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Anaerobic Digestion (AD) uses naturally occurring micro-organisms to digest/degrade organic materials in the absence of air. AD has been used for thousands of years to treat organic waste, including the ancient city of Ur in Mesopotamia where the digesters also provided gas for cooking.
AD is usually classified according to the temperature range that they operate at:
| Process |
Temperature |
Treatment Time |
| Temperature Ranges of Anaerobic Digestion |
| Psycrophilic |
Ambient Temperature |
Months or Years |
| Mesophilic |
30° - 38°C |
12 to 21 Days |
| Thermophilic |
52° - 60°C |
3 to 9 Days |
A schematic diagram of a typical process is shown below:-
Anaerobic digesters currently used to treat farm wastes and sewage sludge are generally mesophilic Completely Stirred Tank Reactor (C.S.T.R.) digesters.
C.S.T.R. digesters are designed to treat liquid wastes and slurries (typically no more than 5% Total Solids (TS) content). Higher total solid content liquids can be introduced, but problems arise through grit and crystals accumulating within the vessel and the system requires ever-increasing power and sophistication to mix the contents. This can lead to pumping difficulties and blanketing of internal heating apparatus which can then lead to loss of temperature control. Mesophilic temperatures are widely used in C.S.T.R. digesters as the Biogas yield usually matches heating energy requirements.
Thermophilic operation, although it has advantages of faster rate of biochemical conversion and better pathogen control, is difficult to achieve with C.S.T.R. digesters. This is because the relatively low solids content in proportion to liquid within the digester, does not yield sufficient Biogas to heat the digester to the required temperature.
There are other types of digestion systems such as plug-flow, batch and anaerobic filter. These however are not as widely used as C.S.T.R. digesters.
The Portagester ® plant is designed to overcome the above problems.
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