Composting (Aerobic Digestion)
Aerobic treatment more commonly known as composting can be defined as:
‘Composting is the biological decomposition and stabilisation of organic substrate under conditions that are predominantly aerobic and that allow the development of thermophilic temperatures as a result of biologically produced heat.’
The composting process works when sufficient quantities of carbon, oxygen, nitrogen and water are present. Microbial communities utilise the carbon and nitrogen as food and generate carbon dioxide, water and heat as a consequence. The heat released during the composting process kills off pathogens such as Salmonella, weed seeds and bacteria. When the sources of food have been used by the microbial communities the composting process is complete. The product of this process is a stabilised, organic matter rich material containing beneficial nutrients including nitrogen, phosphorus and potassium. The compost is manufactured to the national compost standard PAS 100 and the Compost Quality Protocol.
The Science of Composting
Most organic materials can be represented as a mixture of three basic groups of chemicals:
- Lipids and carbohydrates
- Proteins and amino acids
- Ash, lignin and cellulose
In the presence of oxygen and water a wide range of bacteria, actinomycetes and fungi act upon the above chemicals and so drive the composting process.
The lipids and carbohydrates are broken down via a number of intermediates into carbon dioxide and water. At the same time energy is released in the form of heat. It is this biologically produced heat, which raises the temperature of the compost and kills off unwanted pathogens and weed seeds. The proteins and amino acids are degraded into a number of chemicals including carbon dioxide and water. The more resistant components of the organic material – ash, lignin and cellulose are not degraded significantly and add to the final bulk of the compost product.
During the different stages of degradation within the composting process various microbial communities increase and take prominence. The different communities all contribute to the composting process enabling greater degradation and the existence of new communities. Those communities that die become part of the organic mass and are food for the new communities. The composting process is most effective when the oxygen, moisture, carbon and nitrogen are in balance and in the correct ratios. When the ratios and balances are significantly out of balance the composting process becomes inhibited and slows down and potentially reduces the quality of the end product.
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