Bog filtration -- can you have too much filtration from bogs?

[Mmathis]

And, is there a point where you might not have enough fish waste to make the bog(s) prosper?

My pond is 3000+ gals. I only have goldfish [though might be adding a couple of smallish butterfly koi that need to be re-homed]. My water quality is always good. Right now, am using a 100 gal SKIPPY filter, but it's soon to be replaced by a bog. Well, actually, by 2 bogs.

I have one bog for the box turtles, but it's not connected to the pond yet [I'm filling it from the faucet --with an anti-chlorine cartridge -- once a week or so, and letting it overflow the sides each time]. It's about 3' x 6'.

We are about to start work on the major bog, but am now wondering how big to make it. I know there is a formula for how large the bog should be [30% or something like that), but I don't want to make it so big that it won't thrive.

 

[Waterbug]

Kind of depends on what you mean by prosper and thrive. Like if you wanted the bog for growing plants on top then I suppose a bog could be too big for the fish load.

If you wanted the bog for converting ammonia it couldn't be too big, but it would just be a waste of $$$, space and work to oversize it. If the bog were undersize for the flow the water would just travel over the surface, it would be a stream, and that would convert many times more ammonia, plus grow string algae which would consume ammonia directly so in that sense a bog could be too big.

In most Water Gardens a lot, maybe most, waste comes from dead algae, plant parts, soil form pots, leaves blowing into the pond. So really fish aren't generally a big part of the equation. Bogs act as settling chambers for the most part collecting a lot of that waste. As they fill up they contain that same stuff that collects in all ponds, muck. That creates a habitat for all kinds of bacteria and little bugs and of course is great for growing plants. In that sense Wildlife Pond people would consider that a thriving system. Koi Pond people would consider it evil. Water Garden people don't seem to care much either way, it's all good. So in this case a bigger bog would take longer to fill with muck to get a thriving system. But they can be jump started too by adding waste.

There's never been any testing of bogs for performance so no way to know if there is a too big or too small. So that just leaves opinion. For example my last pond with a bog was 5000 gals total and a 900 gal bog. The pond was fine, so I could choose to give all that credit to the bog and say "a bog must be 18% of the pond". But I don't think that's really much of a guide. I'm pretty sure the pond would have performed exactly the same, probably better, without a bog. But I love the plants.

IMO bogs should be fun. I love bogs. But I don't consider them to be a filter. I mean, yes they do some filtering, especially for the first few years, but so does everything in the pond. Recently it seems like people are looking for technical info on bogs like they're some kind of filter. It's totally fine that people call them filters and think they're filters, I sure don't want to debate untested devices, but when we start talking about the technical attributes of bog I think we start missing the point of bogs. They're just a fun part of a Water Garden.

If you really want filters for specific purposes there are filters for all those things. All very well tested, well understood, very well performers and many cheap and easy DIY. But that's a whole deal. Most Water Gardens don't need all that jazz.

 

[Mmathis]

Thanks, WB! You really hit on where I was coming from -- I want the bog for filtration, but I also want it for the beauty of the plants. I'm excited about the plants, but was afraid that if I made it too big [in comparison to the pond], there wouldn't be enough fertilizer to keep the plants happy.

 

[Waterbug]

Filtration yes...but filter what is always the real question.

 

[shakaho]

I suppose that if a bog filter (I assume that's what you mean rather than a bog) was bigger than the pond and the pond was understocked, the filter might be too big to grow vegetables. Vegetables are very demanding in terms of nutrients. Ornamentals are much less demanding. You will probably find that some plants that grow beautifully in someone else's bog filter won't grow in yours and vice versa.

Still, people who do aquaponics often grow huge amounts of vegetables without a huge fish load. A surface area of 30% of the pond surface is recommended if you have koi by Nelson Water Gardens, who have been the source of information for most people with successful bog filtration. Do read their list of plants to be avoided as well as their building instructions. There are some that will just choke out any others that you want to grow.

 

[addy1]

Mine is around 30% surface area, maybe more. The plants started doing their best after the fish population increased. I have not tried vegetables, have thought of it, but never have gotten around to it.

Every plant put in the bog grows very well, spreads very well , even day lilies, but at least it is easy to yank them out.

 

[crsublette]

And, is there a point where you might not have enough fish waste to make the bog(s) prosper?

Short answer...

Yes!


Ultimately, all guidelines are more of a recommendation rather than a rule of law and this must be kept in mind. Any attempt to take guidelines to actually mean something beyond this is actually asking to entirely redefine the term.

The unfortunate angle that is not taken into consideration by surface area recommendations is the pond's depth. So, I do not view surface area recommendations with much validity.


My recommendation would be for you to use as much space as you want to use, and, with these bog filters, the more the better.


If you need specifics, then I would say 1 cubic foot of half inch Pea Gravel, with good plant growth and potentially algae growing in it, is good for 2~5x 10 inch goldfish. However, this is just an "educated" guess.

I would also be sure you have very good water flow through it and a simple foam sock pre-filter to it so to remove any large debris so that the large debris does not clog the bog faster. When the gravel becomes clogged and there is continual plant decay, then there can actually be more ammonia created rather than removing the ammonia.



Long answer...

To an extent of any anecdotal "testing" done, it is shown in the aquaponic hobby. Bog filters, also called gravel media grow beds in aquaponics, is used quite often for backyard enthusiasts since gravel, then followed by lava rock, is incredibly cheap grow bed medium.

For biological nitrification filtering, these hobbyists go by the term "specific surface area" (SSA). SSA is the measurement of the number of square feet per cubic foot of material; in other words, this is measuring the amount of square footage surface area there is of a type of medium within a volume. Once the SSA is determined, then multiply this by the volume of the grow bed so to calculate the biological surface area (BSA). This is all determined by the size of medium in the grow bed.

If the medium is a mixture, then all linear math is thrown out of the window due too the unknown variables; in other words, then it takes an "educated" guess. Also, uncontrolled introduction of debris due to weather and plant decay also makes any suggestions incredible tough. So, the best anyone can do for you is to give you an "educated" guess, such as sometimes described by recommendations of "30% of the pond surface area".


For more info, then checkout Bright Agrotech's (an aquaponic business) video about SSA and BSA, and, for anyone interested, here's a thread about aquaponics (is auqaponics viable?):

Void space ratio is the amount of empty air between particles. The lower ratio indicates more potential of "fouling"; in other words, the filter will be clogged if there are no worms and other critters to dislodge the mulm so to be moved out by water flow. However, eventually, mulm is created even by worms and this mulm will clog the grow bed and this is more likely to happen if the water flow is too slow. Mulm is essentially extremely tiny bits of rock and mineral that can no longer be decomposed nor digested by any microorganisms; in other words, mulm is essentially "soil".

Bare minimum you would need around 2.5 sq. ft. of BSA per 1 gallon of water at quite low fish stocking densities and very low feeding rates. So, if your pond has 1 pound of fish per 10 gallons, then this means you need at very least 25 sq. ft. of BSA. Most common for backyard gravel media grow beds use this rate.

Recommended level is 10 sq. ft. per 1 gallon of water or about 100 sq. ft per 1 pound of fish. This is a quite hefty amount of material and this is where wet/dry filters come into play so to improve conversion rates thus allowing less material to be used.

No matter how folk try to change the argument, the microorganism's growth will only expand according to the amount of area they are allowed to grown on. There are biological environments that will give them better conversion rates, but this is not the same as saying the SSA or BSA is irrelevant.


Thus, using conventional backyard gravel media grow bed recommendation and assuming you are using 0.57 inch Pea Gravel, which has 85 SSA, then this tells us 1 cu. ft. of Pea Gravel provides for 3.4 pounds of fish. This does not include the plant's and various algaes directly consuming the ammonia. So, when plants and algae is present, then 1 cu. ft. of Pea Gravel could provide for 3 times this amount.

While using CNYKOI koi weight calculator, a 10 inch koi is around 0.5 pounds. At these small sizes, from the little research I've done, goldfish's odd body shape actually weight more than koi per inch except koi grow twice as big, in inches, than goldfish. So, this is something to take into consideration. So, if a 10 inch goldfish weighted 2 pounds, then it would take only 0.58 cu. ft of Pea Gravel per 10 inch goldfish. Remember, this does not take into account of plants and algae. So, 0.58 cu. ft of Pea Gravel could potentially manage the ammonia from 3x 10 inch goldfish.