Dark fermentation is the
fermentative conversion of organic substrate to
biohydrogen
Biohydrogen is hydrogen, 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, including biological waste. Furthermore some photosynthetic micro ...
. It is a complex process manifested by diverse groups of
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
, involving a series of
biochemical
Biochemistry, or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology, ...
reactions using three steps similar to
anaerobic conversion. Dark fermentation differs from
photofermentation in that it proceeds without the presence of
light
Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
.
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 (or waste water) 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 do ...
as a potential substrate for
biohydrogen
Biohydrogen is hydrogen, 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, including biological waste. Furthermore some photosynthetic micro ...
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.
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
Wastewater treatment is a process which removes and eliminates contaminants from wastewater. It thus converts it into an effluent that can be returned to the water cycle. Once back in the water cycle, the effluent creates an acceptable impact on ...
from dark fermentation processes using selectively enriched mixed
culture
Culture ( ) is a concept that encompasses the social behavior, institutions, and Social norm, norms found in human societies, as well as the knowledge, beliefs, arts, laws, Social norm, customs, capabilities, Attitude (psychology), attitudes ...
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 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, 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 sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
, which results in 80-90% of the initial
COD
Cod (: cod) is the common name for the demersal fish genus ''Gadus'', belonging to the family (biology), 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 ''Gad ...
remaining in the wastewater in the form of various
volatile organic acids (VFAs) and solvents, such as
acetic acid
Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main compone ...
,
propionic acid
Propionic acid (, from the Greek language, Greek words πρῶτος : ''prōtos'', meaning "first", and πίων : ''píōn'', meaning "fat"; also known as propanoic acid) is a naturally occurring carboxylic acid with chemical formula . It is a ...
,
butyric acid
Butyric acid (; from , meaning "butter"), also known under the systematic name butanoic acid, is a straight-chain alkyl carboxylic acid with the chemical formula . It is an oily, colorless liquid with an unpleasant odor. Isobutyric acid (2-met ...
, and
ethanol
Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...
. 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; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
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 gaseous renewable energy source produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste, Wastewater treatment, wastewater, and food waste. Biogas is produced by anaerobic ...
*
Biohydrogen
Biohydrogen is hydrogen, 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, including biological waste. Furthermore some photosynthetic micro ...
*
Biological hydrogen production (algae)
*
Biomass
Biomass is a term used in several contexts: in the context of ecology it means living organisms, and in the context of bioenergy it means matter from recently living (but now dead) organisms. In the latter context, there are variations in how ...
*
Electrohydrogenesis
*
Fermentation (biochemistry)
Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...
*
Microbial fuel cell
References
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External links
Bio-hydrogen production from wastewater
Biofuels technology
Catalysis
Environmental engineering
Hydrogen biology
Hydrogen production