Digestion is the reduction of solid organic materials through decomposition by microbes, which results in the production of liquids and gases. The digestion process may be aerobic or anaerobic – depending on whether air (containing oxygen) is present in the process.
These processes are often conducted ‘in-vessel’, that is enclosed in a container or vessel. Although a simple process, in-vessel systems generally require high capital outlay for complex equipment. They have the advantage however, of a high level of control over the organic material. Other advantages include a reasonably small operational footprint and short processing time.
The in-vessel design makes exposure of the feedstock material to high temperatures for the requisite time easier to achieve, reducing the risk of producing an unacceptable product. High temperatures also kill weeds, seeds and plant pathogens, making this kind of system suitable for use in environmentally sensitive areas.
Anaerobic digestion is the decomposition of organic matter by microbes in the absence of oxygen. This process results in a solid by-product (digestate) and a gas (biogas). Historically, anaerobic digestion has been used extensively in the wastewater treatment industry for stabilisation of sewage sludge. It has more recently been adapted and applied to process the organic fraction of municipal and commercial wastes.
Anaerobic systems are generally more complex and more expensive than aerobic systems. They often require more space for larger digester vessels and to aerobically-mature processed material. Small plants require several thousand square metres of land.
As with the aerobic process, pre-sorting is also required to remove contaminants. Some technologies use water for separating contaminants. Heavy contaminants sink, light contaminants float and the organics are suspended in the water.
Anaerobic digestion is commonly a component of MBT technologies. These technologies generally incorporate a mechanical sorting process and a biological digestion process such as that shown in Figure 5.
Decontaminated feedstock is added to the digestion vessels with water. The anaerobic digestion process may be either ‘wet’ or ‘dry’, depending on the proportion of solids to liquids in the digester (reactor). Sometimes bacteria are also added to start the process. The vessel is sealed and the process allowed to take place for between 10 and 20 days. Once processing is complete, the solid digestate product is removed from the vessel and dewatered to produce solid and liquid fertiliser. Several weeks of additional aerobic processing outside the vessel may also be required. Processing is mostly done by batch although there are some continuous flow systems.
In an anaerobic process the bacteria absorb heat so less heat is generated. Heat may even need to be applied to ensure digestion. The process temperature can vary, and is generally controlled in order to promote the growth of a specific type of microbe population. Mesophilic anaerobes proliferate at temperatures in the order of 35°C and thermophilic anaerobes are dominant at temperatures around 55°C.
The process produces ‘biogas’ and a liquid fertiliser (digestate). The biogas produced in the anaerobic digestion process comprises mostly methane and carbon dioxide with some impurities such as moisture, H2S, soloxane and particulate matter. Methane and carbon dioxide are both greenhouse gasses. This biogas can be burned in an internal combustion engine to generate electricity. This can be done on-site in ‘co-generation’ plants where the gas is used as a fuel for internal combustion engines that drive electrical generators. Some of the electricity generated can be used to run the facility and the remainder sold into the grid. Heat from the engines can also be used in drying processes. The biogas may need to be purified to certain extents depending on its use.
Biogas is a clean burning fuel, producing only carbon dioxide as a by-product. However, it has a lower calorific value compared to other fuels as shown in Table 15 below.