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An acidification tank (hydrolysis) is sometimes required upstream of the methane fermentation unit;acidogenesis is carried out by extremely diverse species of micro-organisms: mesophiles or thermophiles, strict or facultative anaerobes.

This first stage results in a mixture of volatile fatty acids (VFA): acetic, lactic, propionic, butyric acids, etc, from neutral compounds (ethanol), gaseous products (CO2 and H2) and possibly from ammonium and orthophosphate.

It is used:

  • when the retention time in the methane fermentation reactor is short (e.g. Anaflux)
  • when the effluent contains high levels of sulphates (> 500 mg · L–1) and depending on the value of the SO42–/COD ratio;
  • on certain substrates, difficult to hydrolyse.

The tank can also play a secondary role as a pollution flow regulator. The tank is covered in order to reduce oxygen intrusion and is usually mixed.

The output of the methane fermentation tank is partially recycled back to the acidification tank in order to increase biomass production, and especially to stabilise the pH by injecting the high alkalin treated effluent.

Acidification is normally completed within a couple of hours (2 to 6 hours) using the suspended growths system. Although, there is no fundamental reason why attached growth process cannot be used. With some substrates (very rich in suspended solids), the acidification period

could be extended up to 24 hours.