The Composting Association, UK, in its ‘Large-Scale Composting – a practical manual for the UK’, defines composting as: “The controlled biological decomposition and stabilisation of organic substrates, under conditions that are primarily aerobic and that allow the development of thermophilic temperatures as a result of biologically produced heat. It results in a final product that has been sanitised and stabilised, is high in humic substances and can be beneficially applied to land.”

Or, less formally - composting is a natural process that turns organic waste into a clean and useful product. 

This process can be effectively accomplished in the backyard by occasional application of a fork. Home composting can effectively contribute to waste reduction but for collected waste in commercial or municipal quantities, a rather more engineered solution is required.

WHAT IS COMPOSTING?

Composting is an application of the natural processes of decay. The distinction being that sufficient organic material is gathered so enough energy (food) is available to support a large and active microbial population, which, through respiration, generates sufficient heat to accelerate the process.  This also achieves a selective sterilisation of potential pathogens and seeds that is variously known as sanitisation, pasteurisation or disinfection.

Composting in the UK is controlled by a number of regulations and legislation drivers.  The main regulatory instrument is the Animal By-Product Regulation which requires all wastes that have come in contact with or contain cooked meat to be composted in specialised “in-vessel” or “enclosed” composting systems or biogas plants.

The composting process itself is driven by a succession of microbial processes. Broadly the process may be considered as four phases: Heating, thermophilic decomposition, mesophilic decomposition and curing.

HEATING

In the heating phase microbial respiration, in the presence of oxygen, causes the temperature in the compost mass to rise. As this occurs there is a change in the microbial population from those that thrive at ambient temperatures (mesophiles) to those that prefer elevated temperatures up to around 55-65°C (thermophiles).

THERMOPHILIC DECOMPOSITION

While sufficient readily metabolisable material (energy) remains in the waste, the action of thethermophiles maintains the high temperature. From a practical standpoint this ensures that pathogens and weed seeds are destroyed and results in rapid waste treatment.  It is essential to strike a balance between allowing sufficient heat to accumulate to provide optimal conditions for compost microbes and the elimination of seeds and pathogens and removing sufficient heat to prevent overheating.

MESOPHILIC DECOMPOSITION

As the readily available sources of energy (nutrients) declines the compost cools (as the rate of microbial respiration slows) and a new population of mesophilic microbes, preferring warm temperatures, becomes established. The compost remains warm and active, and considerable degradation continues. Indeed, the mesophilic phase of composting is likely to be the longest and support the greatest numbers and diversity of microbes.

CURING (OR MATURATION)

Eventually, all the readily degradable material has been utilised, the temperature returns to ambient, and active composting has ceased. At this stage the compost is usually referred to as ‘immature’. This is because it contains relatively high levels of ammonia and other compounds that are toxic to plants. Microbes able to oxidise ammonia to nitrate are not able to survive at elevated temperature. During curing these microbes re-colonise and phytotoxic compounds are dissipated and stabilised.  The requirement for and length of CURING period will depend entirely on the needs of the end market.

OPTIMISATION

Good engineering can optimise the composting process, but it must be recognised that the process is driven by the biology of microbial degradation. With a well-engineered system it is possible to generate “immature” compost in around 20 days. From a processing point of view it may be possible to reduce the time within the composting unit below this, but more active curing mechanisms will be required.

Very short residence times of a few days are sometimes used, but these achieve only sanitisation through the thermophilic composting phase, and the resulting material remains highly biologically active and in need of considerable amounts of further composting.