Organic material like food waste and kitchen and garden waste (KGW) is processed in various ways to create products or raw materials. This is done by composting or fermenting. Composting is an aerobic process with active bacteria that need oxygen to live. They decompose organic material and convert it to an earth-like mass, i.e. compost. If the decomposition of organic material occurs under anoxic conditions, it is called an anaerobic process. Bacteria are then active that convert the organic material into (for example) methane, water, carbon dioxide and nitrogen. Methane is particularly attractive, because it is also the principal element of natural gas and biogas. Methane is excellently usable as a fuel for the production of sustainable energy. The sieved material released in the CADoS process is an excellent raw material for fermentation.
Bio-fermentation involves filling steel reactor tanks of a few thousand cubic metres with organic material. By keeping the material in motion and ensuring ideal conditions like moisture content and temperature, the bacteria convert the material into products including methane. The released biogas is captured. Several fermentation techniques exist. Some systems use a lot of water and keep the material moving by means of agitators. A dry digester uses less water and pumps keep the fermenting material in motion. A third system is a horizontal digester, whereby a horizontally rotating agitator keeps the material moving slowly. All systems produce biogas that is usable as an energy source.
The biogas obtained from fermentation contains between 55 and 60% methane; the rest is mainly carbon dioxide. The gas is usable to good effect in special motors that provide electricity or heat. Upgrading biogas to green gas optimises the yield because no energy is lost in the form of heat, as in the case of electricity production. What’s more, every cubic metre of green gas supplied to the gas grid saves 1.8 kg of CO2.
Bioplastics from fermentation
Attero has a special two-stage fermentation installation at Venlo. Kitchen and garden waste is sprinkled with water in composting tunnels. This starts a natural process in the material, i.e. hydrolysis. Bacteria present in the waste convert organic compounds – like fats, proteins and carbohydrates – into fatty acids, amino acids and alcohol. These are the building blocks of organic compounds. One of their properties is that they are soluble in water. The tunnel is sealed off from the outside air. This enables staff to influence the climate in the tunnel in a way that optimises conditions for the bacteria and speeds up the process. These volatile fatty acids are the source of nutrition for the bacteria for the production of bioplastics. Attero thus has a unique link between KGW and bioplastics.
Production of bio-based raw materials for industry (including the chemical industry) is important with a view to sustaining the Dutch economy. The CADoS project is facilitating this sustainability process.