Twin Compost Bin

this is a year-round, easily-dismantlable and movable composting bin. It has two compartments: one for matured compost, and one for fresh vegetable waste. you can let your compost mature for one year while you fill the other compartment; come spring, you then use the ripe compost in the planting of your garden (ymmv depending on your climate/seasons and quantity of composting matter).

Step 1: Materials Needed

you need some wood and some basic tools.

the wood we used was cheap, unfinished 1"x8" (nominally 1" thick by 8" wide, but actually 0.75" by 7.25") spruce typically used for decking, under finished siding or sub-flooring; for a longer-lasting - but considerably more expensive - bin, consider using cedar or pressure-treated wood (do *not* use pressure-treated wood that's older than a couple of years, as it may contain arsenic; the stuff you find in stores now does not contain arsenic and is deemed safe for projects like this).
corner posts were 4' long 2"x2"s, pressure-treated (nominally 2" , but actually about 1.5"); they probably don't need to be quite this long, but this will depend on how deep you want your bin to be and how soft your ground is.

for tools, you will need:
measuring tape and pencil;
speed square;
saw (either hand saw, jig saw or table saw);
hammer and chisel (or flat blade screwdriver);
optional coffee can or similar to slip over corner posts when hammering into ground to prevent damage to top of post.

Step 2: Size Matters

consider how big you want your bin to be.

from what we have read, one cubic yard/meter is ideal: any bigger and it can be hard work to turn it; any smaller and it can be hard for the pile to generate and store heat that helps in the degradation of the vegetable matter. consider the size of your household and how much you will compost. we are a household of two vegetarians, and chose the size of each bin to be 2'x2'. we already had an idea of what size we would need from our old compost heap, which was the bottom of a barrel (in both senses...), around 2' diameter. if you are very pressed for space, you could consider making the mature compartment of the bin smaller, since the compost does reduce in volume when it degrades.

in order to have an interior volume of about 2' square, we made the overall length of the long sides:
24" bin 1 +24" bin 2 + 2" end 1 overhang + 2" end 2 overhang + 1" notch + 1" notch +1" notch = 55"
and the length of the short sides:
24" bin + 2" end overhang 1 + 2" end 2 overhang + 1" notch + 1" notch = 30"

we have a small car, so ended up having the lumber yard cut the lengths on site for us. we also bought precut corner posts (typically used for deck railing balusters, hence the angle cut on end) which were ready to be hammered into ground as-is.

so, to make a bin this size you will need:
4 x 55" lengths;
6 x 30" lengths;
4 x corner posts.
our total cost was less than $15.

Step 3: Location Location Location

depending on your climate, you generally want your bin to be out of the direct sunlight (to avoid the compost drying out and therefore not decomposing as desired). you probably also want it somewhere fairly convenient for filling with your kitchen scraps, but not too close to where you spend quality time, as it can smell, especially after being turned. we have a tiny back yard, so situated it in the corner in the shade of some small trees. you want reasonable drainage so that the area doesn't become waterlogged.

you obviously want the ground to be level, so we filled it in a little where needed and tamped it flat.

Step 4: Cutting to Size

you basically need to make two notches each in the short boards and three each in the long ones.

measure in 2" and 2.75" from each end of all boards; find the center of the long boards and measure a 0.75" wide channel there; then measure a depth of 3.625" (half of the 7.25" depth of the board) on each notch. the shaded area in the fourth picture is what you end up cutting out.

we have a table saw, so it was quick and easy to make the two vertical 3.625" cuts, but a jigsaw or handsaw works fine too. you can snap off the remaining notch and might get lucky, but will probably have to clean up the cut with a chisel (or flat blade screwdriver) anyway.

note: err on the side of making your notches slightly too big rather than slightly too small - it will make assembly easier and gives some wiggle room.

Step 5: General Assembly

lay the interlocking boards perpendicularly into each other at the notches until you have your basic rectangle (well, twin squares) for the first course.

check the positioning on the ground (and fill in depressions/level off humps in ground, if necessary).

take your first corner post and hammer into ground (start in the least accessible corner and work your way out so that you have less hurdles to deal with). we used a coffee can to protect the top of the post from splitting after being hit with the hammer, but you can also use an old piece of 2"x4" or flat metal bar. repeat for the other three posts.

add the second course of boards as per the first.

Step 6: Fill 'er Up

there are other places where you can get composting lessons, so we will keep this brief.

it's good to have a bed of leaves to start with, for both the matured compost and the fresh pile you are about to start. luckily for us, it's fall in new england, so there's no shortage. chuck in any worms you can find too - they always help break down the vegetable matter.

we added our old mature compost to one side and covered with leaves (will help to insulate over the winter, though this isn't essential) and fresh kitchen waste to the other.

the compost on the left will be ready for use next spring when we start planting the garden. once we've emptied that, we then start filling it with fresh waste while the other side matures for a year. and so on, and so on.

update 10/11/08: added a pic to show how it looks now. it has weathered pretty well, and has stood up to having been moved once. should be a good few years left in her yet.

update 04/19/09: added a pic of the first fully developed 1-2 year old compost: really nice and loamy, clean and smell-free. despite this picture being taken only two weeks after the last of the snow had melted, the compost was very dry and crumbly, not a soggy wet mess. we used the blue tray to screen out the larger pieces (mostly twigs, walnut shells, peach pits and the like).

update 08/15/12: we replaced all the wood and built a new one. in truth, the pressure treated corner posts didn't really need replaced, but we had some new cedar ones lying around anyway, so why not. the biggest problem wasn't that the wood on the sides had rotted as much as most of the "tongues had got knocked off the ends of the boards over the years for one reason or another. if they had some kind of metal plate reinforcement, you'd probably get closer to ten years out of these basic pine boards.

Step 7: Possible Mods

some random thoughts/variations/modifications:

you can use wider boards if you can find them; ideally, we would have used two courses of 1"x10" boards, but could only find 8" wide. in the end, they turned out to be perfectly adequate.

we will update this instructable over the years to see how the wood holds up. we don't have great hopes for it lasting much more than 5 years or so, but still consider it a worthwhile investment for $15. pressure-treated wood would have cost around $50, and cedar likewise .

if you are building a bin that is much bigger than this, it might be better to use slightly thicker boards for extra support. these cheap spruce boards might not be effective at standing up to the pressure of a bigger pile and the forces exerted on them when you're turning the compost. thicker corner posts would also help in this case.

disassembly is super easy (the opposite of assembly, but you don't need to remove the corner posts), and it might make sense to just take it apart come spring when you want to shovel out the old compost. also, you can move the whole compost bin to a different location relatively easily any time you want.

a triple or quadruple bin is possible. if you live in a temperate region where you can grow (i.e., use your compost) year-round, and have a good decomposition rate for your compost, this could make sense.

we deliberately left our corner posts long in case we have the need to add a third course of boards. if there wasn't a possibility of that happening, we'd probably cut them down to size. besides, they could be useful for hanging things on.

we would have loved to have used old wood we had lying around for this project, but alas, all we had was painted (and probably containing lead, at that) boards, which we deemed unsuitable.

Comments

[simonborganic]

Thanks for taking the trouble to record all these photos and instructions - very inspiring. I tend to turn my compost more than you, and it seems to accelerate the composting process. And I tend to use my compost before it has completely composted down - this is to feed the soil with all stages of decomposition. I am however thinking of making a sieve so I can take out the fine compost and leave the larger pieces for the next batch. As a suggestion, when you empty one bin, turn the other bin over into the empty one. this allows you to thoroughly mix the inside with the outside and top with bottom so it starts to work again. I would use a fork for this instead of a spade - lighter and easier to pick up a load to sprinkle it over the new pile and less likely to chop up the worms - the guys that do all the work. By the way; I've tried all sorts of bins including tumblers and I'm going back to the basic style of bin that you have described so well here. so save yourself the trouble of experimenting - stick with this one - they do a very good job. And yes a lid helps - a sack or carpet is excellent - the worms like to be protected from the sunlight.

[camp6ell]

thanks for your constructive comments. i think most years we have turned over the full bin into the empty bin; this year, for no good reason, we didn't. does chopping worms in half not give you twice as many worms? :-) i'm kind of half serious here, right? this year, what really struck me was that there's no bad way to compost - whatever you do, however little you do, it still works - pretty pleased with our formula now, which involves very little work.

[rapidprototyping]

leaf mold makes the best compost then comes pine duff nationwide think of the tons of leaves landfilled this is wrong as thats our replacent nutrients we are depleating soils by throwing out these valuable resources. composting takes place in the rivers and creeks through leaf destruction on its way to the ocean this helps filter and clean our water. while vegitable composting contributes to the compost diversity it may possibly add some residual pestacides or herbacides minute amounts but still adding things at molicular level should be of concern. Our water may contain thousand of bad substances the water treatment plant can not extract coming or going. I use wire and form a hoop of wire fencing material easy to relocate and in different sizes. Alarge yard cart help get thing from yard to bins and turning is recomended but i don't turm mine at all just sheet mulch it on the gound but not around the base of the same trees that shed the leafs just incase it has problems things break down over time black gold it's called

[AmyLuthien]

Yes, they did stop using arsenic in 2003 for residential use.  Arsenic is however still used today for commercial applications.  Wood intended for residential applications typically is treated with amine copper quat  and copper azone.  Whether or not those are safe in the long run really hasn't been determined.  Personally, I would not take the chance. 

[AmyLuthien]

True, unless you used a piece you've had lying around for a few years . . . ;)

Cedar is the definitely the way to go though, or redwood.

[AmyLuthien]

Handy!  But I'd point out that you probably don't want to use pressure treated lumber for your compost bin if you're going to spread the resulting compost on any vegetables/herbs you intend to eat.  Pressure treated lumber is impregnated with chromium, copper sulfate and arsenic and highly toxic . . .

[CostaTim]

Hi! I had the old pallets. He made a box of them. Obil PVC. I think that would be better if the new facilities will fall down. When filling the box, the board vytaskivayutsya. And fresh waste come in contact with the worms and microbes old compost.

[gingertech]

nice idea never really thought of the dual bin idea, i have a plastic store bought composter alas this would have been much cheaper lol, quick ideal; perhaps a lid to keep the heat in and stop things growing in it?

[gannonbill]

could you add some old windows that you just sit on top, that would keep heat in and the sunlight would still be able to get into it

[camp6ell]

yeah, a lid would probably help speed things up. it's working out ok just now without it though - probably takes 6-9 months to decompose, which is totally fine as we only need a year for it to happen (when we will next use compost in garden). funny you say to stop things growing in it... we got a great big squash plant growing out of it this year from compost seeds, so just left it and have had quite a few delicata squash.

[gingertech]

lol how odd:)

[6monkeyRS]

How long does the leave or material take to decompose? I have tried this for a year, but it took a long time. We have several tree around my house, and get a lot of leave in Fall-Winter, and would like to make something like this, for Spring Garden. This is a good idea, I can build this around the area that I want to set my garden. Then just move it around from year to year.

[camp6ell]

for us, in the north east of the us, we give it a year. we could probably get away with less as long as everything was chopped up pretty small before it went in there (and conversely, corn cobs or whole loaves of bread might take more than a year if left unattended). basically, we empty one side and use it in the garden in the late spring when we are planting, and then start to fill the empty side, leaving the other side to decompose for the next year. so, in fact, the compost ends up being between 1 and 2 years old. it's frozen for a good 4 months of the year, so we don't touch it, but in the other 8 months, it gets turned 2-3 times when we feel like it. leaves tend to take a little longer for us to decompose, so we try to not use too many. in the pictures, you can see them on top and bottom of the piles for insulation.