harder to plant things in the top of the filter, perhaps I can put a few things but not as I like to cover the top with plants.


i took some pics and put info on them so you can see the set up:


This is an overview and shows the water return from the filter pit (far right) to the 'falls' at the left.  There is a pvc pipe embedded in the wall for the water return, they put holes in the pvc at the end to allow it to run out along the rocks. The red line shows about how it goes.  It's about 16 feet end to end.  I've got the water in the pond very low to keep it from getting into the filter pit.  Normally the water level would be just under the stepping rocks between the filter pit and the pond on the right.


This shows the filter pit.  It's the same depth as the pond, about 3.5 feet.



another view of the filter pit:


from the other direction:




What do you think?  I'm concerned that there won't be any real filtration and that the bio media will not get sufficient oxygen.  I could put an air stone down in there?   I'm open to all suggestions, I just don't want to run 2 pumps or a pump much larger and thus more costly for electric. 

Bad thing about how it is built, the pvc in the rock wall.  Plus, they used too large a diameter of pvc, I think it is over 1".  They used about 4 90° turns as well.  They didn't understand the concept and I was unsuccessful in explaining it.  It's done though and I don't want to break out the rock in the wall again to replace the pipe. 

[Johns]

First of all, they did you a favor in using a larger pvc pipe.  Where before your pump was in the pond and the water going into the filter was under pressure.  As I understand your explanation, you are going to put your pump in the bottom of your filter and the pond water will the flow into the filter area via gravity, which always requires a larger diameter pipe.  Otherwise your idea seems sound, except that I would place more bio media in the filter above the pump, but below the "eggcrate" you mention.  I would use the coarse "blue" fiberous air conditioner filter materal (NOT fiberglas) for this as well as above the eggcrate.  Ordinarily you would have to clean the top filter material more often than that below.
One other note:  Be sure to incorporate a float switch to shut off the pump if the pond water gets too low to allow water to flow into the filter area.

The "eggcrate" material I use is made from flourescent light "lens" material.t

[Jonna]

Good point Johns, I hadn't thought of a shut off but I will need one now.  I have a half piece of matala mat and I will check out what kind of AC filter stuff is available here.  I keep thinking that all the debris will just be floating above the filter.  I suppose I could use a pool net to skim it every day.