All I'm really clear on is that you want the water to go to the bog BEFORE the bio-filter.
Oops. Not really true. First, the bog is a bio filter. There is a chance that a pond would have such a large ammonia load that some ammonia would make it through the bog and therefore to another bio filter, lets call it a Skippy just to conceptualize. So if there were a Skippy after the bog there would no ammonia for it to convert. Of course the same might be true the other way around, Skippy first then bog the bog might not get any ammonia. But then ammonia conversion is rarely 100% per pass, it takes several passes. But in any case the order of bio filters doesn't affect ammonia conversion.
This gets a bit confusing in Water Garden circles because generally all filters are called bio filter and rarely does the ponder know what it is they're trying to remove. A filter is just thought of as needed, or that it will "clean" water whatever that means.
Whoever told you the bog should be first might have been thinking the bog should be used as a settling chamber. It is true that if you pumped poop into the bog it would probably get trapped there, but it would still decay. So not really a very good settling chamber. Any decent bio filter would never clog so it shouldn't matter if poop goes through the bio filter first or not. In either case any design that allowed waste to accumulate in any bio filter, including a bog, is a poor design imo. However, bogs to collect more stuff and do clog, but generally are so large that the clogging takes a long time.
1. Does the bog sit higher and flow into the main body of water or other way around? Seems like I've seen it mentioned both ways, which is confusing me -- or it may be that I don't follow the terminology(?). If so, how do you know which way is best for your needs?
Bog works the same either way. Most common is bog higher.
2. Can you run a bog and a bio-filter off the same pump? Why, why not, & how? And, yes, because of my box turtles being so messy, I'll want the extra filtration.
Either way, there's no difference. Moving ammonia past bacteria is what converts it. One pump splitting to 100 filters or 100 pumps pumping to a single filter. However blowing water through a filter isn't an efficient use of electric so most people want a slow flow through filter. This concept has morphed into a myth that slower flow is a requirement for conversion but that's not true.
3. I always see mentioned a percentage of your total pond (some say total pond volume; some say total surface volume...) that can be "bog." Is that a minimum or a maximum number? If you go over that number, would it matter? The way I understand it is that percentage or ratio is for optimum "filter" performance.
For converting ammonia the size of the bog has no relationship with the size of the pond. The bog has to handle the fish load which really means the amount of food being fed. The entire concept of bio filters in Water Gardening comes from the Koi Pond hobby. Unfortunately the logic for bio filters didn't really come along with the filters and in Water Gardening bio filters are magical things that seem to fix everything if you have one of any size. Few people even measure ammonia levels.
The reason bio filters are so important in Koi Ponds is the amount of food they will feed fish in warm months. They'll feed 3% of body weight per day which can be 10 to 20 pounds of food per week. I doubt there is a single Water Gardener on the planet that has used 10 pounds of food in an entire year, and many won't use 10 pounds of food in years.
Most Water Gardens do not need any bio filter at all.
So there isn't really any way to determine the proper size of a bog because there is no data. Koi Pond owners measure and keep data but don't use bogs any more. 15-20 years ago they did, they were called gravel filters. But good data was hard to come by because gravel filters were too natural to provide good data. And Water Gardeners use bogs but don't measure or keep data. They just use a gut feel type estimate, but there's no data to show if this works. If you build a 100'x100' and it works it doesn't mean a 1'x1' bog wouldn't have worked just as well. Bigger is certainly more opt to do whatever it is you want done.
Other use for a bog
Bogs have another use in Water Gardens and that is in killing unicell algae. It's more hit and miss than ammonia conversion for complex reasons, but a bog filter has a good chance of killing unicell algae and creating clear water. Add to that a bog's ability to trap small bits and they can produce very clear water. Key word is "can", not will. The chance of clear water can be improved by increasing the length of a bog. This can be done in any foot print by winding a bog channel. A bog is just a subsurface stream. It can be very shallow, even 4", but I like 8" to 12".
Length, not size
If you have a 10'x10' bog and pump the water in one end and it flows out the other the size of the bog is 10', the direct line from pump to out flow. Doesn't matter how wide the bog is. The water doesn't travel in exactly a straight line but close enough. If the pump spreads the water over the width of the 10x10 the size of the bog is still 10' as each source of water travels about 10'. That's good for ammonia conversion, but not for killing algae.
For killing algae you want the algae to be in close contact with the killer bacteria. As the algae goes by one killer bacteria the algae takes a hit and weakens. Passing another weakens it more, etc. Given enough hits the algae wall pops and the lucky bacteria eats the creamy center. I think a long ride through the dark bog would weaken the algae too, less sun, less energy, less they're able to produce the chemical that attacks the algae killing bacteria, but I haven't seen any studies on that so just my own personal theory.
The algae and bacteria fight each other with chemicals. A bog can give the bacteria the upper hand. This is unlike ammonia conversion because ammonia doesn't fight back. Ammonia converting bacteria always wins, there are few variables. You can almost set your watch by the conversion rate.
High end Koi Ponds use UV filters instead of bogs because UV is 100% effective. They don't like hit or miss because they're right on the fence. In their case green water can actually kill fish. In Water Gardens green water generally helps fish.
Keep it simple
All this sound really confusing? Learning the basics can greatly reduce the confusion. Unfortunately the basics are almost completely missing from the Water Gardening hobby and that makes it more confusing unless you go with the flow and just set up a single Skippy which works because it isn't needed.
Basic #1, know what it is you're trying to filter. Ammonia? Solids? Suspended particles? Unicell algae? Tannins? Etc. That allows for a specific discussion and keeps a butt load of complexity that has nothing to do with the specific goal from entering the discussion. Forget about "combo" filters.
Basic #2, use standard terms. Bio filter is ammonia conversion. Settling chamber removes solids from the water column before decomposition. UV kills unicell algae. Etc. When a person says bio filter to remove poop the discussion gets really confusing.
Basic #3, measure. To remove something you first have to have that thing present. For example, most Water Gardens measure zero ammonia without any man made bio filter. Then they talk about needing to add a bio filter. That's confusing. How does one discuss the proper size or configuration of something that does nothing? Makes my head spin. Measurement is also used to determine if a filter is performing. This concept is also almost completely missing from Water Gardening. If the pond looks good the filter is deemed to be 100% effective. But without measuring results you're just spinning your wheels and that is confusing.
These concepts will allow you to focus in on smaller targets and reduce confusion.
All I'm really clear on is that you want the water to go to the bog BEFORE the bio-filter.
