terraglen
complexifier
HI, new to the forum. In our new house we inherited a ~750 gallon pond that I'm embarking upon renovating. I've been doing a lot of reading, including the amazing, 7 year long, 41 page opus founded by Addy1. I'm trying to distill things into a single post for my own clarity. As you can see, I'm sort of compulsive about understanding a project around my house before starting it.
So here goes, my picture of how a bog filter works, and what factors matter most in the design:
First and foremost: everything that goes into the pond has to come out of the pond. If you have a lot of leaf drop, you should design a leaf net into the pond in advance. Wet leaves suck. Pieces of stick. Bugs. These will just clog up the bog plumbing; it's a biome, not a magical transporter of crap. There must be a mechanical filter in the system, even if it's just a leaf basket over the pump. But a skimmer is better. More on that in a minute.
There are two kinds of things in the water: gas, and material stuff. Most of us understand oxygen and carbon dioxide, which are gas molecules in the water. These are easy; you manipulate the concentrations of these gases by putting the water through a skimmer/waterfall/fountain, anything that churns up the surface in contact with air. This makes the water concentrations be the same as what's in air: more oxygen, but also (and very important for this filter) very little CO2.
Stuff consists of everything too small to be mechanically filtered: dust, pollen, fish food/poop, bird poop, etc. The carbon in stuff can wind up as CO2 and eventually leave the system by surface exchange as outlined above. The nitrogen is another matter, because it is poisonous in its eventual liquid form (ammonia, NH3) and has to be converted (via two bacteria) into nitrate, which is plant food.
So the bog filter is the final step in managing nitrogen that gets into the pond. You remove nitrogen from the system by harvesting the plants. If you allow dead leaves to fall back into the bog, that nitrogen goes back into the system. It also means: The most efficient bog filter system will include fast growing plants that can be harvested frequently.
Nitrogen could also leave the system by conversion of nitrate into nitrogen gas, which would bubble out. This last bacteria requires an anaerobic environment, which occurs down in the muck of a real bog. Unlike everything else in this discussion, which happens best in an oxygen rich aerobic environment. I have seen advice all over the net to the effect of 'you can increase your filtration by making your gravel bed deeper' which is probably true, because you are creating a slimy, anaerobic muck deep in the gravel bed.
Yuck. Do NOT make your filter too deep!
In fact, we want all the stuff to wind up as leaves without allowing a muck zone to build up. That means that the ideal depth for a bog filter is determined by the size of the root ball of the plants within. If I had a sloping gravel bed I would put bigger root balls in the deeper part. Perhaps people who harvest their plants whole could post size of plant and root ball? This might be a valuable resource in choosing bed depth.
But back to the other two aerobic bacteria, which we want to have in our system. The product of one is the feedstock for the second one, which is why it takes some time for the pond to 'cycle'- the plants can't grow until nitrates are present, which requires two bacterial populations two grow in succession. These two bacteria die at water freezing temp, so if your pond freezes it will have to cycle every year. It also means that a brand new pond will benefit from some stuff instead of a totally clean setup, to get it to cycle faster. This might be as simple as keeping the soil on the root balls of the plants and just knocking off the pots.
These two bacteria can also secrete slime, and in fact can turn dissolved CO2 directly into slime (this would be a cool superpower, if you had to pick from what's not already taken). I think this is why people who have aerators of some kind between the pond and the bog describe having clean gravel beds- it's not more oxygen, it's less CO2. A skimmer performs this function, as well as mechanical filtration. Thus, to me, there should be a skimmer between the pond and bog.
Someone on here described a negative edge waterfall into a gravel bed sump, which then feeds the bog piping. This design combines skimmer, waterfall aerator, and mechanical filter into a single, aesthetically pleasing entity where you can just spray leaves off the bed with your hose. I love this idea.
What else is left... oh. If we are going to make the bog inhospitable to our friendly, nitrogen-fixing bacteria then it has to grow someplace else, ie in the pond. These bacteria do not like light and will produce slime to hide themselves. There is CO2 in the pond (from the fish) and so there needs to be nooks and crannies in the pond that are never in the sun. Plumbing is one place, but I personally am going to add shady surface area with rock.
Wife is giving me the evil eye. Have to finish up with size in another post.
So here goes, my picture of how a bog filter works, and what factors matter most in the design:
First and foremost: everything that goes into the pond has to come out of the pond. If you have a lot of leaf drop, you should design a leaf net into the pond in advance. Wet leaves suck. Pieces of stick. Bugs. These will just clog up the bog plumbing; it's a biome, not a magical transporter of crap. There must be a mechanical filter in the system, even if it's just a leaf basket over the pump. But a skimmer is better. More on that in a minute.
There are two kinds of things in the water: gas, and material stuff. Most of us understand oxygen and carbon dioxide, which are gas molecules in the water. These are easy; you manipulate the concentrations of these gases by putting the water through a skimmer/waterfall/fountain, anything that churns up the surface in contact with air. This makes the water concentrations be the same as what's in air: more oxygen, but also (and very important for this filter) very little CO2.
Stuff consists of everything too small to be mechanically filtered: dust, pollen, fish food/poop, bird poop, etc. The carbon in stuff can wind up as CO2 and eventually leave the system by surface exchange as outlined above. The nitrogen is another matter, because it is poisonous in its eventual liquid form (ammonia, NH3) and has to be converted (via two bacteria) into nitrate, which is plant food.
So the bog filter is the final step in managing nitrogen that gets into the pond. You remove nitrogen from the system by harvesting the plants. If you allow dead leaves to fall back into the bog, that nitrogen goes back into the system. It also means: The most efficient bog filter system will include fast growing plants that can be harvested frequently.
Nitrogen could also leave the system by conversion of nitrate into nitrogen gas, which would bubble out. This last bacteria requires an anaerobic environment, which occurs down in the muck of a real bog. Unlike everything else in this discussion, which happens best in an oxygen rich aerobic environment. I have seen advice all over the net to the effect of 'you can increase your filtration by making your gravel bed deeper' which is probably true, because you are creating a slimy, anaerobic muck deep in the gravel bed.
Yuck. Do NOT make your filter too deep!
In fact, we want all the stuff to wind up as leaves without allowing a muck zone to build up. That means that the ideal depth for a bog filter is determined by the size of the root ball of the plants within. If I had a sloping gravel bed I would put bigger root balls in the deeper part. Perhaps people who harvest their plants whole could post size of plant and root ball? This might be a valuable resource in choosing bed depth.
But back to the other two aerobic bacteria, which we want to have in our system. The product of one is the feedstock for the second one, which is why it takes some time for the pond to 'cycle'- the plants can't grow until nitrates are present, which requires two bacterial populations two grow in succession. These two bacteria die at water freezing temp, so if your pond freezes it will have to cycle every year. It also means that a brand new pond will benefit from some stuff instead of a totally clean setup, to get it to cycle faster. This might be as simple as keeping the soil on the root balls of the plants and just knocking off the pots.
These two bacteria can also secrete slime, and in fact can turn dissolved CO2 directly into slime (this would be a cool superpower, if you had to pick from what's not already taken). I think this is why people who have aerators of some kind between the pond and the bog describe having clean gravel beds- it's not more oxygen, it's less CO2. A skimmer performs this function, as well as mechanical filtration. Thus, to me, there should be a skimmer between the pond and bog.
Someone on here described a negative edge waterfall into a gravel bed sump, which then feeds the bog piping. This design combines skimmer, waterfall aerator, and mechanical filter into a single, aesthetically pleasing entity where you can just spray leaves off the bed with your hose. I love this idea.
What else is left... oh. If we are going to make the bog inhospitable to our friendly, nitrogen-fixing bacteria then it has to grow someplace else, ie in the pond. These bacteria do not like light and will produce slime to hide themselves. There is CO2 in the pond (from the fish) and so there needs to be nooks and crannies in the pond that are never in the sun. Plumbing is one place, but I personally am going to add shady surface area with rock.
Wife is giving me the evil eye. Have to finish up with size in another post.
