Wet fermentation, also known as wet fermentation, is a method of biological waste utilisation in which organic waste such as garden waste, kitchen waste or liquid manure is fermented in a humid environment. The process takes place under low-oxygen or anaerobic conditions - the microorganisms responsible for fermentation do not require oxygen.

The result of wet fermentation is a stabilised product that can be used as a fertiliser and has a lower level of pollutants than the original material. However, methane and carbon dioxide are also produced as by-products.

Wet fermentation is particularly suitable for processing large quantities of moist waste such as kitchen waste and manure. However, it is also more susceptible to problems with pathogenic microorganisms and produces higher emissions of methane compared to dry fermentation. In addition, the energy required to operate the plant is higher than with dry fermentation, which worsens the energy production balance.

There are also hybrid processes with semi-moist fermentation, which require a low water content. They are less susceptible to problems with pathogenic microorganisms and methane emissions are also lower than with pure wet fermentation.