A number of barriers may thwart the impetus of the drivers mentioned above. Barriers that can emerge during consideration of commercial AD projects include (1) uncertainty about the feasibility or reliability of the technology (both AD and energy production), (2) uncertainty about the economic and business outcome of the project, (3) uncertainty about public policy that might impact AD (e. g., incentive pricing or lack thereof for renewable energy, environmental rules and regulations and compliance therewith), (4) uncertainty about selling the renewable electricity to the main grid and standby fees, and (5) uncertainty about or fear of liability or penal­ties (from the complexity of compliance with environmental or safety regulations), and damage to corporate or product image from suboptimal or failed AD projects. The barriers to AD implementation are complex and vary in type and importance for each candidate site or enterprise. They can also be iterative whereby resolution of one concern may reveal a subsequent concern that also discourages implementation of an AD project (e. g., concern over disposal of digestate, or potential impact on material flow or heat recovery of the core operations of the business).

Among the major barriers is the concern over the capability and reliability of the AD system to digest the available feedstock(s) and the inability to validate the biogas yields predicted from the feedstock(s) because biogas and energy yield primarily dictates the economic viability of any commercial AD implementation. Although observation of a similar AD system operating elsewhere can alleviate this concern, concerns often exist over possible unpredictable digester failure that can potentially disrupt the core operation of the factory or farm. Such fear is a strong deterrent to AD implementation and can be difficult to overcome. Additionally, con­cerns can arise from the distraction from core business operations brought by the implementation of an “alien” technology (i. e., AD). Another barrier is the lack of supportive public policy that can provide assured markets for methane biogas and incentive pricing based on its fair value for not only the renewable energy, but also the environmental and social benefits. Uncertain long-term value of a feedstock or its value for alternative use (e. g., bioethanol production) can also further discourages commercial implementation of AD.

1.1.1 Different Conditions of the Reactor

There are many types of fed-batch composting (FBC) reactors. However, FBC reac­tors cannot function continuously because the contents, which usually include waste and biomass-carriers such as wood chips, often aggregate after prolonged opera­tion. Therefore, it is necessary to remove excess contents from the reactor regularly because of decreasing decomposition rate, and this requires secondary treatment of the aggregated contents. Since considerable amounts of partially degraded materials remain during the FBC process, secondary treatment of the products is necessary to obtain matured compost or complete degradation of waste to organic mat­ter. Although the mechanism of aggregation in reactors is unknown, this process decreases the decomposition rate [14], and probably changes the microbial commu­nity. Therefore, this study was conducted to compare the optimal and aggregated conditions of a bacterial community in a FBC reactor using 16S rDNA clone analysis.

The current shortages and high prices of gasoline products are making it clear that a sustainable, economical, and environmentally benign process for producing fuel is needed. In the future, lignocellulosic-derived products are poised for sharp growth. According to a recent McKinsey report, the bio-based products market is expected to exceed $182.91 billion by 2015 [34]. Lignocellulosic-derived products may play a pivotal role to match this expectation and future markets seem very promising for ethanol, xylitol, organic acids, and 2, 3-BD. Mechanisms for higher yield and productivity of these value-added products can be developed by exploring the hemicellulose fraction of the cell wall in depth.

The fermentation of pentose sugars is not as easy as that of cellulosic-derived hexose sugars due to the unavailability of appropriate microorganisms and the lack of an established bioconversion process. In-depth studies of methods for hemicellulosic degradation are required. This will assist in limiting the role of fer­mentation inhibitors during hemicellulosic degradation. In the past five years, there has been substantial development in the area of hemicellulose hydrolysis using rou­tine methodologies with known microorganisms. A newer approach to hydrolyzing technologies using a battery of hemicellulase titers needs to be developed to pro­duce high yields of sugar monomers and eventually convert them into value-added products. Isolation and screening of potent hemicellulase-producing microorgan­isms and further development of mutants/cloned microorganisms may improve the production yields of the desired titers on a commercial scale. Genetic engineering may also improve microbial efficiency for the overproduction of industrial prod­ucts using cheaper sources of carbohydrates in fermentation media, the hallmark of commercial fermentation processes. The microbes will be more useful if they have characteristics such as thermotolerance, alkalotolerance, or tolerance of other extreme conditions.

Hemicellulose degradation into fermentable sugars is another area where the scope of research seems enormous. Efforts are underway at our laboratory for the production of ethanol and xylitol from lignocellulose feedstock. Multiple research projects are being sponsored by government agencies to improve the pretreatment process of lignocellulosics for their conversion into ethanol and xylitol [24, 63-69].

In the last five years, there has been comparatively less research into 2, 3-BD pro­duction than into ethanol and xylitol production worldwide. New research insights, such as the development of transgenic plants containing less lignin, may be help­ful for the conversion of biomass into value-added products. Chen and Dixon [70] developed antisense-mediated down-regulation of lignin biosynthesis in alfalfa to reduce or eliminate the need for pretreatment. This may make the hemicellulosic fraction more accessible due to the reduced presence of lignin, which in turn will require a milder pretreatment and less enzymatic load to get the desired yield of fermentable sugars. Releasing genetically engineered plants may raise ethical issues among environmentalists; however, it can be assumed that the generation of new products from hemicellulose will strengthen the economy by saving for­eign exchange reserves and promoting energy independence, which will benefit the environment.

One of the sustainable ways to reduce the cost of waste or wastewater treatment is to generate bio-energy, such as H2 gas, from the organic matter present. Waste biomass contains enough energy to meet a significant fraction of the world’s entire energy demand, if it could be efficiently converted to useful energy forms [19]. According
to one estimation, the energy value of all residual biomass in the United States is 0.2-0.3 TW [20] and conversion of this material to useful forms would meet approx­imately 7% of the USAs’ total annual energy use (~3.3 TW) [19]. The fraction is much higher worldwide, perhaps 25% or more [19]. Major advantages of energy from wastes are the carbon neutrality, renewability, recovery of energy and simulta­neous wastewater treatment. Simple sugars to complex industrial wastewaters have been evaluated to determine their potential as fermentative substrates for the pro­duction of H2. Table 1 shows some of these studies using dark fermentation. Simple sugars such as glucose, sucrose and lactose are readily biodegradable substrates for H2 production but are expensive. Various wastewaters generated from industrial or domestic activities function as good substrates for H2 generation due to the presence of large fractions of degradable organics. Residue like agricultural crops and their waste products, wood and wood waste, food processing waste, aquatic plants, algae, and effluents produced in human habitats can all be used as fermentable substrates

[70]. Many agricultural and food industry wastes contain starch or cellulose, which are rich in terms of carbohydrate content and can also be used for H2 production. The sludge generated in wastewater treatment plants contains large quantities of carbohydrates and proteins which can also be used for energy production. Table 2 shows data on fermentative H2 production using various types of waste.

Unlike wastewater, cellulosic material or solid wastes typically require pretreat­ment to make the organic fraction soluble and bio-available to microorganisms for conversion to H2. Due to its tightly packed, highly crystalline and water-insoluble nature, cellulose is recalcitrant to hydrolysis into its individual glucose subunits [70]. In the pretreatment step, a combination of chemical, mechanical, and enzy­matic processes is typically used. Techniques viz., high temperature, high or low pH, hydrolytic enzymes, microwaves, ultrasound, radiation, and pulsed electric fields are being used for this purpose [19]. Some microorganisms can degrade cellulose effec­tively by using their cellulase enzymes resulting in monosaccharide products that can be converted into H2 with dark fermentation [70].

A very complex community of bacteria and archaeal methanogens drives the entire AD process [36, 65]. Fungi and protozoa are also found in anaerobic digesters [60] although their functions and contributions to the AD process are not known. The cell densities of microbes in anaerobic digesters are among the highest in managed envi­ronments, with bacteria being the most predominant (up to 1010 cells/mL of digester content) followed by methanogens. The entire AD process can be described as a synergistic process of four sequential phases: hydrolysis, acidogenesis, syntrophic acetogenesis, and methanogenesis (Fig. 1). Each phase is mediated by a distinct functional group, or guild, of microbes [36, 91]. During the first phase, some fac­ultative or strictly anaerobic bacteria (e. g., Clostridium spp.) hydrolyze the biomass polymers (e. g., polysaccharides, proteins, and lipids) present in the feedstocks, giv­ing rise to monomers or oligomers (e. g., glucose, cellobiose, amino acids, peptides, fatty acids, and glycerol). This hydrolysis step is catalyzed by the extracellular hydrolytic enzymes such as amylases, cellulases, xylanases, proteases, and lipases secreted by the hydrolytic bacteria. Kinetically, the hydrolysis step can proceed rapidly for soluble feedstocks such as starch. However, for insoluble lignocellulosic feedstocks that contain recalcitrant embedded lignin, the hydrolysis phase is rather slow and often becomes a major rate-limiting step of the entire AD process [2].

The resulting hydrolytic products are immediately fermented to short chain fatty acids (SCFA), CO2, and H2 during the subsequent fermentative acido — genesis by another guild of facultative or strictly anaerobic bacteria (e. g., Bacteroides, Clostridium, Butyribacterium, Propionibacterium, Pseudomonas, and Ruminococcus). The major SCFA formed include acetate, propionate, butyrate, formate, lactate, isobutyrate, and succinate, with acetate predominating. Small quantities of alcohols (e. g., ethanol and glycerol) are also produced. The fermen­tative acidogenesis typically proceeds rather rapidly [10]. In fact, when feedstocks

containing large amounts of readily fermentable carbohydrates (e. g., sugars and starch) are digested at high organic loading rates, the production of SCFA can exceed their consumption, leading to SCFA accumulation and consequential AD upset or even failure [10].

The final phase of AD involves methanogens of the Archaea domain. Methanogens are strict anaerobes and produce CH4 as the major end-product of their catabolism. Most methanogens are fastidious microbes and only grow on a few substrates within a narrow spectrum of environmental conditions (neutral pH, Eh <­300 mV, etc.). Methanogens use a unique methanogenesis pathway to produce CH4 [36]. Hydrogenotrophic methanogens produce CH4 via the reduction of CO2 by H2 or by the conversion of other C1 substrates (e. g., methanol and methylamines), while acetoclastic methanogens convert acetate to CH4. It should be noted that the former accounts for approximately one third while the latter accounts for two thirds of the CH4 produced in anaerobic digesters. This is because acetate is the major end product of the acidogenesis step in all anaerobic digesters [86]. In spite of this, only a few species of acetoclastic methanogens have been known and they are within genera Methanosaeta (formerly Methanothrix) and Methanosarcina. Methanosaeta spp. are obligate acetoclastic methanogens, while species of Methanosarcina also use C1 substrates. Hydrogenotrophic methanogen species are found in gen­era Methanobacterium, Methanospirillum, Methanobrevibactor, Methanococcus,

Methanomicrobium, Methanoculleus, Methanogenium, and Methanothermobacter. All methanogens contain a unique cofactor, F420, that is autofluorescent at a wavelength of 420 nm [38]. Some methanogens, especially hydrogenotrophic methanogens, contain so much of it that they appear blue when viewed under a microscope. Several trace elements, especially nickel and cobalt, are required by methanogens for methanogenesis and growth. For some feedstocks, supplementa­tion with trace elements can significantly enhance methane biogas production and process stability [48]. Because of the low energy yield from the methanogenesis pathway, most methanogens grow slowly, especially acetoclastic methanogens (e. g., Methanosaeta spp. have a generation time of 3.5-9 days) [36]. However, methano — genesis is typically not a rate-limiting step of the entire AD process because the low-energy yield of the methanogenesis pathway forces it to run rather rapidly. Additionally, methanogens are susceptible to a host of factors (e. g., pH, ammonia, and metals) so they are often implicated in instability or sub-optimal performance of AD [17].

The small amounts of SCFA with three or more carbons (e. g., propionate, butyrate, isobutyrate, valerate) and the ethanol produced during the fermentative acidogenesis as well as the long chain fatty acids derived from lipid hydrolysis can not be used directly by any known methanogens. A unique guild of strictly anaer­obic bacteria (referred to as syntrophic acetogens) can oxidize these intermediates to acetate, H2, and CO2 so that they can serve as the substrates of methanogenesis [75, 91]. However, the oxidation of these fatty acids and ethanol under fermentative conditions (referred to as syntrophic acetogenesis) is thermodynamically unfavor­able; and hydrogenotrophic methanogens are needed to reside in close proximity to rapidly consume the H2 produced by the syntrophic acetogens through interspecies hydrogen transfer [23]. Syntrophomonas wolfei and Syntrophobacter wolinii are thought to be important syntrophic acetogens in anaerobic digesters, with the former primarily oxidizing butyrate and the latter oxidizing propionate. With a generation time of greater than one week, syntrophic acetogens grow extremely slowly [24]. As a result, the solid retention time (SRT) in digesters has to be long (15 days or longer) to retain enough syntrophic acetogens. Hence, syntrophic acetogenesis can be a rate — limiting step during AD, and failure or suboptimal performance encountered during AD operation often involves this guild of bacteria, which is exemplified by AD fail­ure when the organic loading rate was too high and the production of non-acetic SCFA exceeded that of their utilization [47]. Thus, syntrophic acetogens are impor­tant members of the microbial community of stable AD processes even though the carbon flux through them is relatively small, and it is critical to maintain a balanced production and consumption of these non-acetic SCFA by avoiding organic over­loading. It should be noted that because they cannot be cultured as single cultures, syntrophic acetogens are not well studied. The recent advancement of genomics and metagenomics offers new opportunities to better understand this important guild of bacteria in anaerobic digesters (see [55] for a recent review).

Several features of feedstocks can have profound effects on AD, such as the con­tent of readily fermentable carbohydrates, particle sizes of insoluble feedstocks (the hydrolysis step is especially affected by particle sizes), nutrient content and balance, and presence and concentrations of inhibitory compounds. Feedstocks rich in starch and/or proteins are easier to digest than lignocellulosic feedstocks. Reduction of par­ticle size of insoluble feedstocks can significantly speed up AD and increase CH4 yields. Microbes need numerous nutrients to grow, with nitrogen and phosphorous being the most important. The optimal carbon (expressed as chemical oxidation demand, COD) to N to P ratios (COD:N:P) for efficient AD differ with different feedstocks and the AD technologies used. For most feedstocks, a C:N ratio of 25-32 is suitable for most AD processes [8].

Microbial aided electrolysis cells (MEC), also called bio-electrochemically assisted microbial reactor (BEAMR), use electro-hydrogenesis to directly con­vert biodegradable material into H2 by applying external voltages in fuel cells in an anaerobic microenvironment [116, 117]. The supplemented voltage helps to decompose acetate spontaneously under standard conditions [116, 118]. Based on a thermodynamic analysis the addition of greater than 0.11 V to that generated by bacteria (-0.3 V) will yield H2 gas at the cathode, but voltages of -0.2 V are needed because of electrode over-potentials [116]. This process, referred to as electro­hydrogenesis, provides a route for extending H2 production past the endothermic barrier imposed by the microbial formation of fermentation end products, such as acetic acid [116]. Microbial electrolysis makes it possible to generate H2 utilizing effluents generated from acidogenic fermentation and opens the possibility of using diluted organic matter varying in composition, such as wastewater, for H2 produc­tion [119]. Membrane-less continuous flow microbial electrolysis cell (MEC) with a gas-phase cathode was also used to produce H2 [119].

1.1 Hemicellulose Hydrolysis

In contrast to cellulose, which is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength. It is eas­ily hydrolyzed by dilute acid or enzymatically using an arsenal of hemicellulase enzymes [19]. In addition, the lignocellulose can be mildly pretreated with chemi­cals prior to enzymatic hydrolysis for better saccharification into fermentable sug­ars. This reduces the crystallinity of the biomass and makes it more amenable to fur­ther coordinated enzymatic reactions [18, 20]. Various pretreatment strategies with dilute acid, alkali, ammonia fiber explosion, hydrogen peroxide, steam explosion, wet oxidation, liquid hot water, sodium sulfite, etc., have been discussed [3, 21].

The balance between the drivers and the barriers for a potential commercial AD project is primarily centered on the financial economics of the AD system to be installed, which includes the capital and operating cost, the cost of the feedstock (e. g., the cost of acquisition, transportation, preparation, or alternative disposal), and the revenues and credits that can be realized from AD operations. To tip the balance towards the drivers, the capital and operating cost need to be minimized while the revenues and credits need to be maximized. More efficient, cost-effective, reliable, and versatile AD technologies are needed to strengthen key drivers and diminish many of the uncertainties related to AD technologies. However, a bet­ter understanding of the microbiological underpinning of AD processes is required to develop such AD technologies. Additionally, incentives from governments and public support are also important to encourage AD implementation.

In a long-term FBC reactor, a problem that is likely to occur and which needs to be monitored is the significant abrasion of the bulking agents. Nagao et al. noted that both plastic bottle flakes and wood chips were capable of maintaining a high rate of decomposition. In addition, the bacterial community was examined by 16S rDNA clone analysis and the difference in the community between the two bulking agents was compared [15].

1.1.2 Small-Scale and Large-Scale Reactor

As mentioned above, a variety of bacterial community structures in various com­posting reactors have been reported. Nevertheless, knowledge of the microbial community in large-scale, completely-mixed composting reactors is still lacking. Therefore, this study was conducted to clarify the bacterial community succession during the start-up period of a large-scale, completely-mixed composting reactor by using 16S rDNA clone and DGGE analysis, and to compare it with the bacterial community in a small-scale reactor.

James J. Valdes and Jerry B. Warner

Abstract An emerging concept is the convergence of “green practices” such as systemic sustainability and renewable resources with military operational needs. One example is developmental tactical refineries. These systems leverage advanced biotechnology and thermochemical processes for energy production and provide sustainability to military forward operating bases for tactical purposes.

Tactical refineries are designed to address two significant problems in an overseas crisis deployment. The first problem is access to dependable energy. Recent military operations in Southwest Asia have shown that, despite advanced logistics and host nation resources, access to fuel, particularly during the early months of a crisis, can be difficult. Further, even temporary loss of access to energy during military operations can have unacceptable consequences. The second problem is the cost and operational difficulties for waste disposal of materials created by military operations. Delivery of food, supplies, equipment and material to forward positions creates huge volumes of waste, and its removal inflicts a costly and complex logistics and security overhead on US forces.

As a simultaneous solution to both problems, deployable tactical refineries are being designed to convert military field waste such as paper, plastic and food waste into immediately usable energy at forward operating bases, on the battlefield or in a crisis area. These systems are completely novel and are only becoming feasible by taking advantage of recent advances in biotechnology and thermo-chemical science. In addition to providing operational benefits to US Forces, these systems will pro­vide significant cost savings by reducing the need for acquisition and distribution of liquid fuels via convoys which are vulnerable to attack. Tactical refineries would also serve a useful role in other military programs which support disaster relief or post-combat stabilization.

J. J. Valdes (B)

Department of the Army, Research, Development and Engineering Command, Aberdeen Proving Ground, MD 21010-5424, USA e-mail: james. valdes@us. army. mil

O. V. Singh, S. P. Harvey (eds.), Sustainable Biotechnology,

DOI 10.1007/978-90-481-3295-9_5, © Springer Science+Business Media B. V. 2010

Keywords Biofuel ■ Tactical energy ■ Synthetic gas ■ Fermentation ■ Downdraft gasifier

1 Introduction

The initial challenge was to mate the waste streams produced by small tactical units with technologies that were net energy positive at that scale. The TGER system was the result of a high level of optimization “from the trash up” and required a thorough scientific analysis and technology selection process with full consideration of the context within which it would be operating.

There are numerous waste to energy technologies, each with varying efficiencies and capabilities to digest complex waste streams [1]. Figure 1 breaks the problem set down to net power output (x axis) verses the type of waste (y axis), and shows the range of applications from landfill to onsite or tactical utilities. Incineration, for example, will handle all waste types including hazardous materials and metals, but has only 10% net power output at best and is most suited to large static operations such as landfills. By contrast, biocatalytic (i. e. enzymatic) approaches have much more limited ability to handle waste but are relatively efficient (~75%) in terms of net power output [2].

Biocatalytic approaches are therefore more suited to operations in which the waste stream is predominantly food waste and biomass. These two technologies occupy the extremes of this energy return spectrum.

WASTE TO ENERGY TECHNOLOGIES

The Tactical Garbage to Energy Refinery (TGER) design is a “hybrid” that uti­lizes both biocatalytic (fermentation) and thermochemical (gasification) subsystems in a complementary manner to optimize overall system performance and to address the broadest possible military waste stream. The hybrid design is based on detailed analysis of the waste stream combined with a modeling and simulation program unique to the TGER. Given the objective waste stream which includes both food and dry material wastes, a system which included a biocatalytic format for organic wastes such as food and juice materials, and a thermochemical format for solid wastes such as paper, plastic and Styrofoam, would have significant advantages over unitary approaches.

The Energy and Material Balance mathematical model showed that conversion of materials and kitchen wastes to syngas and ethanol would provide sufficient energy to drive a diesel engine and generate electricity. A downdraft gasifier was selected to produce syngas via thermal decomposition of solid wastes, and a bioreactor con­sisting of advanced fermentation and distillation was used to produce ethanol from liquid waste and the carbohydrates and starches found in food waste.

Both dry and wet field wastes (with the exception of metal and glass) are intro­duced into a single material reduction device which reduces both the wet and dry waste into a slurry. This slurry is then subjected to a “rapid pass” fermentation run which converts approximately 25% of the carbohydrates, sugars, starches and some cellulosic material into 85% hydrous ethanol. The remaining bioreactor mass is then processed into gasifier pellets which are then converted into producer gas, also known as “syngas”. The hydrous ethanol and syngas are then blended and fumigated into the diesel engine, gradually displacing the diesel fuel to an estimated 2% pilot drip. The design process model is shown in Fig. 2.

HYBRID TECHNOLOGY

IN- LINE BIOREFINERY DESIGN PROCESS MODEL

— Thermal component provides heat and power to run biocatalytic — Petroleum based plastics recalcitrant until gasifier

— Residues from Bioreactor path are channeled to gasifier — Bioplastics can degrade immediately

— System starts on diesel fuel; then create/introduces Producer Gas and Vaporous Ethanol to displace diesel to minimum drip for pilot ignition

Fig. 2 In-line biorefinery design process model

Adding the advanced fermentation process to the design of the TGER added no significant energy costs, as heat generated by the engine’s exhaust drives the distilla­tion, which is carried out in an 8-foot-high column packed with material over which fractionation of ethanol and water occurs. The additions of a few small pumps used to transport the ethanol solution from the fermentation tank to the distillation column and finally to the ethanol storage tank, were the only additional power requirements. The combination of the two waste-to-energy technologies allowed for the remedia­tion of a broader spectrum waste stream, both solid and liquid, the ability to extract much more energy from the waste, and operation of the generator at full power due to the anti-knock properties of the hydrous ethanol.