I had to google it because I did not know what sono tubes were. They look like cardboard (?) tubes used to create formwork concrete columns. The web site makes some noises about how super extra strong they are, and as you say, there's some kind of waterproof coating at least. I don't know how concerned I'd be about something toxic in there - I'd be more focused on the fact that they are some kind of fiberboard or paperboard and they will eventually break down when they get wet.

So then the question becomes - what are you hoping to achieve by making a worm tower? Worm towers seem like a cool idea, but I don't know what they really accomplish that might not be better done with a conventional compost pile and/or a worm bin. Generally they seem quite small (at least the ones I've seen made out of a 5 gallon plastic bucket) so they probably wouldn't handle even as much food waste as a worm bin.

I think, the initial idea of the worm tower is to have the worms eat the food and than distribute their dropping throughout the garden. However, most of us here have the composting worms which are NON burrowing and the castings will be in and a little around the hole. In time I imagine once the level moves up the tube will have to be cleaned out. Unless of course you are talking about the tunnel.

From what I've read of worm towers, they aren't necessarily stocked with composting worms - you just set it up, start adding food material, and the native worms turn up. You could put in composting worms as well but I don't think it's required.

I'm sure the native worms do turn up, but I'm not sure how far they actually distribute their castings from the tower. My guess is that almost all the castings wind up in the tower or within a very short distance from it. Non-composting, earth-dwelling worms will probably go in and out freely, but as they move away from the tower whatever went in the front end will come out the back end before they get very far - worms don't exactly hold it in until they get to a rest stop. You probably need to clear the tower out periodically and distribute the castings by hand if you want to spread the wealth around.

I'm making a worm tower mostly because the idea intrigues me. I realize there's some debate about their effectiveness, but I'm interested in the idea. Also, we have city-wide compost pick-up so I don't need something that handles a large volume of food waste.

I picked up some pine boards to make one. The sono tubes interested me because they're very cheap and already "assembled".

So far the worm tower seems to be working. I nailed together 4 pine boards to create an open ended square "tunnel" 2 ft (tall) x 10 inches x 10 inches. I drilled 5 holes, each 1 inch in diameter, on each side (on the part of the tunnel that would be under the soil). I then buried it in my garden with the open ends sticking straight up and down, with a foot of the tunnel above the soil and a foot below.

I toss in your traditional composting items: banana peels, coffee grounds, vegetable peels, clippings from plants, etc. At first there wasn't much movement, but now I notice the level of scraps in there drops quite quickly. When I remember I throw in a few handfuls of shredded leaves. It doesn't smell, but does attract a lot of ants.

Is it actually improving my soil? Is it really attracting more worms? Who knows, but it's fun and my child loves to go and feed the worms.

I think I'll pull it out in the fall, since it would likely be destroyed by our winters. It'll be interesting to see what's going on below the soil.