AD can be used to treat both the biodegradable fraction of mixed household MSW, typically as part of a mechanical biological treatment process (MBT), and source- segregated household and commercial organic waste which may contain animal by- product (ABP) materials such as food waste. The process is operated under controlled conditions with the anaerobic digestion taking place within sealed tanks. This is undertaken on a scale ranging from small farm-based AD plants to large industrial AD plants. The range of technology also varies from simple systems to very sophisticated and highly mechanised and automated systems.
The process has not always been deployed successfully for use in the treatment of ‘black bag’ mixed household MSW; emerging techniques for anaerobic digestion of the organic fraction of MSW are considered in Section 4.7.5.
Organic waste will be received at the site, inspected for compliance against waste codes and then treated to remove packaging and/or prepare it for the digestion process. Successful pretreatment systems exist for household biowaste and packaged food waste from stores. For wet AD processes, water will be added to create a slurry. The feedstock is anaerobically digested in a tank over a period of time, generating biogas. The biogas is captured and used to recover renewable electricity or heat. Following the completion of the digestion process, the digestate may be stored to allow stabilisation before being used either in liquid or dewatered form as a fertiliser or soil improver on agricultural land or for land restoration. The digestate is mechanically screened to the required size grade for final use and to remove any residual physical contamination such as plastic which was not removed at the pretreatment stage. For a conventional AD plant, the electrical output based on the energy content of the organic feedstock is 18%98.
A common variation on wet AD processes are dry AD processes. Dry AD processes are operated under controlled conditions with the anaerobic digestion being undertaken either in a ‘tunnel’ or ‘box’. Due to the more capital-equipment-intensive nature of the dry AD process, it is typically undertaken at scales in excess of 25,000 tonnes per year. The process normally uses specialised machinery including shredders, and screens make the process more efficient, introduce greater process control and reduce costs through greater mechanisation. Waste is commonly fed into the digestion vessels using walking floors. The biogas and digestate produced by dry AD processes are used in the same way as for wet AD systems.
4.7.2 Energy efficiency
The energy output from an anaerobic digestion plant depends to a great extent on the biomethane potential of the feedstock. High-energy feedstocks such as glucose or kitchen waste will have much higher energy yields than feedstocks such as grass cuttings. Those organic feedstocks with the highest biomethane potential contain 10 times more energy than the lowest biomethane potential feedstocks, such as sewage sludge.
98 ISWA CE Report 5, p. 25.
In terms of converting the available feedstock input energy into heat and power, the following characteristics distinguish a high-efficiency plant99:
The overall net annual average energy efficiency of a mesophilic AD plant which operates at around 40°C will be better than that of a thermophilic AD plant which operates at higher temperatures of around 70°C, even though more biogas will be produced at higher temperatures.
The highest waste energy utilisation can usually be obtained where the heat recovered by the combustion of the biogas can be supplied continuously to a heat consumer in combination with electricity generation. However, the adoption of this output is very dependent on plant location and the availability of a long-term user for the supplied energy.
Where co-generation is not practical, high energy efficiency can be obtained by upgrading the biogas produced to biomethane and utilising this for transport fuels or by injecting the biomethane directly into the grid.
From an operational point of view, the sooner that biowaste can be input into an AD plant, the better the energy yield will be as fresh matter has a higher biomethane potential.
Basic anaerobic digestion leaves much of the energy content of the feedstock untapped. Advanced AD systems (which use a variety of techniques as described below) to extract more biomethane and residual energy from the waste will offer higher overall energy efficiency.
Where AD digestate can be spread to land in lieu of manufactured fertilisers and the organic waste nutrient content is recycled, significant GHG savings can be made. Fertilisers derived from fossil fuel sources are energy-intensive in their manufacture and, when applied to land, emit nitrous oxide which as a greenhouse gas is almost 300 times more potent than CO2 in its warming potential.
The range of energy efficiency (based on the organic waste energy input) in AD plants is shown below in Table 2.57100.
Table 2.57:Net annual average efficiency of AD processes
Net annual average efficiency (%)
Electricity only CHP mode (80% heat load factor) Gas network / liquefaction to biofuel
18 – 23101 36 > 40
Energy efficiency may be further increased by linking AD with other processes as described under emerging AD and biological techniques.
99EBA Interview, May 2016. 100ISWA CE Report 5, 2015.
4.7.3 Anaerobic digestion - Proven improvement techniques