Friday, 9 December 2011

A composition

Composting seems to be a good idea in general. My dad was always a keen composter and we grew up with a couple of the classic square frame designs made from interlocking planks that would grow up as we emptied lawn mowings and things into it, and the other one would shrink as compost was used around the garden. We were all trained, so that if ever we ate melon, we had to cut up the skins into small peices so they would break down more quickly. We took turns taking out the compost bin from the kitchen.  

At one point I was going to do an A-level project harnessing the energy of compost to make hot water. I think it was going to use a heat pump, although I ended up dropping that A-level in favour of more academic ones so I could get into a university engineering degree. A tragedy.

If you're living in an apartment it may not be very practical to compost your kitchen waste, but with a garden, producing it's own variety of foliage and requiring fertilization, composting seems like a good idea in our specific case too. Although over seven different kinds of rubbish are collected here, and burnable rubbish is used at an efficient incinerator that heats a swimming pool and spa, composting seems more sensible than putting garden waste into plastic bags to be driven away by a truck.

The garden behind the new house was getting a bit overgrown with weeds, so it seemed like a good idea to start composting now, so that when we start our garden in earnest next spring we'll have something to help our plants grow.

The first composter I made was an old bin with a broken bottom. I sawed the bottom off, and drilled holes every ten centimetres or so up the sides. This filled up very quickly and the pile of weeds next to it began to grow, so a larger scale solution was necessary.

I decided on a hexagonal version of the classic interlocking plank design. I'm sure there were many reasons for this, but I'm not entirely sure of a good reason. Perhaps you can think of one.

The hexagon has a slightly larger ratio of area to circumference than a square. This means that for a given amount of materials, you get a larger volume of compost. Nature's solution--the circle--has the highest possible ratio, which is why so many things are cylindrical or spherical. As well as being the most efficient use of materials, the corresponding lower surface area to volume means that round things end up with lower heat loss. This may be a factor in compost heap design as the heat they generate speeds up the work of the bacteria.

The difference between squares and hexagons is pretty tiny, barely significant in fact, at less than 10%. 

Ignoring the widths of the walls, the ratio of unit area to circumference for a square is 1/4, or 0.25; 

for a hexagon it is 1/2√(2√3), around 0.27;

for a circle it is  1/2√pi, around 0.28.

And all the materials I used were offcuts and discarded packaging materials from the house, so didn't cost me anything. The only materials I did buy, in fact, were screws, and the hexagon uses more of these than the square. Also the hexagon involves more work as each layer has six, rather than four components, each needing work on each end. The work itself is also more complicated for the hexagon, involving angles of 60 degrees rather than right angles.

So the hexagon, while theoretical a more efficient shape in terms of material use, in fact cost more in terms of materials, and took more work of a more complicated nature to make. 

But time is free if you're having fun. 

The hexagon may be a little more stable than the square, and there's something aesthetically pleasing about the hexagonal shape. 

You can see how each part was constructed below. One of the major revelations for me was the power of power tools. I started off with a hand saw, and a hammer and nails. The carpenter let me borrow his power saw at one point, which he set to cut fixed lengths with ends at 60 degrees. This was quicker and easier. In view of the strange angle at the ends, the nails were not working so well, and on the carpenter's advice I started using screws. I also used my electric drill with a screwdriver bit. I've never done any serious amounts of woodwork, and only ever used a regular screwdriver, but the difference in speed was amazing.