Bog construction question


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After reading the posts about building a bog filter, I think I must be missing something. Is it as simple as building a 12-14 inch box with a surface are of about 30% of the main pond, bottom feeding a slit PVC pipe at one side, filling with pea shingle and some plants, and clear water comes out of the slit PVC pipe at the other side?

Is there a limit or ideal flow rate on how much water you can push through it? (LPH)

The other thing I am having trouble getting my head around is the water level in "the bog" ..over the gravel, level with it???
 
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No a bog can be any shape . The rule of thumb is what ever the surface of the pond is is that the bogs surface is at least 30 % of that.
For simplicity you build a box 10 foot by 5 foot and 2 feet deep.
Take 2 inch pvc for argument sake you can use smaller for smaller ponds and you create a baffle on the bottom . Two pipes that are evenly spaced. In this case one pipe is 15 inches from the wall of the bog and this would be about as fas as you would want the spacing to be. Less is better. So you have wall 15 inch space along the bottom 2 inch pvc pipe cut one third the way through the pipe ever six inches again on the bottom only. 30 inches between the pipes 2 inch pipe and then 15 inches to the wall . Cover it all in 3/8 " and or 3/4 inch pea or river rock for at least a foot thick. The Water is pumped to the bottom through the sliit cut pipes. And the water is forced up through the rock . That's it. You can add risers on tge end for clean outs . And you want to leave about 4 to 6 inches from the top of water level to the top of the bog frame. Water drops out a lower side to the bog where you make a waterfall an added bonus.
 
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The level of water vs pea gravel is not important - you don’t want the water to be too deep for marginals. My bog, the water level is about 1 inch below the grave, the important thing is: the water to flow throw as much of the gravel and roots as possible.
 
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thanks for that guys, most helpful. I might be none the wiser, but I'm much better informed! :):):)

Is there any advantage in taking (in the above example) a vertical feed of say 4 x 1/2" pipes (capped and slitted!) from the 2" horizontal feed pipe? In order to spread the feed water over a wider area of the side of the 'box'?

The other bit I think I missed was the input and output. Feed in at the bottom, feed out from the top? like an overflow arrangement back to the waterfall?

I'm pretty much convinced this is the way to go, I've had another look and I think I can squeeze in a 'box' of the correct size while dodging manhole covers etc. The only doubt I have left is flow rate. I imagine that the bog requires a fairly low feed rate (hundreds or tens of litres per hour?) I had envisaged running a higher volume of water over the waterfall. Bypass filtering perhaps?

Is there an ideal flow rate calculation for bog filters? It will be 3 square metres.
 
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mrsclem

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@poconojoe has a great picture of the pipe layout on his bog that shows how it works. Basically, water flows in thru 2" pipe to a T and then parallel pipes with slits. You can cap them off under the gravel or use 90 degree elbows to bring the pipes up to the surface to flush it out if needed.These are the ends of my bog pipes, uncap them in the spring when I start the bog up, flush out a few seconds of muck then clear water. Put the caps back on till nest year.
 
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thanks for that guys, most helpful. I might be none the wiser, but I'm much better informed! :):):)

Is there any advantage in taking (in the above example) a vertical feed of say 4 x 1/2" pipes (capped and slitted!) from the 2" horizontal feed pipe? In order to spread the feed water over a wider area of the side of the 'box'?

The other bit I think I missed was the input and output. Feed in at the bottom, feed out from the top? like an overflow arrangement back to the waterfall?

I'm pretty much convinced this is the way to go, I've had another look and I think I can squeeze in a 'box' of the correct size while dodging manhole covers etc. The only doubt I have left is flow rate. I imagine that the bog requires a fairly low feed rate (hundreds or tens of litres per hour?) I had envisaged running a higher volume of water over the waterfall. Bypass filtering perhaps?

Is there an ideal flow rate calculation for bog filters? It will be 3 square metres.
"Feed in at the bottom, feed out from the top?"

That's right. You pump the water into the bottom and allow it to rise through the gravel.

"The only doubt I have left is flow rate."

Very few authority type figures talk about this, but the ones I've heard have said a good flow rate is to shoot for 10 minutes of transit time. So, from the moment a gallon of water enters the bog, it should take at least 10 minutes for that same gallon to reach the top and flow over back to the pond. What that means in terms of GPM on a pump varies based on the size of the bog. You can do some calculations to try to estimate it, but I think most just install a ball valve on their input plumbing and choke it down until it reaches the flow rate they're looking for.

If you were to build an Aquascape style wetland filter, they have some tables that will tell you what GPM to shoot for based on the size of filter you're building.

"I had envisaged running a higher volume of water over the waterfall. Bypass filtering perhaps?"

Yup. A common way to get more water over the falls is to put a Y fitting in front of your input plumbing. The line that goes to the bottom of the bog gets a ball valve to choke flow down to the rate you need for filtering, and the rest gets dumped out on top of the bog, which just flows over the waterfall.
 
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thanks for that mrsclem, found and assimilated :)

My first mistake was thinking the water moved from one end to the other.. from left to right through the box rather than bottom to top. makes more sense now...

Thanks too Combatwombat;

a quick calculation (@45gpm, 4.3ft per sec, gives a transit time of 2.35 seconds).. hmm, a bit too quick lol.. bypass filtering next on the reading list!

Also means I dont think I can dump the bypass water onto the gravel... a sump reservoir at the head of the stream to the waterfall perhaps??

wow.. thanks again everyone, I've learned more here in a day than I did pottering bout the interweb. thank you for your time, its much appreaciated
 
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Pictures will help indeed. Here you go...

 
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yea, i mentioned the smaller pipes when I thought the water flowed horizontally rather than vertically... design amended. :)
 
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is there any specific reason for the 12-14" depth of the bog? I can understand a need for a minimum depth, but is 16 or 18" not better?
 
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You'll get (and read) lots of "must be" about bog building. My bog is 4 feet deep - works great. Some have 12 Inch deep bogs - also works great. Some use Aquablox or other matrices, others fill it with 100% pea gravel. They all work. Flow rate? I have no clue how fast my bog flows. We just kind of eyeball it and turn it up or down based on the situation - when it's full of plants it may need a bit more "oomph". So a ball valve for adjusting flow is always a good idea. But in any case, mine works so I don't worry about the math.

Many many "experts" out there, but this is a relatively new concept in garden ponds - our pond is 9 years old and you rarely heard or saw anyone using constructed wetland filters when we built ours. In fact, many "pros" laughed at us and told us we'd be sorry if we didn't include a box filter. Now everyone is building them and recognizing that it's one more way to make the pond less work and more enjoyment.

The basics are always the same - pump water into the bottom of the space, fill the void above the plumbing with rocks, gravel, and maybe Aquablox, figure out how the water will exit the bog and voila!
 

addy1

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Mine is 2.5 feet deep, more or less, 27 feet long around 4 feet wide, I send in 6800 gph more or less into the bog.
The slit pipes are 27 feet long, slit about 8 inches apart, about, two of them joined by a T at the input.
Mine is just the pvc pipes, pea gravel, plants. Others have been fancier. But mine works and works great easy set up.

I did no clean out, but a lot did.

Tons of plants, the water gets back into the pond via a flat rock water fall. The flat rock is about 3 feet wide.

I never have green water or string algae attack, water test never off, (have not done any in years) got bored testing perfect water.
I never treat the water with anything.


Finger drawing of a bog:
bog.JPG
 
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thanks for that Lisak1. I've been overthinking it a bit :) 2" pipe and gravel seems to be the way I'm heading.. (Although I do have some 41mm pipe left over from another job.) I'll get the bog filter as large as I can in the space available.. but away from the pear tree.

Current plan is to have my TNB strainers feeding the 10,000 pump, from there via 2" pipe to the T piece and ball valves. Getting it into the bog should be easy enough, mainly by stealing the designs seen elsewhere around here ( :) ) Getting it out is still a question to be answered.. the output from the bog is about 3m away from the waterfall into the pond... a stream seems the obvious choice, just need to figure out the logistics of getting the bog filter water and bypass water (which will be at different pressures and velocities into the stream, from there a simple job of getting it over the waterfall... how hard can that be? LOL.. Some sort of header tank perhaps?
 
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Yep, if you research it, you'll read many conflicting statements.
Some say any deeper than 12" of gravel and you'll have a problem due to lack of oxygen which will prohibit healthy beneficial bacteria from colonizing. That has been debunked by Lisa as she stated in her last post. Her bog is 4 feet deep!

I think as long as you adhere to the very basics of it, you will be fine.

I used wide sweeping elbows instead of standard short ones where I could and a wye instead of a Tee, thinking there would be less constricting flow. I really don't think that was neccessary because when I open one of my clean-out stacks, the flow is massive.

My pump is a 3600gph and I think it flows nicely. The bog works great.
 
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Nice one addy, lots of useful info there,
 
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Depending on your climate, you may have to shut the bog off for the winter.
If you shut your bog pump off you might get a natural backflow if you don't break the suction. This will drain water out of your bog and you don't want that. Plants will dry up and die.
A union can be used at the high point of your bog feed or inlet. This way you can break the suction by opening the union. I used a Fernco rubber coupling, but I'm going to change that to a more secure PVC union.

Oh, I got away with running mine all winter with no problems. I thought the water from the bog would pour over the top of the frozen pond and escape onto the ground. That didn't happen because the flow out of the bog kept an opening in the ice. I kept my deicer nearby that area just in case, but it wasn't necessary.
 
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In a normal winter there are perhaps 5 days of the year where water will freeze, in a bad one maybe 15.. I'm thinking a couple of ball valves at the split between bypass and bog filter would allow the bog 'loop' to be isolated from the main flow.. have to see how it goes with that. one of the advantages of the location is that it is sheltered quite well from the north winds.
 
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