DIY Compost Tumbler

I'm not too interested in gardening (my wife does teh gardening, though I work on a couple of veg beds), but I like composting. Beyond saying that if you possibly can (if you physically have teh space) you really should compost your compostable waste, I'm not going to discuss how to compost - you can find that on the web. I have a couple of big bins (about 1000l each), but wanted a tumbler, so I made one.

Performance

Before describing teh build process, though, you might be interested in how well it works after nearly two years (two summers, one winter).

The tumbler was an experiment - I've seen the marketing promotion that suggests that with a tumbler you'll get beautiful compost out almost as quick as you put anything in, but I didn't want to spend too much. Of course, if you cost all the time locating bits and building it, it's not all that cheap.

To be honest, I find it useful in my composting, but it doesn't make very good compost. I suspect that needs some explanation:

What I do is that anything heading for the compost goes into the tumbler (and I turn the tumbler a few times every time I put anything in it). Howevere, it doesn't compost fast enough - actually, I think it goes slower than if I just put stuff on a heap (though I turn my heap regularly - probably more than most people would), and the tumbler fills up. It's also too wet, even though I put in quite a bit of shredded paper (which I get from work) and occasionally tear up cardboard for it too.

I think I have two problems - foremost is that not enough air gets through. My main reason for thinking this is that when it has filled I tip it all out and chuck it on my heap (which is in a big square plastic bin), then dig some out the bottom of the heap and cover over the new stuff. This then gets hot very fast. I think the tumbler mixes stuff up and inoculates all the bits with micro-organisms, but they are held back by lack of oxygen. When it goes on the heap, that is resolved and away it goes.

The second problem is the wet - I have quite a few holes in the bottom but they clog up and don't drain as well as they should. Possibly it's excess water rather than insufficient oxygen that stops the tumbler working.

The tumbler has two advantages though:

So I keep using it, but as a mixer really, rather than a composter. I find the best composter to be a big bin which I keep digging over (most weeks I'll dig a load out the bottom and chuck it back in the top, to keep it slowly turning over).

Build

The starting point was a 150 litre grey plastic barrel with a screw lid. This was second-hand, having previously held unspecified food product, from ibcextreme on ebay (who also has an ebay shop. I don't know about the rest of the products, but the barrel was delivered very quickly, was exactly as specified and in excellent condition. I chose the seller simply because they had the barrel I wanted, but I'd have no hesitation buying from them again.

The points I wanted from the barrel were:

As you can see, this barrel meets all those requirements.

Unfortunately, I forgot to photograph the barrel as a whole before I started carving it up, so this photo is stolen from the ebay listing (I'm guessing he won't mind).

The plan was for an axle through the middle (with the barrel upright). A tube vertically up the middle (supported by the axle) vented out the bottom with air-holes in the tube to let air in, and some small holes under the lip of the lid to let air out. As (if) the contents heat hopefully convection will drive air circulation. The top holes, being under the lip, will avoid most rain getting in.

I chose this plan because it meant minimal holes in the outside of the barrel, so neat appearance, less prone to scattering its contents (because the material inside will mostly fall away from the middle as tumbled, not out the holes). I also thought it would make the whole thing easier to rodent-roof (squirrels and presumably less savoury furry things chew on my compost bins)

Step one is the tube up the middle. I had some remnants of plastic downpipe and fittings, and some bits of aluminium mesh. This is a socket used for joining two pipes, which was good because it had a lip at one end. Cut a square of mesh and bend it to a tight fit by hand.

The mesh is jammed into place, then liberally glooped with hot-melt glue.

Hole-saw through the bottom of the barrel. This was harder than expected - the hole-saw bit at first, but then seemed to polish the bottom of the groove and progress ground to a halt.

It turned out the bottom was about 10mm thick, and I eventually got through by laying the barrel on its side and crawling in to cut from the other side.

Once through, the socket was dropped into place, and liberally glooped (from both sides) with more hot-melt glue.

Socket in place at the bottom of the barrel...

into which a length of down-pipe simply slots. This is plastic 68mm (I think) down-pipe. As you can see from the accumulated dust, it's a piece I had lying around from something I did some years ago.

Drill holes for 8mm threaded rod (bought from hardware shop) and 3/4" OD (1.5mm wall thickness) steel tube to be the axle. The tube was another ebay purchase - from 10gbottles ebay shop. Again, accurately described, quickly delivered, and I'd buy from the same seller again.

The axle and rod pass through the central tube to support it (with the addition of some nuts and washers, not in this photo). The threaded rods are intended to break up the compost as the barrel is rotated. Also, the downpipe has been trimmed to length here.

Air circulation holes drilled in the central pipe.

Threaded rod cut to length ...

and capped with penny washers and dome nuts. There are also some T-nuts I had which went on the top threaded rod either side of the central pipe in order to restrain it. I had to buy the nuts, the rest was bits-and pieces lying around.

A couple of steps were not photographed:
I made a cap for the central pipe out of a socket, some scrap plastic sheet and silver duct tape.
I drilled the top air holes in the barrel, three each side above the axle tight under the lip of the lid and pop-riveted (plus hot-melt glue) aluminium mesh on the inside to screen them.

That's it for the tumbler itself, which was three evenings work after my daughters were in bed, and required the purchase of the barrel, the axle, the threaded rods, and the dome nuts. Obviously, if I had to purchase all the other bits, it would have been rather more expensive - probably as much as just buying a cheap commercially-produced unit.

To tumble it needs a support frame. I made one out of 90x25mm decking boards and some bits and pieces. It's screwed with deck screws and glued with weatherproof adhesive, then painted with some dark wood preservative. The wood I used was already pressure-treated, but I wanted it darker (to make it less conspicuous in the garden). The frame was a Saturday morning's work, with assistance from my four-year-old elder daughter.

The bearings are 50mm lengths of 22mm copper pipe a tight push-fit into holes drilled in the timber frame legs. The axle protruding from the barrel is a reasonable fit through the copper pipe.

More 8mm threaded rod passes right through the axle ...

and is capped with penny washers and wing-nuts, which hold the axle in the frame. The nuts are done up tight, clamping teh washer onto teh end of teh axle, but the axle is trimmed so that there's a little bit of play - with everything assembled there's about 1mm of gap between teh inner face of teh washer and teh outer face of teh frame. It tumbles very easily.

Fully complete, the unit is installed in place at the end of my garden, behind a row of trees and some trellis fencing, which is why I wanted it all dark - it disappears into the shadows viewed from most of the garden.

There's a spacer bit of timber between barrel and support frame legs on each side so the barrel is held away from the legs, to prevent the assembly acting like a guillotine on (especially little people's) fingers.

Test

So, now I need to see if it works, and the first thing is to see how well it mixes, so:

Add some compostables - in this case kitchen vegetable scraps and a big handful of shredded paper. Shredded paper is what I use when a heap gets too wet, but here I'm using it as a bright indicator.

There's a step not described in the construction sequence visible in this photo - the barrel doesn't just hang on the axle, the axle is a tight push fit through a hole in a length of the same timber used for the support frame, and then there are a number of short but fat screws through the barrel wall from the outside into this timber on the inside.

Cover with a couple of shovels-full of half-made compost from another bin. Note that vegetable scraps and paper are now concealed.

Put the lid on, two complete revolutions, and open up...

Excellent! Even the quick two turns of the tumbler seems to mix it all up, and now there's a fairly uniform mixture of scraps, paper and compost. I'm very pleased with this.

When it's more full, I expect it will mix less quickly and easily, but we'll just have to wait and see how it performs in practice.


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