4.1 What is (co-) composting and how does it work?

In the composting process, organic materials, including sewage sludge or solid waste, undergo controlled aerobic decomposition. This carefully managed procedure yields a stable, humus-like, and odour-free end product known as compost.

The term “co-composting” specifically refers to the practice of combining multiple organic materials, such as sewage sludge and green waste, to enhance the composting process and create a more balanced and beneficial end product.

Overall, composting is a natural and efficient way to recycle organic waste, reduce landfill usage, and promote sustainable soil management practices as compost enhances soil fertility and structure.

The composting process begins with the introduction of organic materials into a composting system. Aerobic microorganisms, including bacteria, fungi, and other microbes, play a crucial role. These microorganisms require oxygen to thrive. As they consume the organic matter, they break it down into simpler compounds. The microbial activity generates heat as a by-product. This increase in temperature is a significant aspect of the composting process. The heat helps create a thermophilic environment, promoting efficient decomposition and pathogen elimination. To ensure effective pathogen deactivation and thorough decomposition, it is essential to maintain the compost’s temperature within the optimal thermophilic range of 55 – 65°C for a specific duration, typically 3 to 5 days.

The composting process results in a notable reduction in volume, approximately 50% or more. This reduction is attributed to the breakdown of organic materials and the release of gases such as carbon dioxide.

Achieving the required temperature conditions during composting involves maintaining the water content and carbon to nitrogen (C:N) ratio of the composting material within specific ranges. Also, there must be enough free air space to provide the oxygen needed for aerobic microbial activity. Faecal sludge is composted with a suitable bulking agent to achieve these conditions. Materials commonly used as bulking agents include organic household or market waste, agricultural waste, wood chips, and sawdust. The recommended ratio of bulking agent to faecal sludge is typically 2–5 times, depending on the C:N ratio and water content of the sludge and bulking agent. The final product is a pathogen-free, dark, humus-like material that can be added to soil to increase its organic content and improve water retention.

For further information, please click on the Materials tab at the top of the page.