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Pond liner thickness, bell siphon, tight outflows - beginner questions

Here are some more beginner's questions:

I'll be lining my grow bed in my first ever AP system with pond liner. Do you have any recommendations on the thickness of pond liner?

Any tips on how to make good, tight outflows through the liner?

I'll try to build a bell siphon like this one:

Any tips or warnings?

Thank you!


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I like the Firestone Pond Guard EPDM pond liner for many purposes.  It folds easily and is tough stuff as long as you keep the chewing critters and insects away from it (I had trouble with lumber and liner grow beds in Florida and termites.)

Durascrim is nice but only for very rectangular troughs of the correct width since it is not nearly so easy to fold nor does it stretch like EPDM will.

I personally would avoid PVC liners since I don't like the chemicals used to keep vinyl flexible or the ones to keep the flexible stuff from getting too brittle in the sun.

to plumb through a liner you can use good bulkhead fittings or even uniseals.  You just need to make an appropriate thickness backing out of a piece of harder plastic and cut the right size hole in the backing.  For bulkheads you have to cut a very nice hole in the liner the right size and then install the bulkhead through the liner and backing.  Make sure the backing has support to hold it in the right place since stretching liner can fail the seal.  For Uniseals, you will make the right size hole in the backing and mark the hole in the liner but instead of cutting a round hole you will make an X cut or * cut in the liner centered on the hole and a tiny bit smaller than the hole in the backing.  Then you push the uniseal through the liner/backing and shove the pipe in.  Make sure there is support for the backing as pushing the uniseal through can be hard anyway.  The thicker the backing the harder it is.

Do you know the thickness of these liners in mm? I will maybe not find the same brands here, but I could compare. Thanks for the warning about pvc's. I assume that's pretty common stuff.

I can attest to DuraSkrim being a pain in the butt to fold,  (especially the first corner or two... lol I can't wait to line my bigger rafts :). But I like the fact that it is fiber reinforced LDPE, it's UV stabilized, and not that I put much faith in that sort of thing, but it's been deemed 'Food Safe' by the Gov't of the US. And Friendly AP in Hawaii managed to get an Organic certification using that particular liner. Its 20mil (which is 0.508 millimeters)... You might have a hard time sourcing it in Sweden though. I had to import mine and shipping was as much as  the two roll cost.

It's probably no good for media beds with sharp rocks like lava...

TC's EDPM pond liners are probably the way to go for a number of reasons. (It was just too risky for me personally here in Serbia, as I've found that many things turn out to be something other than what they were sold as). 

the EPDM firestone pondguard liner is what they call 45 mil thick but I don't know how much that is in milimeters

45 mil is 1.43mm thick...

This calculator has come in handy many, many times...for many things :)

How well does billboard liner holdup?I think its 17mil.

Hmm...there have been some rather "passionate" discussions on that topic...In a practical sense, many used billboard liners will be damaged enough by the time you get them to leak. While many will not. On the passionate debate have the leaching toxicity issues...And NOT just from the inks (which is what most folks latch onto). Having worked a a billboard print shop in the US a while back, there are horrible, horrible known and dangerous toxins that are used to prep the bilboard liners (most so that certain inks will adhere better). This is why I quit that place after 4 months (I had started getting into silk screening t-shirts at the time and got the job figuring I could learn some stuff. That and the fact that the photo-emulsion fluid that they threw away at the end of just one shift would last me many months $$$ :)

Personally, I'd never use the stuff. I didn't get into AP because it is a cheap way to grow food, but rather it seems a good way to grow quality food free of many toxins, pesticides, herbicides etc...I would not wish to replace one group of toxins in my food with another (potentially worse) group of toxins.

This is just my personal, biased opinion though. We all need to make our own choices. Hopefully they will be informed choices, and not based solely on the dollars...

This goes double if anyone but the AP operator/owner/constructor themselves will be consuming food grown from the system. For me this includes Family, neighbors, or customers...I just could not feel right about it.

That being said, if you were building such a system for yourself only, at least have the sense to turn the print side down! I think I remember reading somewhere of folks wanting to do the opposite because they liked the 'perty colors'  their billboard liner had. 

The billboard liners are hit and miss.  You might get lucky and get one that doesn't leak but then again, they are often old and brittle with age and small abrasions and pinholes are likely.  Using multiple layers doesn't help.

And then there are the possible leaching issues.

1mil = 1 thousandth of an inch.

I have been using a 12mil liner for a gravel bed for over two years.

TCLynx said:

the EPDM firestone pondguard liner is what they call 45 mil thick but I don't know how much that is in milimeters

Thank you for a lot of good information! Next, I'll go out there and find out what's available here, and see how it matches your liners. They are in fact thinner than I expected.

My box - I'll take a photo of it one of these days - is made from a very hard and compact board underneath. I don't know what it's called, it's wood based, but very compact and strong. I found it in a waste container. The sides are made from plastic planks made out of recycled plastic. These things are in themselves not "food approved", or even water proof, but won't rot or be eaten by insects. It's rectangular, so I need to make folds in the corners.

Wow Vlad didn't know they where so toxic.I've seen  a few people using these in raft setups in TX.I would never use with ink side up that's just tacky,lol. I'm getting some for free... maybe ill find another use for it.I'm using PT wood which has copper and I don't want that to leech into my water and kill my fish.What precautions can I take to prevent leeching?... will staining or some kind of barrier, aside from the liner be necessary?

Now Alex, I'm not saying that every single billboard ever printed is going to leach toxins if used in an AP setting. Just that the potential exists, why risk it? It would be nice, but impossible to know "what billboard" your getting. What inks were used, what pre-treatment it went through. That kind of info would help in making a choice, but since that is un-realistic, I would personally stay away from it entirely. And your right, it is kinda tacky :)

As long as no water is in direct contact with the wood, my guess is your good to go. Whatever liner you end up using should take care of that. I went with a UV stabilized, fiber reinforced, shrimp and fish safe LDPE liner from Raven industries called DuraSkrim. It's 20 mil thick, white and is made to last a decade (or more) out in the sun...Comes in a bunch of sizes/widths...


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