State of the Art and Progress in Production of Biohydrogen

Hydrogen (H2) is a versatile, clean burning, and renewable energy currency that can potentially displace the use of petroleum-based fuels in the transportation sector, which accounts for 74% of the total projected increase in liquid fuel consumption over the next 30 years. Demand for hydrogen is also expected to increase as it starts penetrating the transportation sector as a fuel: about 40 million tonnes of hydrogen per year would be required to fuel about 100 million fuel cell-powered cars after full market penetration. Hydrogen fuel can be produced from a diverse array of potential feedstocks including fossil fuels, water, and organic matter using various chemical and electrochemical methods. Biological hydrogen (biohydrogen) production, which employs the use of hydrogen producing microorganisms via light dependant or fermentative processes. Research on biohydrogen has increased dramatically in the past 6 years, with great emphasis on dark fermentation, but there are many scientific and engineering challenges that must be met by current and future biohydrogen researchers if these technologies are to be technically feasible and economically viable.

In both green algae and cyanobacteria, H2 production is coupled to photosynthesis, which uses abundant water and sunlight as the source of electrons and energy, respectively. This process has immense potential for renewable H2 production in future scaled-up processes. To generate H2, green algae and cyanobacteria employ two different types of phylogenetically unrelated hydrogenases: [FeFe] enzymes in the former case and [NiFe] enzymes in the latter. Hydrogenases contain distinct metalloclusters at their catalytic centers that require different suites of proteins for their assembly and maturation. Research in recent years has identified the genes encoding the hydrogenase proteins along with their maturation machineries, underlying regulatory controls, and enzyme catalytic mechanisms. One of the major challenges to be overcome is the deleterious effect of O2 on hydrogenase structure and function. This chapter will examine the status of these topics in both classes of microbes, with the expectation that a more in-depth understanding will better guide the development of a robust system for sustained photobiological H2 production.

The development of renewable fuels of the future is important for the replacement of depleting oil and natural gas reserves. Hydrogen is one of the most promising clean fuels, since its combustion yields only water. One of the visionary methods to obtain hydrogen at the expanse of solar energy is the use of photosynthetic microorganisms. Hydrogen production in phototrophs is coupled to the oxygenic and anoxygenic photosynthesis involving hydrogen-evolving enzymes, hydrogenases and nitrogenases. At the present time the efficiency of hydrogen photoproduction is not sufficiently high. Most hydrogen-evolving enzymes are inhibited by molecular oxygen, which creates a major barrier for the sustained hydrogen photoproduction in oxygenic phototrophs, such as green algae and cyanobacteria. However, several strategies have been applied to solve this problem, including spatial and temporal separation of water splitting and hydrogen evolution, and regulation of water splitting activity and respiration to maintain anoxic conditions. Anoxygenic photosynthesis can be used to drive hydrogen photoproduction in integrated systems including fermentative anaerobic organisms. In this review different mechanisms for hydrogen production in photosynthetic organisms and the latest advances in this area are discussed.

Photofermentative hydrogen production is a bioprocess in which photosynthetic purple nonsulfur bacteria grow heterotrophically on organic acids like acetic acid, lactic acid and butyric acid and produce hydrogen using light energy under anaerobic conditions. Two enzymes are specifically involved in hydrogen production, namely nitrogenase and hydrogenase. While nitrogenases produce hydrogen under nitrogen-limited conditions acting as ATP-dependent hydrogenase, hydrogenases have the ability for both production and consumption of molecular hydrogen depending on the type of hydrogenase and physiological conditions. Photofermentation process can be achieved in a wide variety of conditions such as in batch or continuous mode, upon artificial or solar illumination, utilizing various carbon and nitrogen sources including food industry wastewater and dark fermentation effluents. Panel and tubular photobioreactors are the most applicable bioreactor types since they ensure simple design, reasonable material and production costs and high light energy utilization. Physiological parameters such as pH, temperature, medium composition and light intensity control the yield and hydrogen productivity of the bacteria. Hydrogen productivity and yield can also be increased by using genetically modified bacterial strains or immobilization of bacteria. Genetic studies focus on development of mutant strains by disrupting the uptake hydrogenase genes, altering pigmentation and blocking alternative by-product biosynthesis. Techno-economic evaluations show that photofermentative hydrogen production process is very near to the commercialization stage, however demo scale experience is necessary to solve some problems such as low rate of hydrogen production and the cost associated with photobioreactor scale-up. Furthermore, recent studies are trying to integrate photofermentation to dark fermentation to have an enhanced hydrogen production yield. Finally, the whole process could end up with a fuel cell application where the produced hydrogen is stored for future uses.

Biological methods of H2 generation are preferable to physico-chemical methods for several reasons: i) biological systems can use renewable sources of energy (sun, organic wastes); ii) biological processes are carrying under ambient pressures and temperatures; that is why they are safer; and iii) biological systems are self-supporting, self-repairing, and self-reproducible in principle. Different biological systems have own advantages and peculiarities. Combining them, the individual strength of each may be exploited and their weaknesses can be overcome. Different strategies of their integration are discussed in this chapter based on literature data. Some methods of integration are promising but still they have not been experimentally supported. The integration of dark fermentative H2 production using organic wastes in the first stage and H2 photoproduction by photosynthetic anoxygenic bacteria using an effluent from the fermentation as the second stage attracted much attention last years. This review evaluates published data with attempts to reveal the most important factors affecting the productivity and efficiency of these dual systems.

Biohydrogen production through dark fermentation could be a promising route to the generation of a renewable fuel source. Dark fermentation is attractive since its deployment would probably be based on known reactor technology, and, in a first application, could use various carbohydrate rich waste streams, followed by lignocellulosics, a vast largely untapped resource. A thorough understanding of the enzymes and pathways is involved would help develop strategies, especially metabolic engineering, for improving rates and yields of hydrogen production. Here, the various hydrogenases and their active sites and biochemistry are discussed followed by an examination of known hydrogen producing pathways.

The production of hydrogen which is predicted to be the fuel of the future has a number hurdles to overcome. One of the constraints is cost of production. However, cost of production can be significantly reduced considering biowastes as a potential and cheap renewable substrate for hydrogen production. The wastes are a result of the many activities of humans leading to wastes generation and includes household, municipal, agricultural and industrial wastes. Further, the microbial diversity available enables us to select the right microorganisms to ferment a particular biowaste in obligate anaerobic or facultative anaerobic single stage or hybrid system to covert the carbonaceous components to biohydrogen. The systems can be operated with an axenic culture or a consortia of microbial cultures. Factors that affect microbial growth need to be optimized and the systems can be operated in a continuous mode. Further, this approach leads to dual benefits, that is, a simultaneous biohydrogen – a green fuel production and pollution load reduction for the industry. Cost saving technologies are always attractive to industry where cost of waste management is high. The aim of this review is to summarise the possible technologies that combine utilisation of biowastes with production of biohydrogen. This will be attractive to industry as the cost of management of the biowastes can be off set by the alternative green fuel produced during the process. When biowastes are utilised as energy substrates for biohydrogen producing microbes.

Various kinetic models were developed to describe the fermentative biohydrogen production in a batch system. These models took into account the effects of substrate degradation (and inhibition), temperature and pH on biomass growth as well as on the rate of biohydrogen production. This chapter summarizes various such models describing the effect of different parameters on the biological hydrogen production process. It was found that the modified Gompertz model was most suitable to describe the progress of biohydrogen formation process. Substrate inhibition was well described by the Andrew model, which is the modified form of the classical Monod model as well as by the Han- Levenspiel model. The modified Logistic model was used to describe the growth of the biomass as well as the rate of biohydrogen production. The effect of temperature was well described by the Arrhenius model, but up to the critical temperature only. The Ratkowsky model was able to describe the effect of both temperature as well as pH on fermentative biohydrogen production.

Of the many ways hydrogen can be produced, this chapter focuses on biological hydrogen production by thermophilic bacteria and archaea in dark fermentations. The thermophiles are held as promising candidates for a cost-effective fermentation process, because of their relatively high yields and broad substrate palette. Yet many challenges remain to be faced, including improving productivity, tolerance to high osmolality and growth inhibitors, and reactor configuration. This review consolidates current insights in the quest for high yields and productivities within thermophilic hydrogen production. Important is to understand how environmental parameters affect the redox- and energy metabolism of the microorganism(s) involved. This knowledge is required for designing an optimal bioreactor configuration and operation.

The present chapter discusses the opportunities and challenges faced by microorganisms when they produce molecular hydrogen (H2) as a major fermentative electron sink. We will focus on sugar fermentation to look at the thermodynamic implications of selecting hydrogen as a main electron sink, how competing fermentation pathways compete with hydrogen for electrons and their effect on both flux and yields of hydrogen. The signatures of these pathways can be observed in the genomes of these organisms. We will contrast the putative enzymes and pathways available to different fermentative organisms on the basis of an ever-increasing collection of available genomes. A description of the molecular toolbox available to various phyla and specific organisms will lead to a better understanding of the key reactions involved in electron flow and will lead to rational strategies of molecular engineering to optimize hydrogen concentrations and yields from dark fermentation.

Dark fermentation has the potential to convert carbohydrate-rich waste-streams into hydrogen (H2). However, current yields of H2 production during fermentation fall far short of theoretical maximum values. If biohydrogen production via dark fermentation is to become practical, yields must be increased. The application of recombinant DNA techniques to direct metabolism towards the production of industrially valuable substrates is an emerging field of study. Metabolic engineering seeks to “improve” cellular function through the modulation of enzymatic, transport, or other regulatory functions of the cell. In contrast to traditional strain improvement approaches involving mutagenesis followed by the screening of colonies for a desired phenotype, metabolic engineering often involves the introduction of heterologous genes or regulatory elements that are often employed to confer novel metabolic configurations.

Microbial electrohydrogenesis is a novel process capable of generating high hydrogen (H2) yields from organic waste streams and fermentation end products. A small voltage is applied to a Microbial Electrolysis Cell (MEC) to force the microbial oxidation of organic material at the anode and drive the chemical reduction of protons at the cathode. In this chapter, the microbe-electrode relationship is studied and three mechanisms for electron transfer to an electrode surface are explored. The thermodynamics of electrode reactions and cell potentials is described to calculate the systems theoretical limitations. Deviations from ideal behavior are characterized by quantifying the energy losses associated with overpotentials at the electrode-solution interface and ohmic resistances inherent in electronic and ionic conduction. Finally, strategies are discussed to overcome the technical challenges electrohydrogenesis currently faces. The purpose of this chapter is to introduce the fundamental principles of microbial electrolysis, to identify the biological and electrochemical challenges currently under investigation, and to discuss the future role microbial electrohydrogenesis will play towards sustainable H2 production.

Cell and enzyme immobilization is a widely employed technique in many industrial applications. Over the recent years, researchers have been also focusing on benefiting from the merits of immobilization techniques in biohydrogen studies. In this chapter, various immobilization techniques and their application results in terms of enhancing biohydrogen production via different processes, namely anaerobic dark and light fermentation, are presented. For this purpose, firstly, principles of various immobilization techniques and protocols are discussed and secondly the literature reports on the results of use of immobilization techniques are reviewed in regards to their biohydrogen productions.

Hydrogen is seen as a “fuel of the future” that will replace the petroleum-based economy. It is a versatile energy carrier with the potential for extensive use as a transportation fuel, in power generation, and in many other applications. Hydrogen is currently produced from fossil sources (steam reforming of methane), but technologies utilizing renewable sources are urgently needed for sustainability. Biological hydrogen (biohydrogen) production is one of the challenging areas of technology development for sustainability. There are a wide range of biohydrogen technologies, including direct biophotolysis, indirect biophotolysis, photo-fermentations, and dark-fermentation. The current scientific results are promising, but substantial improvements in biohydrogen production through research advances (i.e. improvement in efficiency through genetically engineered microorganism, development of bioreactors etc.) are needed. In this study, attempts have been made to highlight not only the advantages, but also the bottlenecks that limit biohydrogen production.