Anaerobic Digestion Explained

Process	Temperature	Treatment Time
Temperature Ranges of Anaerobic Digestion
Psycrophilic	Ambient Temperature	Months or Years
Mesophilic	30° - 38°C	12 to 100 Days
Thermophilic	52° - 60°C	1 to 7 Days

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:

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). have been successfully used in the Water Industry for many years. This is due to the nature of the input material, use of good quality front end screening, well built anaerobic digesters and expert management.

Single-stage C.S.T.R. digesters can be used for energy crops, farm and food wastes, materials with higher total solid content, but be aware of problems that can arise through grit and crystals accumulating within the vessel and the need for 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 energy from the biogas 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, but some have potential for faster processing..

The patented Bioplex ® process uses two or more stages to create optimum conditions for each step of the digestion process, increasing efficiency and reliability whilst having the ability to treat a wider variety of feedstock.