Several barriers exist that make many industries and farmers reluctant to imple­ment AD to convert their biomass wastes to methane biogas. Improvements in AD technologies with respect to efficiency, reliability, and cost-effectiveness will overcome some of the barriers related to the technologies. Other barriers can only be debased from policy and public supports. While it is broadly realized that the building of a sustainable society requires both renewable energy and protection of the environment, the valuation of such is incomplete. AD is one of the few tech­nologies that help achieve both goals. Even the bioethanol and biodiesel industries generate biomass wastes, which need AD to extract otherwise wasted energy and to reduce environmental pollution. Therefore, AD should be regarded as a unique, indispensable, renewable energy-producing biotechnology that protects the environ­ment. With continued improvement of AD technologies and supports from both the public and the government, AD will become more cost-effective, energy-efficient, reliable, and widely implemented. AD will evolve into one of the most environment — friendly biotechnologies that produce cost-effective bioenergy in the next five to ten years.

Acknowledgement Research in our laboratories is supported by grants awarded from DOE and the Ohio Third Frontier Project. We thank Dr. Mark Morrison for his helpful discussions during the preparation of this chapter.

Dimensions (LxWxH)

Weight

Waste residuals per day (Ash):

Emissions

Consumable electric power produced Water supply

Manpower to operate

1.1.1 Consumables:

Biological package, fuel, water, charcoal, and downdraft gasifier filter bags

Lactrol (Antibiotic): 1 g/day ($0.26/g)

Glucozyme (Enzyme): 50 g/day ($0.89/50 g)

Amylase (Enzyme): 50 g/day ($2.05/50 g)

Yeast: 200 g/day ($4.39/200 g)

Total cost for biological package: $7.59/day

Downdraft gasifier filter bags need to be replaced every 2 weeks 50 lbs of charcoal per month

The organic loading rate (OLR) of the wastewater also influences the H2 production pattern, apart from other wastewater characteristics. H2 yields were inversely pro­portional to the glucose feeding rate, while the highest H2 yields were observed at lowest glucose loading rate [78]. Glucose concentrations exceeding 2 g/l (as co-substrate) showed suppression in H2 production [21]. A marked reduction

in H2 production rate was observed with an increase in OLR when chemical wastewater was used as the substrate [22]. H2 production was also found to decrease with an increase in OLR when dairy wastewater was used as substrate [31]. Similar observations with substrate loading have been reported in the literature [101, 102]. Decreased H2 production may also be due to end product inhibition by over­accumulated (supersaturated) soluble metabolites in the liquid phase at high OLRs [102]. However, each wastewater has its own threshold value, which relates to the system microenvironment and desired output [4,22,26, 31]. Feed consisting of only glucose as substrate showed a low H2 yield, while feed with chemical wastewater admixed either with glucose or domestic sewage as co-substrates showed a positive influence on the H2 generation rate [21, 71]. Domestic sewage addition showed a positive affect on the acidogenic fermentation process due to supplementation of additional micronutrients, organic matter and microbial biomass in the direction of enhance the process efficiency.

These wastes are characterized by varying water contents, but high VS contents (>95%). However, these parameters vary considerably. Most food wastes have bal­anced nutrients and large amounts of readily fermentable carbohydrates and thus are among the most suitable feedstocks for AD. According to a recent study, 348 m3 of CH4 can be produced per dry ton of food wastes within only 10 days of AD [93]. Food wastes amount to approximately 43.6 million dry tons each year in the USA [81]. This represents a potential of 15.2 billion m3 of CH4 per year. During food processing, a significant portion of foodstuffs also ends up in wastes or wastewaters. For example, 20-40% of potatoes are discarded as wastes during pro­cessing. National data on the amount of food-processing wastes are not available.

The state of California generates more than 4 million dry tons of food-processing wastes each year [54], potentially producing 1,200 million m3 of CH4. This trans­lates into an annual potential of several billions m3 of CH4 in the USA. Except for the wastes from animal meat processors, most food-processing streams are rel­atively poor in nitrogen, but rich in readily fermentable carbohydrates. As such, food-processing wastes can be co-digested with other nitrogen-rich feedstocks (e. g., municipal sludge or animal manures) to enhance AD system stability and CH4 production [46].

Approximately 250 million dry tons of MSW are produced annually in the USA. The organic fraction, such as paper, yard trimmings, and food scraps, is biodegrad­able and can be converted to methane biogas. Although the composition of MSW varies dramatically depending on society, season, collection, and sorting, OFMSW accounts for more than 50% of the MSW in most societies. Most OFMSW has little moisture or readily fermentable carbohydrates and is relatively deficient in N or P, but has a relatively large BMP (300-550 m3 CH4/ton) if digested adequately [25]. The OFMSW generated annually in the USA has a CH4 potential of 37.5 billion m3.

Crop residues amount to an estimated 428 million dry tons each year in the USA. Although the majority of crop residues is typically left in the field, approximately 113 million dry tons are recoverable and available for conversion to methane biogas [69]. Crop residues typically have relatively low water contents, high VS contents, and variable contents of readily fermentable carbohydrates. Most crop residues are non-leguminous and are poor in available nitrogen. The BMP of crop residues varies from crop to crop (from 161 to 241 m3 CH4/ton) (124). If subjected to proper AD, at least 20 billion m3 of CH4 can be produced annually from the crop residues available for biogas production in the USA. Similarly for other nitrogen-poor biomass, co­digestion of crop residues with animal manures or municipal sludge substantially improves CH4 yield [50]. In theEU, 1,500 million dry tons of biomass are available each year for biomethanation within the agricultural sector, with half of this being crops intended for bioenergy production [5]. It should be noted that production of bioethanol and biodiesel from energy crops only utilizes a fraction of the biomass, and implementation of AD by the bioethanol industry can generate substantially more energy (up to 30% of the total energy of the initial biomass) [3, 74]. This also holds true for many other biomass-based processes producing non-food products.

All these types of feedstocks likely contain bulky materials, such as peeling, papers, stems and leaves. Pretreatment, especially reduction of particle size by grinding or milling, is typically required to enhance AD [40]. Other pretreatments such as alkaline pretreatment [53] have also been evaluated to further enhance the hydrolysis step in laboratories, but few of them have been implemented in full — scale AD plants. As mentioned earlier for the AD of livestock manures, co-digestion with other nitrogen-rich biomass (e. g., municipal sludge or animal manure) can also substantially stabilize the AD process and increase CH4 production [50, 94].

The above mentioned wastes have relatively low water contents. They can be digested using some wet AD processes (e. g., CSTR and CMCR) after dilution. The Lemvig Biogas plant in Denmark is one example of such wet AD. It is a centralized biogas plant consisting of three thermophilic CSTR with a total volume of 7,000 m3

that digests various types of organic industrial wastes, source-sorted MSW, and manures [7]. The biogas produced is used to generate electricity and heat.

Apparently, dry AD is advantageous for these low-moisture feedstocks because it eliminates the need to dilute the feedstocks to a fluid state and produces a low — moisture digestate, which is easier to transport and disperse [90]. The DRANCO technology is a dry AD technology successfully used to convert low-moisture organic wastes (e. g., OFMSW and crop residues) to methane biogas [21]. The DRANCO technology requires the feedstock to be shredded and milled first to reduce particle sizes (<4.0 mm in diameter). A digested sludge or digestate is then mixed with the feedstock in a 6:1 to 8:1 ratio in a mixing compartment. The mixture is heated by steam (to 30-40°C for mesophilic AD or 50-55°C for thermophilic AD) and then pumped into the digester at the top. The feedstock descends by gravity while digestate is withdrawn at the bottom. The biogas rises and exits the digester through the roof of the digester. The retention time in a DRANCO digester averages 20 days with a pass-through time of 2-4 days. The DRANCO technology is marketed by the Organic Waste System (OWS) in Belgium (http://www. ows. be/index. php). According to OWS, the DRANCO technology has a number of advantages including high solid digestion, high loading rates (10­20 kg COD/m3 of reactor/d), high biogas productivity (100-200 m3 of biogas/dry ton of feedstock), small digester volumes, no maintenance or failures inside the digester, less energy consumption, well controlled external inoculation, and kill — off of pathogens and seeds. The largest DRANCO digester started operation in 2006 in Vitoria, Spain. This digester has an effective volume of 1,770 m3 and a capacity of 120,750 tons/yr of primarily OFMSW. It produces 5,962 tons of biogas, which can generate 6,000 MWh of electricity, and 12,580 tons of compost per year. As of this writing, most of the DRANCO digesters in use are located in Europe, and the capacity of dry AD has exceeded that of wet AD of solid wastes [21]. The ECOCORP (www. ecocorp. com), BEKON (www. bekon-energy. de), Kompogas (www. kompogas. com), and Linde (http://www. anaerobic-digestion. com) processes are emerging dry AD technologies mostly used in Europe for dry AD of solid biomass wastes.

A new two-staged AD process was evaluated by Parawira et al. [67] in digesting solid potato wastes under mesophilic and thermophilic conditions. This process uses a solid leaching bed reactor for hydrolysis and acidification while an UASB reactor is used for methanogenesis. High loading rates (36 g COD/L/d), high methane yields (0.49 L/g COD removed), and stable operation were observed under mesophilic conditions. The utility of this new process remains to be validated for other types of feedstocks containing significant amounts of lignocellulose.

Although H2 produced from dark fermentation process is considered as a viable alternative fuel and energy carrier of the future, H2 storage, purification, low pro­duction rates and the requirements of separate fuel cell systems for the generation of energy (electricity) are some of the inherent limitations. Alternatively, the microbial fuel cell (MFC) facilitates in situ conversion of energy in the form of bioelectricity from wastewater treatment by dark fermentation [111, 147-158]. MFC is a hybrid bio-electrochemical system, which converts the substrate directly into electricity by the oxidation of organic matter in the presence of bacteria (bio-catalyst) at ambi­ent temperature/pressure [155, 156]. The potential developed between the bacterial metabolic activity [reduction reaction generating electrons (e-) and protons (H+)] and electron acceptor conditions separated by a membrane manifests bioelectricity generation. In an acidogenic microenvironment, single and dual chambered MFC systems were evaluated for the production of bioelectricity using various types of wastewater viz., chemical wastewater, designed synthetic wastewater, domes­tic sewage and vegetable waste employing mixed cultures as anodic biocatalysts [147-158] (Table 7). The higher activity of intracellular e — carriers which will help in the translocation of e — from bacteria to the outside of the cell might be the rea­son for higher current generation observed under acidic pH operation [156]. Apart

from power generation, the MFC also demonstrated an enhanced substrate degrada­tion rate along with good color and total dissolved solid (TDS) removal efficiency compared to conventional anaerobic treatment [156]. MFCs can also utilize acid — rich carbon effluents generated from acidogenic processes as primary substrate for bioelectricity generation along with additional treatment efficiency.

One of the main industrial uses of microorganisms has been alcoholic fermenta­tion. The giant “microbial libraries” in current vogue can be studied for microbes that convert cheaper carbohydrates into value-added products, which can serve as raw materials for the fermentation of hemicellulosic-derived sugars into valuable commercial commodities [30]. The bioconversion process holds more promise of utilizing both hexose and pentose sugars from lignocellulosic materials. Microbial

conversion of hexose sugars into chemicals is well established; however, the ability of these organisms to ferment pentose sugars is somewhat less so. The use­ful exploitation of lignocellulosics by fermentation can be enhanced by efficient utilization of the pentosanic fraction along with hexoses.

Yeasts that have been studied extensively for use in xylose fermentation include Pachysolen tannophilus, Candida shehatae, Pichia stiptis, and Kluveromyces marxi — anus [3]. The optimal performance of these microorganisms is usually controlled by the air supply. Other yeasts investigated for their xylose-fermenting ability include Brettanomyces, Clavispora, Schizosaccharomyces, several other species of Candida viz. C. tenius, C. tropicalis, C. utilis, C. blankii, C. friedrichii, C. solani, and C. parapsilosis, and species of Debaromyces viz. D. nepalensis and D. polymorpha. Maleszka and Schneider [31] screened 15 yeast strains for their ability to utilize D-xylose, D-xylulose, and xylitol for ethanol production under aerobic, microaero­bic (low aeration), and anaerobic conditions using rich undefined or defined media. In almost all cases, ethanol production by P. tannophilus and species belonging to Candida and Pichia was better on rich media under microaerobic conditions [3,4, 31].

Several pentose-utilizing fungal species like Fusarium oxysporum, Rhizopus sp., Monilia sp., Neurospora crassa, Paecilomyces sp., Mucor sp., Neurospora crassa, and F oxysporum and bacterial species like Bacillus macerans, B. polymyxa, Kiebsiella pneumoniae, Clostridium acetobutylicum, Aeromonas hydrophila, Aerobacter sp., Erwinia sp., Leuconostoc sp., Lactobacillus sp., Clostridium ther — mocellum, C. thermohydrsulfurium, C. thermosaccharolyticum, and C. thermosul — furogenes utilizing pentose, hexose, and lignocellulose hydrolysates for ethanol production have been extensively reviewed [32].

S. Venkata Mohan

Abstract The global impact of increasing energy demands, depleting reserves of fossil fuels and increasing pollution loads on the environment due to the utilization of energy produced from fossil fuels have received considerable notice in recent years. Generation of energy from fossil fuels is generally convenient but the deplet­ing reserves and associated global warming are major problems. One potential alternative is a shift from fossil fuel to a hydrogen (H2) based economy. H2 is con­sidered to be a clean energy carrier with high-energy yield (142.35 kJ/g) and upon combustion it produces only water. H2 can be produced by the biological routes of bio-photolysis, photo-fermentation and dark fermentation or by a combination of these processes. Dark fermentation offers the particular advantage of using wastewater as a substrate and mixed culture as catalyst. Wastewater contains high levels of biodegradable organic material with net positive energy. One way to reduce the cost of treatment is to generate bio-energy, such as H2 gas by metabolically utilizing organic matter, at the same time accomplishing treatment. This chapter mainly focuses on the evaluation of fermentative H2-generating processes utilizing wastewater as substrate and mixed culture as biocatalyst. A particular insight was also laid on to discuss the process based on important operating factors involved and to delineate some of the limitations. Various strategies such as multiple process integration, microbial electrolysis, polyhydroxyalkanoate (PHA) production, bioaugmentation, self-immobilization and metabolic engineering were discussed in overcoming some of the limitations in the direction of process enhancement.

Keywords Biohydrogen ■ Anaerobic ■ Dark fermentation ■ Wastewater treatment ■ Acidogenic ■ Pretreatment ■ Bioelectricity ■ Microbial fuel cell (MFC) ■ Microbial electrolysis ■ Bioaugmentation ■ Polyhydroxyalkanoates (PHA) ■ Mixed culture ■ Immobilization

S. Venkata Mohan (B)

Bioengineering and Environmental Centre (BEEC),

Indian Institute of Chemical Technology (IICT), Hyderabad-500007, India e-mail: vmohan_s@yahoo. com; svmohan@iict. res. in

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

DOI 10.1007/978-90-481-3295-9_7, © Springer Science+Business Media B. V. 2010

1 Introduction

Hydrogen (H2) is a potentially sustainable energy carrier as because it produces only water and has a high energy yield of 122 kJ/g; 2.75 fold greater than that of hydrocarbon fuels and can be made from renewable resources, although at present nearly 90% of H2 is produced from steam reformation of natural gas or light oil fractions at high pressure and high temperatures.

Biological H2 production proceeds through two main pathways: photosynthesis and dark fermentation. Photosynthesis is a light-dependent process, comprised of direct biophotolysis, indirect biophotolysis and photo-fermentation, while anaero­bic fermentation, also known as dark fermentation, is a light-independent catabolic process [1-4]. Photosynthetic microorganisms, such as algae, photosynthetic bac­teria and cyanobacteria manifest H2 production in photosynthetic processes [5-6] while, fermentative microorganisms generate H2 during the acidogenic phase of the anaerobic digestion. Fermentative processes yield comparatively better H2 produc­tion than the photosynthetic process and do not rely on the availability of light. They also utilize a variety of carbon sources such as organic compounds, wastes, wastew­aters or insoluble cellulosic materials, require less energy, are technically much simpler, have lower operating costs and are more stable [3, 7-12]. Fermentative microorganisms also generally have rapid growth rates. Dark fermentation is a practically a more feasible process for the mass production of H2. H2 generation via biological routes is relatively pollutant free, requires low energy inputs and is therefore considered as a potential alternative to the conventional physical/chemical methods used for H2 production. Most of the biological H2 production processes are operated at ambient temperature and pressure, thus are less energy intensive. Research on photo-biological routes of H2 production was initially reported with specific strains and defined medium. Subsequently, dark fermentation gained impor­tance due to its feasibility of utilizing wastewater as a fermentative substrate and mixed cultures as biocatalysts. The process simplicity and efficiency are strong features.

Set-up/breakdown time: three days total to operate the system through one full cycle

1.1.2 Safety and health risk:

Received safety release from the Army Test and Evaluation Center for prototypes, certifying the prototypes safe for human use. TGER will require further safety evaluation to be cleared for soldier operation

1.1.3 Target MTBEFF:

TGER is composed of several subsystems, each with their own mean time between essential function failures (MTBEFF). The gasifier was the worst performer of the subsystems, with a MTBEFF of about 6 h. This has caused us to look at other gasi­fication technologies to replace the current gasifier. The pelletizers in the material handling subsystem were the next worst performer. The pelletizers were undersized for the amount of throughput which caused some maintenance problems and break­downs. The pelletizer MTBEFF was about 48 h. This problem should be resolved with pelletizers that have the right specifications. Applying the proper upgrades to the gasifier and replacing the pelletizers the target MTBEFF will be 1 month

Nitrogen is a necessary component in proteins, nucleic acids and enzymes and is second only to carbon as a requirement for bacterial growth [103, 104]. Nitrogen in an appropriate concentration range is beneficial to fermentative H2 production, while at a much higher concentration can inhibit the process performance by affect­ing the intracellular pH of bacteria or inhibiting specific enzymes related to H2 production [105-107]. Optimal nitrogen concentrations of 0.1 g N/l were reported for effective H2 production [104]. Substrate degradation efficiency was also found to increase with increasing nitrogen concentration from 0 to 0.01 g N/l [104]. Appropriate ratios of C/N are fundamentally important, with the optimum being 47 [103]. Phosphate helps to maintain the system’s buffering capacity during the H2 fermentative process [91]. Using phosphate as an alternative to carbonate as a buffering supplement should increase the H2 gas fraction [108]. An increase in the carbonate concentration increases the CO2 fraction in the gas phase due to carbonate dissolution. Adding phosphate at a proper concentration is a useful strat­egy for optimal H2 production [108]. Na2HPO4 affected the H2 production in a concentration-dependent way with the optimal concentration being 0.6 g/l. Using a proper carbonate and phosphate concentration formulation, the H2 production rate can be enhanced by 1.9 times which might be due to a shortening of the microflora lag-phase [108].

Wastewaters generated from food — and beverage-processing industries often have little SS but high concentrations of soluble organic compounds (up to 50,000 mg/L of biological oxygen demand, BOD) such as starch, sugars, and proteins. Some common examples of these high-strength wastewaters come from cheese factories, wineries, breweries, distilleries, slaughterhouses, potato processing, and ice cream factories. The organic compounds in these wastewaters can be readily degraded and converted to methane biogas, but the initial major objective of anaerobic treatment of such wastewaters was to degrade and reduce the organic pollutants in the wastew­aters to satisfy governmental discharge requirements. With the push for bioenergy, the focus of anaerobic treatment of high-strength organic wastewaters has been shifted to methane biogas production. High-rate AD (HRT <24 h) has been com­monly used to both reduce the organic strength of the wastewaters and recover the energy as methane biogas. UASB reactors [32] are among the most popular digesters used by many industries. A new variant of UASB reactors is the expanded gran­ular sludge bed (EGSB) reactors. The advantages of EGSB over UASB, such as improved mass transfer and digestion rate, enhanced ability to handle high-strength influents, and high hydraulic loading rates (HRT <2 h), have been well recognized [82]. Therefore, during the last decade the number of EGSB reactors built exceeds that of new UASB reactors [32].

In addition to UASB and EGSB reactors, the following digesters have also been successfully used in the AD of these high-strength wastewater streams: CSTR (sin­gle staged, e. g. [74], or two-staged e. g. [72],), anaerobic contact filter reactor [83], anaerobic filter reactor [1], down-flow fluidized bed reactor [34], internal circulation (IC) reactors [29], and anaerobic hybrid reactors [14]. The sand-bed filter reactor manufactured by NewBio E Systems, Inc, [91] is another promising AD technology for such wastewater (unpublished data). Compared to the digesters used to digest feedstocks with high SS, most of these reactors have much higher loading rates. Thus, they have smaller footprints, but they need to be operated by well-trained digester operators. Detailed descriptions of each of these reactors and vendors is beyond the scope of this chapter, but interested readers should consult other recent books [79, 91] or reviews [10, 77]. Anaerobic treatment or digestion of specific high-strength wastewaters have also been extensively reviewed (e. g., see [56, 57, 66] for distillery wastewaters; and [18] for meat — and potato-processing and dairy wastewaters).

It should be noted that performance data from an existing AD digester can only be regarded as broadly indicative of how a similar AD technology may perform elsewhere, especially with respect to stability, efficiency of organic removal, biogas yield and quality. Only through studies using laboratory — and pilot-scale AD reactors on the feedstock of interest can the most suitable AD technology be identified for that feedstock.