ANAEROBIC DIGESTION

The term AD covers several different processes; the most common are shown below and can be classified according to whether they are:

• Mesophilic, 25-45°C; or thermophilic, 50-60°C
• Wet, 5-15% dry matter in the digester; or dry, over 15% dry matter in the digester
• Continuous flow or batch
• Single, double or multiple digesters
• Vertical tank or horizontal plug flow

Mesophilic systems operate at 25-45°C while thermophilic systems operate at 50-60°C or above. Thermophilic systems have a faster throughput with more rapid biogas production per unit of feedstock and digester capacity and there is greater pathogen kill. However, the capital costs of thermophilic systems are far greater as more energy is needed to heat them and they generally require more management.

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 greater gas production per unit feedstock. However, wet AD has a lower set-up capital cost.

Most digesters are continuous flow as opening the digester and restarting the system from cold every few weeks is a management challenge. They generally provide more biogas per unit feedstock and operating costs are lower. Some dry systems are batch flow and, to overcome peaks and troughs in gas production, multiple batch digesters with staggered changeover times are specified.

AD occurs in several stages; some systems have multiple digesters to ensure each stage is as efficient as possible. Multiple digesters can provide more biogas per unit feedstock but at higher capital and operating costs with a greater management requirement. Most digesters are single or double digesters.

Vertical tanks simply take feedstock through a pipe on one side of the tank whilst digestate flows out through a pipe on the other side. 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 too early nor remain within for an uneconomically long period.

The optimum system will be determined by what feedstocks are available, the output required, ie, energy production or waste mitigation, space and infrastructure.

A typical AD project can be divided into three planning phases; design, build and operation. The first stage is to identify a suitable site and determine what the feedstock will be.

Upon choosing a site the following needs to be evaluated:

• Site layout and space to ensure sufficient space and access for construction, operation and storage
• Feedstock supplies and maintenance access
• Visual impact in terms of topography and natural screening
• Regulatory and environmental requirements regarding buildings and water course proximity

Planning permission is necessary for most anaerobic digestion installations. Small scale digesters using only on-farm waste may be passed as ‘Permitted development’, but local authorities must be consulted in the early stages of planning for confirmation.

Any installation accepting third party waste will need full planning permission.