Composting is a biological process through which organic matter is converted into stable, humus-like compounds. The evaporation of water and the decomposition of organic matter lead to an increase in dry-matter content and a decrease in volume. The process is also known as ‘biothermal drying.
At Twence, the entire process of composting organic waste from kitchens and gardens takes place in sealed spaces, using the so-called ‘closed method’.
Upon arrival, the ‘fresh’ material is first made damp and sifted. Bulkier elements are reduced in size. The material is then put up in composting tunnels for pre-composting at 48°C. Maintaining the right temperature, the right humidity and a sufficient supply of oxygen ensures that the bacteria, moulds and other microorganisms will be able to do their work. In this pre-composting phase, the temperature is temporarily raised to 600 Celsius to kill off any harmful germs and render seeds from weeds ineffective. After ten days, the shovel operators empty the tunnels and bring the material into a large hall for the post-composting phase. Fresh air is constantly circulated from below the compost to allow the compost ripen further. This phase takes another ten days. Throughout the entire process, biofilters containing bacteria and moulds serve to purify the air that comes out of the composting tunnel.
Finally, the material is sifted and impurities such as plastic, iron, stone and glass are removed. All that remains now is compost, an excellent soil-enhancer for agriculture and horticulture.
In contrast to the composting of organic waste from kitchens and gardens, ‘green’ composting takes place in the open air. The material lies on aerated floors and is regularly turned and moistened. In this way, it takes two to three months to convert landscape waste from parks and pruning into compost.
After the composting process, any impurities this ‘green’ waste material might contain (e.g. stones, glass, iron and plastics) are removed and the material is sifted. This results in two end products:
green compost (consisting of elements of 0-10 mm in size) and coarser wood components (larger than 10 mm in size). The latter material can be used as a secondary fuel. Indeed, it will be used as such in our future Biomass Power Plant. By means of this process, Twence is able to treat 30,000 tons of green waste each year.