Journal of Medical Microbiology and Diagnosis-Biotechnology Open Access Journals

Agricultural Wastes-To-Green Energy in Egypt

Safenaz Shaaban and Mahmoud Nasr*

Sanitary Engineering Department, Faculty of Engineering, Alexandria University, Egypt

Submission: February 14, 2018; Published: April 10, 2018

*Corresponding author: Mahmoud Nasr, Assistant professor, Sanitary Engineering Department, Faculty of Engineering, Alexandria University, 21544, Alexandria, Egypt.

How to cite this article: Safenaz S, Mahmoud N. Agricultural Wastes-To-Green Energy in Egypt. Adv Biotech & Micro. 2018; 8(5): 555750. DOI: 10.19080/AIBM.2018.08.555750

Keywords: Agricultural biomass; Biohydrogen; Environmental conditions; Fermentation

Agriculture Wastes as Biomass Feedstock

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%) [1]. 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 [2]. The hydrolysis phase should be able to disrupt the lignin component of the plant material and release a sufficient amount of valuable sugars [3]. The extracted sugars can be managed to produce green energy either alone or in co-digestion with other organic materials.

Hydrogen Gas

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 [4]. 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) [5]. Amongst them, biological routes providea cost-effective and environmentally friendly solution for hydrogen production [6].

Biological H₂ fermentation

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 [7]. 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 [8]. The application of nanoparticles for encouraging the bioactivity of microorganisms to produce high hydrogen yields has been reported [4]. Nanoparticles can stimulate hydrogen productivity 0 the surface effect and the quantum size effect.

Dark Fermentation

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 [9]. Most studies have reported that culture medium pH of 5.5-6.5 provides the optimum condition for hydrogen fermentation [10]. 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 [11]. 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 [10]. 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).

Photo-Fermentation

The soluble microbial products obtained from darkfermentation can be subsequently utilized as a substrate to generate more H₂viaphoto-fermentation [4]. 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 [12]. 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 [13]. 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 fermentation [14].

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 [4].

Recommendations

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.