Flex Pipe in Bog Filter - Yea or Nay?

[YShahar]

Shalom Y'all,

I'm slowly getting the bog filter built. I hired a friend who works in construction to help me put up a cinderblock retaining wall where the bog is going to go (and we ended up building it one cinderblock too high, but that's another story). I bought a bunch of used milk crates for the matrix and I'll be digging the interior out over the next week or so. Liner has been delivered, but I'll need to buy another roll of underlayment (and sadly, it comes only in 100 meter rolls, of which I'll have a use for about 10 meters worth).

In parallel, I'm working out what I'm going to need for the plumbing. The outside measurements are 160 cm by 320 cm, and the inside will be about 10 cm smaller all around, so I can probably get by with a single length of pipe down the center, running under the matrix blocks. Buying PVC pipes and fittings isn't straightforward here, and I'll have to go to numerous different suppliers to get everything I need, so I'm looking at the simplest solution possible. And that's what leads to this question! I came across this video:

Here, they've used flex pipe for the bog plumbing. Now I've got a lot of 50mm flex pipe left over, since it came in a 25 meter roll, so this might be a cheap-and-cheerful solution. So what are the pros and cons. Here are a few that come to mind right off the bat:

Pros:

• I already have enough of it to go down the length of the bog.
• Using flex pipe means having more gradual bends (so less flow lost to friction)
• I won't have to buy as many fittings, since I can mold the flex as needed.

Cons:

• The pipe could collapse if weight is placed on it (though having it under the matrix should protect it from this).
• Holes or slits may be more liable to clogging (not sure of this, but it seems to me that the flexible nature could lead to deformation that might close up some of the holes).
• Pipe may not last as long (again, not totally sure of this)

Does anyone have any thoughts on this? What factors might I be missing here?

 

[Tula]

I can see the advantage to flex PVC if your bog is an interesting shape. I don't know how flex PVC holds up to cutting / drilling the slits / holes, as we used rigid PVC.

 

[Laaf]

Are you going to take out a few blocks to have an outflow area? It looks like it will be a great filtration system for your pond.

Once the gravel is installed it will be a big pain to redo the bog pipes if they collapse or clog. I would use non flexible with the hope it will last for many years.

If you were buying new materials, what would you use? You can use the flexible piping for water flow to the solid piping. So you can still use it.

 

[combatwombat]

I don't think I would do it unless I didn't have another choice. Is rigid PVC expensive/hard to come by where you are?

If you do it, I'd go with more small holes than fewer big ones or slits. The small holes will be more prone to clog but big ones or slits, I think, would compromise the structural integrity of the pipe too much... which is already an order of magnitude weaker than rigid. Would definitely include the cleanout.

 

[GBBUDD]

ll need to buy another roll of underlayment (and sadly, it comes only in 100 meter rolls, of which I'll have a use for about 10 meters worth).

DON'T BE AFFRAID TO DOUBLE OR TRIPPLE THE UNDERLAYMENT LAYER . AND I STRONGLY ADVISE doing so on all the rock you have . also make sure you fill in 90 degree angles at the bottom of the wall no hard 90's to prevent stretching. and the 90 degree corners at the top of the wall you'll want to round over to keep it from cutting the layers a simple grinding cup wheel will do the job or if your real ambitious use a brick or similar.
underlayment inside and on the outside of the liner is a must.

The ridged will be a better bet but i believe if your careful and do not cut the reinforcing strand in the flex pipe you should be fine. just be careful when placing your rock once the pipe is buried and surrounded it would be hard for the pipe to collapse as the weight will disperse as you build up.

Ah with a matrix area above there's no worries when you make the trench for the pipe you can make it so there is literally no weight on the pipe at all the milk crate can span over the trench. as far as lasting i don't believe that's a problem it should last decades providing its out of the sun.

Though only plastic i had all kinds of sharp areas on the matrix blocks i used a quick belt sand to any spots and i'd entertain zip ties to each other to keep them from shifting

 

[YShahar]

I can see the advantage to flex PVC if your bog is an interesting shape. I don't know how flex PVC holds up to cutting / drilling the slits / holes, as we used rigid PVC.

That's what I was thinking: that it would work better for an irregular shape. I had originally planned to make the bog in the triangular area beyond our garden wall. But then I realized that there wasn't enough ground on the edge of the cliff to ensure that the whole thing didn't go sliding down the hill. So Plan B was to situate it where my old pond was -- an area that is more stable, but has the disadvantage of being mostly shady. So now the shape will be rectangular, roughly 3 x 1.5 meters for the inside diameter. No real need to use the flex piper for that shape, other than the fact that I've got a whole lot of it!

I'm thinking that if I do use it, I would drill holes rather than slits. Still not totally sure it's the right solution though.

 

[YShahar]

I don't think I would do it unless I didn't have another choice. Is rigid PVC expensive/hard to come by where you are?

It's fairly pricey and hard to find, though I think I've found a plumbing store that does carry it. With any luck, I can get not only the pipe, but also most of the fittings there.

If you do it, I'd go with more small holes than fewer big ones or slits. The small holes will be more prone to clog but big ones or slits, I think, would compromise the structural integrity of the pipe too much... which is already an order of magnitude weaker than rigid. Would definitely include the cleanout.

That's what the guy in the video I linked to did--used lots of holes. But yeah, I'm kind of leaning toward going with the rigid PVC. Keep it simple!

 

[YShahar]

DON'T BE AFFRAID TO DOUBLE OR TRIPPLE THE UNDERLAYMENT LAYER . AND I STRONGLY ADVISE doing so on all the rock you have . also make sure you fill in 90 degree angles at the bottom of the wall no hard 90's to prevent stretching. and the 90 degree corners at the top of the wall you'll want to round over to keep it from cutting the layers a simple grinding cup wheel will do the job or if your real ambitious use a brick or similar.
underlayment inside and on the outside of the liner is a must.

Yes, I'll definitely double up the underlayment on the bottom (maybe also on the inside). The plan is for the liner to be well inside the cinderblock walls. I'll be wrapping the liner over the milk crates and backfilling with sand. The blocks are there mainly to hold up the sand and give some height, but they'll mostly be covered over with dirt on the inside and rock along the top.

The ridged will be a better bet but i believe if your careful and do not cut the reinforcing strand in the flex pipe you should be fine. just be careful when placing your rock once the pipe is buried and surrounded it would be hard for the pipe to collapse as the weight will disperse as you build up.

Ah with a matrix area above there's no worries when you make the trench for the pipe you can make it so there is literally no weight on the pipe at all the milk crate can span over the trench. as far as lasting i don't believe that's a problem it should last decades providing its out of the sun.

That's the idea: to have the milk crates sitting well above the pipe (even if I go with rigid PVC in the end).

Though only plastic i had all kinds of sharp areas on the matrix blocks i used a quick belt sand to any spots and i'd entertain zip ties to each other to keep them from shifting

Good idea about using the belt sander. I definitely do plan to zip-tie them together. May even use @combatwombat's PVC milk crate strengthener, assuming I can find a supplier for rigid PVC.

 

[YShahar]

Are you going to take out a few blocks to have an outflow area? It looks like it will be a great filtration system for your pond.

I had originally left a space open for the outflow, but then realized that I hadn't taken into account the rocks that will be used to disguise the wall. The extra height would have made the whole thing look much higher than the existing stream, which would have spoiled the naturalistic effect. So in the end, I got out a chisel and sledge hammer and knocked off the top blocks along the front and the two corners. I'll have to build the outflow by building up the sides a bit around it, but that shouldn't be a problem.

Once the gravel is installed it will be a big pain to redo the bog pipes if they collapse or clog. I would use non flexible with the hope it will last for many years.

Yes, after reading over the advice here, I think I'll go with the rigid PVC. I put out a question on a local FB forum, and someone directed me to a plumbing supply shop in Tel-Aviv that supposedly has what I need (finding a place to park in Tel-Aviv is another story!). I really don't want to have to take all the gravel out again!

If you were buying new materials, what would you use? You can use the flexible piping for water flow to the solid piping. So you can still use it.

Yes, that's definitely the plan! I've got a fair bit of the flex pipe left over, so may just have to build another pond! ;-)

 

[combatwombat]

Good idea about using the belt sander. I definitely do plan to zip-tie them together. May even use @combatwombat's PVC milk crate strengthener, assuming I can find a supplier for rigid PVC.

Ha. It's not exactly an engineering feat, but it adds a lot of strength to the weakest part of the crate. Good to do if you can. Will probably be fine if you can't.

I'm not sure I understand the point of the zip ties, especially for milk crates. They interlock and, once backfilled, aren't going anywhere. I think any force that would overcome the interlock would instantly snap a zip tie, too. But better safe than sorry, I guess.

 

[YShahar]

I'm not sure I understand the point of the zip ties, especially for milk crates. They interlock and, once backfilled, aren't going anywhere. I think any force that would overcome the interlock would instantly snap a zip tie, too. But better safe than sorry, I guess.

Sadly, the boxes I have don't interlock at all. They're built to sit nicely beside each other, but there's nothing to keep them from sliding around, hence the zip ties. I'll have only a single layer of them along the bottom, with the plumbing in a trench below. I found someone selling 8 of them used, but I'll need at least 6 more (may be able to squeeze in another two, depending on how much room my cleanout stack takes up.

So now I'm beginning to rethink part of my plumbing design. I had intended to have the water enter from near the head of my existing stream (which is where my pipe comes out at the moment) and have the main outlet be a stream midway down the length of the bog. But after thinking it over, I'm now inclined to run the plumbing directly up this smaller stream (I like having the plumbing inside the liner as far as possible) and come into the bog on the near side. That means moving the clean out stack to the other side, near the head of my larger stream, but that's fine.

I had planned all along to have a fair bit of the water tee off into the existing stream, using a ball valve to decide how much goes to the bog and how much to the stream. I'm thinking that this tee could also double as my siphon break. Running the pipe up the smaller stream would shorten the run of pipe to the bog by about 2 meters overall, though the savings would be at the expense of a slightly longer run to the stream head.

I have the option of having two overflows from the bog--one into the existing stream and one into the smaller one (the purpose of the smaller one is not only aesthetic but also to push water out of a dead zone in the pond.

Need to come up with a sketch and run it by the forum to see if the whole thing makes sense.

 

[GBBUDD]

Sadly, the boxes I have don't interlock at all. They're built to sit nicely beside each other, but there's nothing to keep them from sliding around, hence the zip ties. I'll have only a single layer of them along the bottom, with the plumbing in a trench below. I found someone selling 8 of them used, but I'll need at least 6 more (may be able to squeeze in another two, depending on how much room my cleanout stack takes up.

So now I'm beginning to rethink part of my plumbing design. I had intended to have the water enter from near the head of my existing stream (which is where my pipe comes out at the moment) and have the main outlet be a stream midway down the length of the bog. But after thinking it over, I'm now inclined to run the plumbing directly up this smaller stream (I like having the plumbing inside the liner as far as possible) and come into the bog on the near side. That means moving the clean out stack to the other side, near the head of my larger stream, but that's fine.

I had planned all along to have a fair bit of the water tee off into the existing stream, using a ball valve to decide how much goes to the bog and how much to the stream. I'm thinking that this tee could also double as my siphon break. Running the pipe up the smaller stream would shorten the run of pipe to the bog by about 2 meters overall, though the savings would be at the expense of a slightly longer run to the stream head.

I have the option of having two overflows from the bog--one into the existing stream and one into the smaller one (the purpose of the smaller one is not only aesthetic but also to push water out of a dead zone in the pond.

Need to come up with a sketch and run it by the forum to see if the whole thing makes sense.

The only thing id keep in mind is utilizing the energy as much as possible . that's why bogs are usually the head of the stream , waters pushed to the bottom of the bog same energy makes the water rise for a waterfall and the same energy again is the stream working from gravity.