Composting
Composting is the human technique of converting organic waste materials into soil improvers. It repeats, in a managed way, the natural process of biomass breakdown that sustains all life on Earth.
The key difference between natural breakdown and composting is that the process is deliberately optimised to maximise quality while minimising time. In all other respects, it is a natural process honed over billions of years.
A compost ecosystem
A compost pile houses a web of life in miniature. At the bottom are the bacteria, fungi, actinomycetes and protozoa, which chemically break down the waste material, and the larger organisms, such as worms, mites, snails, beetles, centipedes and millipedes, which physically break it down. Other organisms eat these organisms or their waste products, and are themselves eaten by higher-order consumers.
In the process of breakdown, energy is released, primarily as heat, and the overall complexity of the contents is reduced.
Aerobic versus anaerobic
Composting can take two forms: aerobic and anaerobic. Aerobic composting takes place in the presence of oxygen. This process is faster, hotter and nicer smelling. Anaerobic composting takes place in the absence of oxygen and is slower, cooler and foul-smelling. Ultimately, however, they both produce a useful product.
Aerobic composting is preferable, for these obvious reasons, but must be managed, or the process can quickly become anaerobic. Aerobic organisms, including microorganisms and worms, require oxygen or they will die. A loose physical structure to a compost pile will maintain a route for the entry of outside air, but the composting process destroys this structure. So it is necessary for the composter to work the pile so as to maintain the structure, or the pile will compress and thus exclude air.
Various solutions are available. The simplest is to turn a pile regularly. This also has the benefit of allowing the composter to keep the pile’s temperature in the optimum range. Compost windrows – very long stacks up to several metres high and tens of meters long – are turned every few weeks with compost turners, machines of various sizes and designs that invert the pile’s contents as they pass above it.
For smallish quantities, it can be effective to keep the compost in a tumbler – a perforated drum or barrel – that is turned mechanically every day. Or air can be blown into intermediate quantities.
Turning offers the advantage over aerating of ensuring that materials on the outside, where the temperature is cooler and the material can dry out, eventually end up on the inside.
Hot work
Because the aerobic breakdown of organic matter releases heat, compost piles heat up. The heat has both beneficial and harmful effects. On the plus side, it speeds up the chemical reactions, so the compost breaks down faster. It can also pasteurise the compost, destroying most pathogens. Finally, because hot air rises, it draws in fresh air from below. On the minus side, a compost pile can overheat, killing the composting organisms and thus making room for recolonisation by pathogens, such as Salmonella species, and necessitating reseeding of the compost by the composter. In addition, it can catch fire. So in a commercial setting, it is vital to keep an eye on temperature: too hot and the compost can cook; too cool and the process slows down.
The stages of composting
An aerobic compost pile goes through three stages of composting. The first is the mesophilic stage, when mesophiles (literally, middle lovers, which prefer temperatures of 10–45 °C) start the process. When the temperature reaches about 45 °C, thermophiles (heat lovers, 45–70 °C) take over. Depending on the size of the pile and thus the quantity of organic material present, this second, thermophilic, stage can last several weeks. During this phase, toxic chemicals are broken down, weed seeds are killed and pathogens are destroyed.
When the supply of easily decomposed materials runs out, the temperature drops back to about 38 °C, mesophiles return, and the third phase, the curing phase, begins. After 1 to 4 months, the temperature has dropped back to ambient and the compost is ready for use.
Factors that affect composting
In the next article we’ll look at the various factors that control the composting process, including temperature, composition, the carbon-to-nitrogen ratio, nutrients and moisture content.
Further reading
DECC. 2008. Organics recycling and use for compost purchasers. NSW DECC.
Recycled Organics Unit, UNSW.
Recycling and Waste Management, NSW DPI.
Young CC, Rekha PD, Arun AB. 2005. What happens during composting? Food & Fertilizer Technology Center, Taipei, Taiwan.