Dark fermentation is the
fermentative conversion of organic substrate to
biohydrogen
Biohydrogen is H2 that is produced biologically. Interest is high in this technology because H2 is a clean fuel and can be readily produced from certain kinds of biomass.
Many challenges characterize this technology, including those intrinsic to ...
. It is a complex process manifested by diverse groups of
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
, 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
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
.
Overview
Fermentative/hydrolytic microorganisms hydrolyze complex organic polymers to monomers which are further converted to a mixture of lower-molecular-weight organic acids and alcohols by obligatory producing acidogenic bacteria.
Utilization of
wastewater
Wastewater is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of domestic, industrial ...
as a potential substrate for
biohydrogen
Biohydrogen is H2 that is produced biologically. Interest is high in this technology because H2 is a clean fuel and can be readily produced from certain kinds of biomass.
Many challenges characterize this technology, including those intrinsic to ...
production has been drawing considerable interest in recent years especially in the dark fermentation process. Industrial wastewater as a fermentative
substrate for H
2 production addresses most of the criteria required for substrate selection viz., availability, cost and
biodegradability
Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegradati ...
.
Chemical wastewater (Venkata Mohan, ''et al.'', 2007a,b), cattle wastewater (Tang, ''et al.'', 2008), dairy process wastewater (Venkata Mohan, ''et al.'' 2007c, Rai et al. 2012), starch hydrolysate wastewater (Chen, ''et al.'', 2008) and designed synthetic wastewater (Venkata Mohan, ''et al.'', 2007a, 2008b) have been reported to produce biohydrogen apart from
wastewater treatment from dark fermentation processes using selectively enriched mixed
culture
Culture () is an umbrella term which encompasses the social behavior, institutions, and norms found in human societies, as well as the knowledge, beliefs, arts, laws, customs, capabilities, and habits of the individuals in these groups ...
s under acidophilic conditions. Various wastewaters viz., paper mill wastewater (Idania, ''et al.'', 2005), starch effluent (Zhang, ''et al.'', 2003), food processing wastewater (Shin ''et al.'', 2004, van Ginkel, ''et al.'', 2005),
domestic
Domestic may refer to:
In the home
* Anything relating to the human home or family
** A domestic animal, one that has undergone domestication
** A domestic appliance, or home appliance
** A domestic partnership
** Domestic science, sometimes c ...
wastewater (Shin, ''et al.'', 2004, 2008e), rice winery wastewater (Yu ''et al.'', 2002), distillery and molasses based wastewater (Ren, ''et al.'', 2007, Venkata Mohan, ''et al.'', 2008a), wheat straw wastes (Fan, ''et al.'', 2006) and
palm oil mill wastewater (Vijayaraghavan and Ahmed, 2006) have been studied as fermentable substrates for H
2 production along with wastewater treatment. Using wastewater as a fermentable substrate facilitates both wastewater treatment apart from H
2 production. The efficiency of the dark fermentative H
2 production process was found to depend on pre-treatment of the mixed consortia used as a
biocatalyst
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
, operating pH, and organic loading rate apart from wastewater characteristics (Venkata Mohan, ''et al.'', 2007d, 2008c, d, Vijaya Bhaskar, ''et al.'', 2008d).
In spite of its advantages, the main challenge observed with fermentative H
2 production processes is the relatively low
energy conversion efficiency from the organic source. Typical H
2 yields range from 1 to 2
mol of H
2/mol of
glucose
Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...
, which results in 80-90% of the initial
COD
Cod is the common name for the demersal fish genus '' Gadus'', belonging to the family Gadidae. Cod is also used as part of the common name for a number of other fish species, and one species that belongs to genus ''Gadus'' is commonly not call ...
remaining in the wastewater in the form of various
volatile organic acids (VFAs) and solvents, such as
acetic acid,
propionic acid,
butyric acid, and
ethanol
Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...
. Even under optimal conditions about 60-70% of the original organic matter remains in solution.
Bioaugmentation with selectively enriched
acidogenic consortia to enhance H
2 production was also reported (Venkata Mohan, ''et al.'', 2007b). Generation and accumulation of soluble acid metabolites causes a sharp drop in the system pH and inhibits the H
2 production process. Usage of unutilized
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...
sources present in acidogenic process for additional biogas production sustains the practical applicability of the process. One way to utilize/recover the remaining organic matter in a usable form is to produce additional H
2 by terminal integration of
photo-fermentative processes of H
2 production (Venkata Mohan, ''et al.'' 2008e, Rai et al. 2012) and methane by integrating acidogenic processes to terminal
methanogenic processes.
See also
*
Biogas
Biogas is a mixture of gases, primarily consisting of methane, carbon dioxide and hydrogen sulphide, produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste and food waste. It is a ...
*
Biohydrogen
Biohydrogen is H2 that is produced biologically. Interest is high in this technology because H2 is a clean fuel and can be readily produced from certain kinds of biomass.
Many challenges characterize this technology, including those intrinsic to ...
*
Biological hydrogen production (algae)
*
Biomass
*
Electrohydrogenesis
Electrohydrogenesis or biocatalyzed electrolysis is the name given to a process for generating hydrogen gas from organic matter being decomposed by bacteria. This process uses a modified fuel cell to contain the organic matter and water. A smal ...
*
Fermentation (biochemistry)
Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food ...
*
Microbial fuel cell Microbial fuel cell (MFC) is a type of bioelectrochemical fuel cell system that
generates electric current by diverting electrons produced from the microbial oxidation of reduced compounds (also known as fuel or electron donor) on the anode to oxid ...
References
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External links
Bio-hydrogen production from wastewater
Biofuels technology
Catalysis
Environmental engineering
Hydrogen biology
Hydrogen production