Dark fermentation is the fermentative conversion of organic substrate to biohydrogen. It is a complex process manifested by diverse groups of bacteria, involving a series of biochemical reactions using three steps similar to anaerobic conversion. Dark fermentation differs from photofermentation in that it proceeds without the presence of light.

Anaerobic digestion is a collection of processes by which microorganisms break down biodegradable material in the absence of oxygen.

Dear Sreeja and Navdeep: you forgot to mention, that you used WIKIPEDIA as source of your answers.....

Vit

Both processes are anaerobic fermentations. In fact, dark fermentation (DF) is composed by the first two phases of anaerobic digestion (AD) (hydrolysis and acidogenesis). In dark fermentation (DF) you aim to produce hydrogen while in anaerobic digestion (AD) the objective is to produce biogas which can be further upgraded to biomethane. The differences are in the operating conditions. In DF you have to inhibit the growth of methanogens (which consume hydrogen) and to do so there are some strategies like inoculum pretreatment, operating at low HRT, operating at pH lower than 7, among others. In anaerobic digestion, HRT is usually high and the pH is near 7. Usually DF is folowed by AD to achieve maximum energy production and COD removal. These reviews could be useful:

http://dx.doi.org/10.1016/j.fuel.2016.08.030

http://dx.doi.org/10.1016/j.apenergy.2015.01.045

http://dx.doi.org/10.1016/j.biortech.2016.08.073

http://dx.doi.org/10.1016/j.rser.2015.10.054

Fermentation is a specific term used for the conversion of sugars to alcohols or acids whereas digestion is a broader term used to convert any kind of organic substrate to small molecules particularly biogas.

Dark fermentation is the biological H2 production by fermentative conversion of organic substances in the absence of light. This process is differing from Photofermentation, which is the fermentative conversion of organic substances to biological H2 by a diverse group of photosynthetic bacteria with the presence of light. The most important factor in these fermentations is presence or absence of light.

Anaerobic digestion is the breakdown of organic materials into biogas such as methane and carbon dioxide in the absence of oxygen, naturally or in an anaerobic digester. The most important factor in this process is the absence of oxygen.

Study of the 2 attached files is recommended

Anaerobic digestion is the breakdown of organic materials into biogas in the absence of oxygen.

Dark fermentation is the biological H2 production by fermentation conversion of organic substances in the absence of light.

In a nutshell, dark fermentation is a subset of anaerobic digestion.