Anaerobic digestion is the breakdown of material in the absence of oxygen. It is
legally permitted providing that the enclosed reactor is maintained at 700C for 1 hour
with a maximum particle size of 12 mm across one dimension. Anaerobic digestion
facilities must have a pasteurisation phase to ensure that the time temperature
requirements are met. There are also wider plant requirements, including obtaining
the necessary planning permissions from the local council and waste management
licenses from the regulator.
Facilities are typically operated at mesophilic (25-45oC) or thermophilic (55-70oC)
temperatures. For mesophilic digestion, biogas production increases up to 40oC but
little above this temperature. Thermophilic digestion enhances performance but
requires better control of conditions and more expensive equipment. A third option,
often called low temperature digestion or psychrophilic (5-150C) digestion, enables
simple, low cost, low performance systems to be used.
The quantity of biogas produced from fish waste varies 50-200 m3/tonne. The
composition of the biogas depends on the feedstock but typically contains 55 to 75%
methane, and the remainder is mostly carbon dioxide.
The type of technology chosen depends primarily on whether the slurry has a high or
low solids concentration. The majority of systems in the UK have been designed to
treat low solids wastes like animal manures. The load rate and biodegradability will
also influence the type of system chosen.
The anaerobic digester can produce biogas that is consumed on site for heating and
energy applications. For export and sale of energy the gas can be converted to
electricity by burning it in an engine driven combined heat and power unit. The
electricity can be used on site with the surplus exported to generate Renewable
Obligation Certificates (ROC's) and the heat from the engine recovered for heating
Economics and scale
The economics of digestion are dependen