The raw biogas generated in the anaerobic purification process contains high levels of H2S (hydrogen sulphide). This must be removed to allow the biogas to be used as a fuel. The raw biogas is therefore transported to the base of a washing tower where the H2S is absorbed by a washing fluid that is pumped in the opposite direction. The resultant gas flow contains almost no residual H2S while the washing fluid is contaminated. In the 1980s, Professor Lettinga discovered that under the influence of a small amount of oxygen, bacteria are capable of converting the dissolved H2S into pure sulphur. As a result, the washing fluid can be regenerated, representing not only a considerable cost saving but also helping to reduce the consumption of chemicals. This technology has been converted by the company Paques into an industrial installation, the first full scale example of which was installed at IWE. Following a series of minor adjustments, this first installation is still successfully operating today, and many others have been sold all around the world.