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Should the first bin (ie the one you put the fresh stuff in) of a 3 bin be bigger than the second? And similarly should the second bin be bigger than the third, due to the fact that volume will decrease as composting happens?

Any picture I see of compost bin systems have all three the same size - I'm wondering if this is just convenience of building rather than optimal size for when used.

As background for where I'm coming from here - I've got a L shaped 2.6m x 2m area to put a compost heap and a simple "row" of three bins would be a touch small, around 80cm per bin. I've the choice of making bin one bigger, or putting 3 bigger bins in an L shape, which may be better for compost, but will surely be a pain for moving compost in or out of the second "corner bin", even if I build with removable slats between each bin. My only other thought is to make the bins deeper/taller to make up for the lack of ideal width.

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Do you mean a different bin for each STAGE in the composting process? That means you need to shovel material from one bin to the other. The simplest way would be to fill one bin and when it's filled don't add more material. You simply move to the next one, and so on. When you empty the first bin, the second bin becomes first, the third becomes second and the once-first becomes third. In such a case, the best to do is to build three bins of the same size. In each stage you fill a new bin and you let the previous one compost without adding anything to it. There are bins which you can turn so that the compost is stirred. Just search for "compost tumbler". Here is a home-made one on Pinterest. You can also buy ones which work the same way.

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The 3 bin setup is a common one and you can definitely move the contents through the bins as you describe. It's true that the volume of the first pile will shrink before it gets moved to the second spot, so you could definitely construct them as you describe.

While you don't need to move them from one bin to the next, doing so helps incorporate materials from the edges to the middle and insure that the pile is thoroughly mixed. It seems like a fine solution to the environment you're in.

In this scenario you'd use the piles like this:

  1. Actively add new material to the first pile where it will start the decompose.
  2. Once you filled the first area, move it to the second bin. Once the second bin is fully composted and "done" you move it to the 3rd bin.
  3. You can pull compost out of the 3rd bin as you need it and put it into your garden. This is where it can age and you should keep it as moist as the other piles.

The 3rd bin can definitely be a lot smaller. I wouldn't want to make the 2nd bin too much smaller since it can be hard for the pile to build up heat if it's not big enough and the heat is key to fully composting.

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