In Egypt, dependence on fossil-based fuels (e.g., coal, oil, and natural gas) as a primary source of energy can cause environmental damage and health problems. Moreover, the recent rise in fossil fuels prices has driven the national economy towards a critical issue. Hence, it is worthwhile finding alternative sources of energy that can cope with the extensive utilization of fossil fuels. In parallel, agricultural wastes cause several problems to rural villages/areas in Egypt, viz., they can be incinerated in the field creating air and soil pollution problems. The five crops producing the highest quantities of wastes are rice, sugarcane, corn, cotton, and wheat. These wastes include cobs (corn cobs), bagasse (sugar cane bagasse), straw (rice straw), and peeling (banana and orange peels).The estimated amount of agricultural wastes in Egypt ranges from 22 to 26 million dry tonnes per year. This huge amount should be beneficially utilized rather than being incinerated or disposed in a land fill.
Agricultural wastes are mainly composed of lignocellulosic materials remained after collecting the valuable parts of crops. Lignocellulosic constituents can retain carbohydrates in the form of cellulose (35- 45%) and hemicellulose (25-40%) . Hence, agricultural wastes should be subjected to a hydrolysis/delignification process to break the interpolymer/intermolecular linkages of lignocellulose complex and hydrolyze the cellulose and hemicellulose fractions . The hydrolysis phase should be able to disrupt the lignin component of the plant material and release a sufficient amount of valuable sugars . The extracted sugars can be managed to produce green energy either alone or in co-digestion with other organic materials.
Hydrogen gas, as a clean and renewable source of energy, has been found to be a potential solution for carbon-free
environments. H2 gas has the highest gravimetric energy density of 122 – 142kJ/g, which is 2.75-folds that of hydrocarbon fuels . The combustion of H2 gas produces only water with no hydrocarbons, carbon monoxide, or carbon dioxide in the end product. Hydrogen can be obtained using various forms of energy, i.e., biological (dark fermentation, or photo-fermentation), electrical (electrolysis, or plasma arc decomposition), thermal (thermolysis, thermo-catalysis, or thermochemical processes), and photonic (PV electrolysis, photo-catalysis, photo-electrochemical, or bio-photolysis) . Amongst them, biological routes providea cost-effective and environmentally friendly solution for hydrogen production .
In the biological fermentation process, various bacterial species are capable of converting sugars (carbohydrates) into H2 gas and other value-added products. Biological H2 fermentation is dependent on the hydrogenases enzymes, which can be divided into iron-iron [Fe-Fe] hydrogenases, and nickel–iron [Ni-Fe] hydrogenases . Recently, several types of researches have been conducted to improve the bioactivity of hydrogen-producing microorganisms and increase the conversion efficiency of the substrate into H2. The production rate and yield of bio-H2 are influenced by various environmental conditions such as medium pH, culture temperature, concentration and composition of substrate, inoculum dosage, and fermentation time . The application of nanoparticles for encouraging the bioactivity of microorganisms to produce high hydrogen yields has been reported . Nanoparticles can stimulate hydrogen productivity 0 the surface effect and the quantum size effect.
During dark fermentation, acidogenic bacteria such as Clostridium sp. can convert various organic substrates (as a source of electrons and energy) into H2 gas in addition to volatile
fatty acids (VFAs) and CO2 . Most studies have reported that
culture medium pH of 5.5-6.5 provides the optimum condition
for hydrogen fermentation . Dark fermentation can be
performed under mesophilic (25- 40°C),thermophilic (40-65
°C) or hyper-thermophilic (>80 °C) conditions. The favorable
ratios of nutrients supply have been reported as 11.4:1-200:1
for COD: N and 73:1-970:1 for COD:P . Species of Bacillus,
Clostridium, and Enterobacter have been reported to dominate
the dark fermentation studies as pure cultures in the inoculum.
The mixed culture of sludge should be subjected to a treatment
process to eliminate the H2 consuming bacteria such as
methanogenic and homoacetogenic microorganisms . The
sludge treatment techniques include heat shock, acid or alkaline
treatment, sonication, freezing and thawing, and bromoethane
sulfonate or iodopropane methods.
Theoretically, a maximum of 4 mol hydrogen per mole
glucose can be obtained when acetic acid is the major VFA
component (Eq. 1). However, lower yields are obtained in real
applications because a portion of the glucose is stored in cells for
microbial growth and maintenance. The butyric acid formation
is accompanied by the formation of 2 mol-H2 per mole glucose
(Eq. 2) while propionic acid accumulation consumes 2 mol-H2
per mole glucose (Eq. 3). When both acetic and butyric acids are
produced during dark fermentation, 2.5 mol-H2per mole glucose
can be attained. Lactic acid fermentation does not affect either
the formation or the consumption of bio-H2 generation (Eq. 4).
The soluble microbial products obtained from darkfermentation
can be subsequently utilized as a substrate
to generate more H2viaphoto-fermentation . In photofermentation,
photosynthetic bacteria utilize light as an energy
sourceto assimilate small organic molecules into H2 and new
bacterial cells. However, the improvement of H2 production via
photo-fermentation requires strict control on environmental
conditions. For example, the optimum ranges of pH and
temperature have been reported to be 6.8 – 7.5 and 31 – 36
°C, respectively . Other operating parameters can include
wavelength 400 1000nm and lightintensity 6-10klux.
Purple non-sulfur bacteria (PNS) are favorable
photosynthetic microorganisms responsible for H2 production
during the photo-fermentation process. These gram-negative
bacteria are able to absorb and utilize a wide region of the
solar spectrum and perform high substrate conversion
efficiencies . Several strains such as Rhodo spirillumrubrum,
Rhodopseudo monaspalustris, Rhodobacter sphaeroides, and
Rhodobactercapsulatus have been widely employed for genetic
and physiological studies in H2fermentation production.
Increasing the light illumination efficiency, and avoiding
inhibitory factors (e.g., ammonia nitrogen, and oxygen) are
the main factors used for enhancing the performance of photo
Theoretically, the integration of dark and photo fermentation
processes can achieve hydrogen yield up to 12 mol-H2/molglucose.
More studies have been conducted to improve the rate
of photo-fermentation, thereby promoting the efficiency of the
overall bio-hydrogen production process .
Future studies should be conducted to improve the
bioconversion efficiencies of agricultural wastes into H2 gas.
The integration of dark and photo fermentation mechanisms
should also be examined in details. In addition, the application
of more complex carbon sources for bioH2 fermentation should
be studied. The ideal microbial consortium responsible for
bio-hydrogen production from waste materials should be
determined. Further works should be performed to assess the
economic analysis and feasibility study for bioH2 generation
from waste materials.