I need to do fast, hot composting on a largish scale; I'll get into why in a minute.
The options for fast, hot composting on a large scale are: using a piece of heavy equipment to move piles; hiring an army of workers with pitchforks; or building a giant tumbler.
All of these cost money, and some of them use a lot of fossil fuels.
But as I considered different ways to build my own giant tumbler, I got what seems like a good idea.
The whole point of a tumbler is to mix up and aerate the pile.
I'm thinking that I will lay down a big sheet of concrete reinforcing wire, say 8' by 20', backed with a breathable tarp or landscape fabric, with 2x4 cleats of lumber attached every so often, including a really heavy, well attached cleat at each 8 foot end of the sheet. Then I would pile up a mound of compost across the middle of the sheet, 6 feet long, 3 feet wide, and 3 feet tall. One of the ends of the sheet of material would be brought across the top of the pile and attached to a come along, and then pulled, tumbling the pile as the material moved toward the other end. Every two days, the material would be moved in the opposite direction. The cleats would keep the material from sliding. A breathable tarp could be placed over the pile between tumblings.
As to why I need to actively hot compost large amounts of material; I'm starting a small nursery business, and need lots of compost and potting soil ingredients. I'm hoping to use wood chips as the bulk ingredient, heating the piles with coffee grounds, vegetable waste, and similar materials. The wood chips should be decay resistant enough to eventually screen out and use as the main ingredient in my potting mix, along with a certain percentage of the compost. I may add biochar and other potting soil ingredients to the piles.
If I tried to slow compost, weed seeds and pathogens could create problems. Bindweed tends to colonize slow cool piles here, and is impossible to eradicate. Rodents are attracted to food waste and the ideal lodgings in a slow pile. Material is slow to break down here without active management; turn around time on a slow pile here is at least 2 years, possibly more, since piles I built 2 years ago are not really broken down.
So, by largish you mean 6x3x3 or so? I use a tractor to turn compost but would not hesitate to do a pile like that by hand, especially as it does a better job and lets you really see what's going on. You might be surprised at how quick a pile can be flipped -- just stand on top and toss it over. Bet I could do your pile in ~30 minutes.
Not that I want to deprive you of the fun of building such a thing... it sounds cool. But how on earth are you going to tumble it? It could be very very heavy!
For hot, there is nothing quite like chicken poo. Once in a while I'll want a really hot pile and for that I use straw and a high N manure like chicken. Wood chips take way too long to break down.
If this works, I'm hoping to scale up by building several piles at a time, and increasing the pile size to 4x4x8. And I'm hoping to flip each pile every other day.
As far as the weight, I've estimated it will be about 2 tons. That is the part I'm unsure about; will a come along winch do the job? Or will something break before the thing starts moving? I could also try using a truck with a towing hitch.
About the wood chips being slow to break down; I plan to sift them out of the pile at the end of the cycle. That way I can use them as a substitute for peat moss in my potting mix.
I think you may find it very difficult to move. I get the principles as you described, but you have weight (and 2 tons seems about right) and then lots of friction on top of that. Not saying it can't be done just that the forces involved may be larger than you anticipate. My sense is, from moving a lot of material here on my farm, that it's going to be a major effort to flip them and that you will find it near impossible to do it at the frequency you desire. Don't let me dissuade you though... I'm interested to hear the results I think you will want a 4-6 ton come-along or maybe use a truck.
Also, I wanted to mention hog panels. Might be a ready-made way of getting your tumbler. If you're not familiar with them, they are basically a very heavy duty section of fence.
Have you considered a more traditional tumbler design that rotates on an axle? That isn't so hard to build large and would be much easier to turn. Even automate. I could see how you could use hardware cloth inside the hog panel (or similar) to screen the compost after it's done -- like open a hatch and tumble again.
Oh, and wood chips. Be careful. They are not a substitute for peat or coconut coir and will tie up nitrogen in your mix. How much and the impact obviously depends a great deal on what you are doing, how composted they are, etc. Just a heads up, it may not be an issue for you. For my veggie starts I make a point of screening woody material out. A few years ago we experimented and for the tomatoes in particular there was very substantial difference using a mix with high woody content. YMMV
Currently, I'm thinking of attaching two come alongs, with the chains spread out along the end of the cleat, each of 2 tons or over. I may also attach the chains to a 4x4 and use that as an additional lever, you can find videos of this online.
Hog panels sound like a great idea; I'd use them except that I've already got a big roll of concrete reinforcing wire laying around, so I will use that in my prototype.
What type and how much woody stuff did you have in the compost you were screening for the tomatoes? Did it just go through the pile? Or did you add extra for the test? How decomposed were the woody elements? I'm going to be embarking on a series of experiments trying to use local ingredients to create a potting mix, since neither peat nor coir are really "green." So far, the only ideas I've come up with are compost, rotted wood chips, biochar, and volcanic cinders (the last being not quite local.)
The potting mixes we used in the experiment were a low-grade commercial mix slightly augmented with our best compost and our own mix which contained small wood chips that were partially decomposed -- actually a compost pile that didn't quite have the right ratio and hadn't fully broken down. In both cases woody material content was significant, perhaps up to 25%.
The commercial mix was pretty much a bust. It was so cheap we had though it might make a cost-effective base, but the conclusion was the woody bits were tying up nutrients to the point where it would never work without adding lots of fertilizer components, thus negating the cost savings.
My experience, as you might expect, mirrors conventional wisdom: well-rotted chips are ok but they need to be at the point of being mostly crumbly. The woody stuff in the commercial mix was the least well rotted, as in barely at all.
This doesn't mean you can't compensate by adding a fertilizer to the mix or (in our case) by increasing foliar feeding and or soil soaks with compost tea or fish emulsion.
What I meant about screening was that after that experiment I made a point of screening excess woody material for most starts. That kept my costs and effort to supplement the fertility to a minimum.
Part of the lesson for me was that with a good compost high in organic matter the need to add other materials to improve drainage or consistency isn't nearly what you'd think. Now I pretty much just use compost and if I feel it's too dense or doesn't retain moisture well I'll add some old used potting soil that contains vermiculite -- no more than 5-10%. Or I'll add some old woodchips that are so well rotted you wouldn't recognize them. I just go by consistency.
Whatever you do it really pays to test systematically.
Will you post pictures when you get to the point of trying this out? I'd really love to see how it all works out.
Yes, I will be posting pictures as I go. I will also be posting pictures of a series of experiments trying to make a 100% local potting mix, with side by side growing comparisons with the best potting mix I've come across so far.
Do you know what species the wood chips came from? Why were they in your compost pile? I know what you mean about the commercial stuff, it is usually mostly wood chips!
Yes, a caterpillar tread in reverse does come to mind. I had initially been thinking about using a big "loop" of wire plus fabric, with circular end caps of thick plywood, and internal 2x4 cleats, as a big tumbler that would roll along the ground, probably pulled by a come along setup. But it seemed like it would be hard to roll, the plywood would be expensive for something that would rot eventually, ($120 at least) and the cylinder would deform badly under strain. Also, I could't think of any easy way to load it; it would have been 8 feet in diameter to hold a four cubic yard pile.
Then thought about making it easily openable/ flattenable, so that it would be easy to load. Then I started thinking about leaving out the expensive plywood, and letting the thing take on a squashy oval shape as it rolled; and then I wondered why I would bother to shut it anyway.
All my pee goes in buckets which I put on the compost, or I just pee near plants. It's like 10-1-2 (or so) and it seems a travesty to throw that away. Yay pee!
The wood chips in that compost were primarily pine, with some oak. Mostly from my land, but some came in the horse poo we get (bedding). I use wood chips in the longer term compost piles because the horse manure is usually too high in N and I'm chipping for fuel reduction anyway. Normally they are all long gone by the time I use the compost.
I built the thing, and stacked up a compost pile on one end of it. I took pictures, but they are on somebody else's phone, so I will have to wait to post them. The pile heated up really well; it was mostly made of old vegetable garden waste and straw, with some earth and a lot of fish emulsion added. I was planning to flip it over this weekend, but then we got snow, so the moment of truth will have to wait. When it happens, I will let you all know!