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					Anaerobic Digestion
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
the digester.

Economics and scale
The economics of digestion are dependent on the returns from treatment such a gate
fee for commercial treatment of waste, sale of the digestate products and the value of
energy produced.

A facility designed to treat 10-20,000 tonnes per annum may have an average capital
cost of approximately £3.25-4.25 million, with operational costs of approximately
£100,000 per annum.

Typically in a 600m3 centralised digester the cost of treatment is £38/tonne. In
comparison to aerobic treatment, anaerobic digestion has the advantage of
generating energy for the heat treatment sanitisation of the waste to meet ABPR
requirements.

The plant must be designed to take advantage of all revenue streams - gates fees,
power, heat, electricity and digestate/liquor revenues (even though no net revenue
should be expected from the digestate/liquor it avoids costs). Extracting electrical
power requires a large step up in investment, which goes against small-scale
installations.




28/07/2005
Theoretically, anaerobic digestion can be effective at all scales but large scale can
take advantage of all revenue streams making the technology best suited on a large
or regional basis.

SWOT analysis

Strengths                                      Weaknesses
• Reduced greenhouse gas emissions             • fish protein produces high concentrations
  compared to other waste management             of ammonia, volatile fatty acids, amino
  options                                        acids and hydrogen sulphide. Ammonia is
• Suitable for wet waste                         highly toxic to the bacteria which would
• Produces a sanitised product suitable for      digest the fish offal
  composting/landspreading                     • There is a requirement for liquid storage
• enclosed process so emissions are              prior to and post digestion
  avoided                                      • High capital cost of treatment vessels
• There is a high degree of process control    • High level of management skills
• Indigestible solids can be removed through   • Limited practical information on
  settlement                                     performance with seafood wastes
• Compliance with ABPR                         • High density shell fish waste may give
• Generates energy so reduces treatment          build up of solid materials at the base of
  costs                                          the vessel which are difficult to remove
• Excess energy is eligible for Renewable      • High transport cost for large volumes of
  Obligation Certificates (ROC’s)                high moisture waste
• Wastes that have been digested do not        • Must have access to land for recycling
  emit as much odour                           • Effect of salt content on bacteria is
• The loss of ammonia to the atmosphere on       unknown.
  land spreading is reduced                    • Storage of digestate required prior to land
• Nutrients in the digestate are more            application
  available to plants.                         • Revenue from the digestate unlikely
                                               • Some gaseous emissions are toxic so
                                                 must be controlled
                                               • Currently only 1 commercial system
                                                 accepting seafood waste in the UK (not
                                                 shell)
Opportunities                                  Threats
• ABPR compliance                              • Restriction of access to land in the event
• Generation of energy for use on site             of a major animal disease outbreak for
• Energy use                                       spreading of the digestate
• Rising energy costs                          • Lower cost alternatives are available
• Renewable energy obligation for large        • High level of management is needed
  scale operations                             • End-product standards required
• Exemption from the climate change levy       • Increasing legislative costs

Conclusion
In comparison to aerobic treatment, AD has the advantage of generating energy for
the heat treatment sanitisation of the waste for ABPR. However anaerobic digestion
is not considered an individual solution. It is only suitable on a regional basis
requiring a large facility. These are best operated as waste management businesses
taking in a number of waste streams. Therefore this solution will be reliant on the
development of centralised anaerobic digesters. The timescale for procurement and
construction of large-scale digesters is considered as 2-3 years.

Useful links
   • British Biogen - http://www.britishbiogen.co.uk/
   • The Composting Association - http://www.compost.org.uk/dsp_home.cfm
   • Holsworthy Biogas - http://www.holsworthy-biogas.co.uk/


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Description: Aerobic exercise anaerobic exercise is relatively speaking. During exercise, the body's metabolism is accelerated to speed up the metabolic needs more energy. The body's energy through the body of sugar, protein and fat catabolism come. When not in the exercise, such as jogging, playing badminton, dancing, etc., the body's supply of energy mainly from aerobic metabolism of fat. To fat as the main supply of energy aerobic exercise aerobic exercise is what we say. When we engage in very intense exercise, or the rapid outbreak, such as weightlifting, 100 m sprint, wrestling, etc., then the body needs a lot of energy in an instant, and in normal circumstances, aerobic metabolism can not meet the body at this time demand, so the conduct of anaerobic metabolism of sugar, to rapidly produce large amounts of energy. This state is anaerobic exercise.