Anaerobic purification is a fermentation process. It is a natural process according to which organic material is degraded by bacteria. A key difference between this system and a ‘fermentation, manure fermentation or co-fermentation’ installation is that in these systems solid material is used (manure, sludge, corn, etc.) while anaerobic purification exclusively involves the degrading of dissolved material. As a result, normal fermentation takes much longer (25-40 days) while anaerobic purification is completed in around 5 hours.
A huge variety of bacteria, each with their own specific function, ensure that the carbon chains of organic material are successively broken down at a temperature 25-38ºC into carbon dioxide (CO2), hydrogen (H2) and acetic acid (C2H4O2). In the final phase of the process, methane (CH4) is released from the acetic acid and from the combination of hydrogen and carbon dioxide. However, the volume of excess CO2 is so great that it is not all converted into methane, with the addition of hydrogen. The resultant biogas therefore contains both methane and CO2. Another (common) by-product of the process is hydrogen sulphide gas (H2S) that must be removed before the biogas can be successfully reused.
The bacteria responsible for the process of degradation grow in granules. This process was discovered by Professor Lettinga of the Wageningen University. Anaerobic purification plants have been built since the nineteen eighties, and since that time the techniques for separating water and gas while retaining the bacteria have undergone many developments.
The anaerobic purification process is not capable of degrading all organic components into methane. The difficult-to-degrade substances must subsequently be removed in an aerobic (oxygen-rich) process. Nonetheless, it is important to apply the greatest possible level of anaerobic purification, because anaerobic purification generates energy while aerobic purification requires energy input, produces far more CO2 and results in a bacterial flow (sludge) which (as yet) has no useful application.