The term AD covers several different processes. The most common for the UK are shown below in bold. AD systems can be classified according to whether they are:
Mesophilic or Thermophilic
Mesophilic systems operate at 25-45°C and thermophilic systems operate at 50-60°C or above. Thermophilic systems have a faster through put with faster biogas production per unit of feedstock and m³ digester and there is greater pathogen kill. However, the capital costs of thermophilic systems are far higher, more energy is needed to heat them and they generally require more management.
Wet or Dry
The difference between what is considered a wet process and a dry process is quite small. Effectively, in wet AD the feedstock is pumped and stirred and in dry AD it can be stacked. Dry AD tends to be cheaper to run as there is less water to heat and there is more gas production per unit feedstock. However, wet AD has a lower set-up capital cost.
Continuous Flow or Batch Flow
Most digesters are continuous flow as opening the digester and restarting the system from cold every few weeks is a management challenge. They also generally give more biogas per unit feedstock and their operating costs are lower. Some dry systems are batch flow, however. To overcome peaks and troughs in gas production there is usually multiple batch digesters with staggered changeover times.
Single or Multiple Digesters
As shown in the AD in more detail page, AD occurs in several stages. Some systems have multiple digesters to ensure each stage is as efficient as possible. Multiple digesters can give you more biogas per unit feedstock but at a higher capital cost, higher operating cost and greater management requirement. Most digesters in the UK are single or double digesters.
Vertical or Horizontal Plug Flow
Vertical tanks simply take feedstock in a pipe on one side whilst digestate overflows through a pipe on the other. In horizontal plug-flow systems a more solid feedstock is used as a 'plug' that flows through a horizontal digester at the rate it is fed in. Vertical tanks are simple and cheaper to operate, but the feedstock may not reside in the digester for the optimum period of time. Horizontal tanks are more expensive to build and operate, but the feedstock will neither leave the digester to early nor stay in it for an uneconomically long period.
The best system for you will be determined by what feedstocks are available, what output you want to maximise (e.g. is the goal energy production or waste mitigation?), space and infrastructure.
Read more about AD technology in this brochure, produced as part of the biogas regions project and this report by the NNFCC and Andersons Centre.
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