To enhance biomass productivity, irradiance on the surface of the solar receiver tubes must be maximised. Solar irradiance is the power of electromagnetic radiation on the tubular surface which varies with location and weather conditions. The solar receiver tubes must be configured to maximise the irradiance on the surface tubes; thus the appropriate configuration should be chosen for the designated location.

The surface on which the tubes are located can be painted or covered with white sheeting to take advantage of the albedo effect. The albedo effect refers to improved irradiance by means of reflection [1]. In retrospect, the design of the photobioreac­tor is not as significant as the source of illumination. Generally, light is always the limiting factor with regard to algal cultures [7] .

The annual costs represent a range of expenses incurred in the running of the plant from payroll charges to maintenance costs. The most significant of these costs is depreciation, which is calculated using the FCI based on 10-year plant life as rec­ommended in Peters et al. [26] . All other annual costs were calculated using the methods specified in Molina Grima et al. [21] with the exception of labour, supervi­sion, wastewater treatment, and goods and services tax. Labour was assumed to be constant, with 12 employees working during the day and 3 working at night, each charged at the standard labour hourly rate given in ENR (US $34.16). Supervision was also assumed to be constant, with two managers working during the day and one at night, charged at the skilled labour hourly rate again outlined in ENR (US $44.99). Wastewater treatment cost was also estimated based on the costing data reported by Molina Grima et al. [21]. Finally, goods and services taxes were charged at a rate of 10%, reflecting the tax codes applicable to Australia.

After the goal and scope of a study have been specified, a life cycle inventory (LCI) is typically carried out. The LCI is the accounting stage in which all the physical flows are reconciled with known emissions data to quantify the environmental bur­dens and resource requirements over the entire life cycle [1]. The outcome from this process is typically an exhaustive list of emissions factors; many more than can be reasonably expected or necessary in a report. Therefore, an important step in devel­oping an LCA is the process of simplifying raw LCI data into specific metrics. Table 2 lists the impact metrics used in a few recent LCA papers of algae-to-energy systems. The differences in study endpoints contribute to the difficulties in compar­ing the results. The decision to include some metrics and exclude others can have important implications for the results and interpretation of the study. Most LCA guidebooks divide impact categories into three principle categories: resource use, ecological consequences, and human health [1]. Each category is discussed briefly here in the context of algae-to-energy systems.

1.1 Resource Use

Resource use is the most straightforward of the impact factor categories because the metrics involved are typically simple sums of flows from the environment. For example, total nonrenewable energy use, normalized by energy content, is a com­monly encountered metric. Total land use is an important resource metric that has been hotly debated by the life cycle community because of the important upstream or indirect land use that is required to maintain the productivity of the agricultural region (e. g., land associated with production of fertilizer) or because of land could be used for alternative uses if not for agriculture (e. g., primary growth forest). Similarly, total water use is a resource that is relevant for most biofuel life cycle studies as shown in recent work [9] . An important distinction when it comes to water use is that of consumptive vs. nonconsumptive use. Most energy generation facilities use a large amount of water, primarily for cooling, so even though the amount of water needed for these systems is large, a comparatively small amount of the water is actually consumed [16].

Most models of biofuels systems include, at a minimum, total net energy use as a metric. This is an obvious and important metric because many biofuels such as ethanol consume a considerable amount of fossil fuels to generate a certain amount of ethanol. Recognizing that biofuels are not worth pursuing if there is no energetic gain, many studies have explored the net energy balance associated with alternative energy options. Algae-derived energy is no exception, and several studies report on the energy that is required to produce energy carriers from algae. Whether these estimates are net positive or net negative depends on the modeling assumptions selected in the study. In addition to energy use, there are at least two other impact factors that should be considered when evaluating algae-to-energy systems. The first is land use. Algae grow more efficiently than terrestrial crops, and so quantify­ing this parameter is important as a means to highlight one of algae’s most pro­nounced advantages. Similarly, water use is an important parameter since large-scale algae cultivation is likely to require large volumes of water. How much, and how this relates to the water use of terrestrial crops is likely to be an important factor in water-limited growing regions. Including water as a key metric is important.

T. suecica was cultivated in outdoor bag bioreactors using a modified F medium [16]. Each bioreactor contained up to120L of culture and was aerated with com­pressed air. Temperature and illumination depended on day-to-day weather condi­tions. Microalgal cultures from multiple bioreactors were harvested at the same time (with a concentration of ~0.5 g/L), concentrated via industrial centrifugation, and then mixed together to create a homogeneous culture from which all the bio­mass needed for the study was obtained.

3.2.2 Dewatering

The homogeneous culture was further dewatered in a laboratory centrifuge (Heraeus Multifuge 3S-R, Kendro, Germany) at 4,500 rpm for 10 min. The supernatant was discarded and the resulting microalgal paste was rinsed with deionised water to remove residual salts. The paste was then stored in the dark at either 4 or -20°C until further use.

3.2.3 Extraction Pre-treatment

In experiments where extraction was carried out on dried microalgae, microalgal paste (stored at either 4 or -20°C) was dried at 65°C in an oven (Model UNE 500 PA, Memmert GmbH + Co., Germany) for 16 h. A pestle and mortar was used to grind the dried biomass into powder. In experiments where extraction was carried out on the wet paste, the microalgal paste obtained from centrifugation (stored at 4°C) was used directly. The solid concentration of the paste (22.4 wt.%) was deter­mined by drying a portion of the paste and comparing its pre-dried mass to that of the corresponding dried biomass.

Pavlo Bohutskyi and Edward Bouwer

Abstract Anaerobic digestion is a common process for the treatment of a variety of organic wastes and biogas production. Both, macro — and microalgae are suitable renewable substrates for the anaerobic digestion process. The process of biogas production from algal biomass is an alternative technology that has larger potential energy output compared to green diesel, biodiesel, bioethanol, and hydrogen pro­duction processes. Moreover, anaerobic digestion can be integrated into other con­version processes and, as a result, improve their sustainability and energy balance. Several techno-economic constraints need to be overcome before the production of biogas from algal biomass becomes economically feasible. These constraints include a high cost of biomass production, limited biodegradability of algal cells, a slow rate of biological conversion of biomass to biogas, and high sensitivity of methanogenic microorganisms. The research opportunities include a variety of engineering and scientific tasks, such as design of systems for algae cultivation and anaerobic digestion; optimization of algae cultivation in wastewater, nutrients recy­cling and algal concentration; enhancement of algal biomass digestibility and con­version rate by pretreatment; deep integration with other technological processes (e. g., wastewater treatment, co-digestion with other substrates, carbon dioxide sequestration); development and adaptation of molecular biology tools for the improvement of algae and anaerobic microorganisms; application of information technologies; and estimation of the environmental impact, energy and economical balance by performing a life cycle analysis.

P. Bohutskyi • E. Bouwer (*)

Department of Geography and Environmental Engineering, Johns Hopkins University, 3400 North Charles Street, Ames Hall 313, Baltimore, MD 21218, USA e-mail: bouwer@jhu. edu

J. W. Lee (ed.), Advanced Biofuels and Bioproducts, DOI 10.1007/978-1-4614-3348-4_36, 873

© Springer Science+Business Media New York 2013

1 Introduction

People have been using anaerobic digestion processes (ADP) for centuries, but the first documented digestion plant was constructed in Bombay, India in 1859 [1]. The first usage of biogas from a digester plant was reported in 1895 in Exeter, England where biogas was used for street lighting [2]. Approximately 15 million digesters, including small farm-based digesters, are now operated in China [3, 4]. And about 12 million digesters are located in India [3, 5].

High fuel prices coupled with an increasing awareness of greenhouse gas emis­sions and global warming have promoted an interest in further anaerobic digestion (AD) research and industrial applications. Now, the ADP is viewed not only as a method for treatment of sewage biosolids, livestock manure, and concentrated wastes from food industry, but also as a potentially significant source of renewable fuel. The biogas gross production (Table 1) within developed countries has nearly doubled from 2000 to 2007 [6].

Different agricultural crops and terrestrial and aquatic plants are proven to be an appropriate feedstock for AD [7]. Indeed, the National Algal Biofuels Technology Roadmap 2010 noted that anaerobic digestion is an underutilized technology for algal biofuel production that “eliminates several of the key obstacles that are respon­sible for the current high costs associated with algal biofuels, and as such may be a cost-effective methodology” [8] . For instance, the AD of algal biomass to biogas possesses advantages compared to other biofuel sources and conversion techniques, such as:

• Higher productivity. Algae have a higher conversion efficiency of light energy to biomass compared to plants, up to 5-10% vs. 0.5-3% [9-12].

• Water quality is less critical. Wastewater, brackish water and even seawater can be used for algae culturing in addition to fresh water.

• Noncompetitive to food production. Algae can be cultivated on nonarable lands and in the ocean.

• Carbon dioxide sequestration. Algae convert carbon dioxide into biomass, and culture media can be enriched with carbon dioxide from gases exhausted from power plants or other sources.

• Elimination of several energy consuming steps. The ADP does not require dry­ing and an extraction steps as well as a high extent of algal biomass dewatering.

• Deeper level of algal biomass utilization is possible. The ADP can convert all fractions of organic matter, including lipids, proteins, carbohydrates, and nucleic acids to biofuel.

• Partial recycling of nutrients with AD effluent. Anaerobic digestion is a natural conversion process that releases nutrients in a potentially usable and recyclable form. The supernatant liquid with higher nitrogen and phosphorus content can be used as a fertilizer for algae culturing. Moreover, the solid phase can be used as a biofertilizer in agriculture or as a livestock nutrient.

• Integration with other technologies is possible. For instance, the ADP can be used as a co-technology for algal residues utilization after biodiesel, green diesel,

bioethanol, and hydrogen production. Furthermore, a variety of organic wastes and by-products can be co-digested with algae to produce biogas.

• Environmental friendly process. No toxic materials are produced during ADP.

Nevertheless, the process of methane production from algae has several limita­tions that need to be overcome to become an attractive technology for producing renewable energy:

• High capital cost of algae production and AD units.

• Relatively low algae productivity. Algae growth rate is relatively limited by low efficiency of photosynthesis, photoinhibition, and carbon assimilation.

• Incomplete digestibility of algal cells. The algal biomass partially contains recal­citrant organic matter that cannot be hydrolyzed by the conventional ADP.

• Conversion rate is relatively slow. Generally, biomass residence time in the ADP varies between 10 and 30 days.

• In some cases, algal biomass has an unbalanced C:N ratio. A low ratio can lead to the accumulation of NH4+ in an anaerobic digester to inhibitory levels while lack of nitrogen can limit anaerobic conversion and methane production.

• High sensitivity of the ADP. Methanogenic organisms are sensitive to fluctuations of environmental and operational parameters.

This chapter provides a literature review and analysis of biogas production from algal biomass though ADP. In the first part, we describe morphological, ecological, and biochemical characteristics of cyanobacteria and three major algae groups as well as their current commercial applications. The second part provides background on ADP and focuses on the algae anaerobic digestion research in the past several decades. Finally, we discuss prospective methods for enhancement of algae produc­tion and anaerobic digestion with emphasis on metabolic manipulations, genetic engineering, algae pretreatment, co-digestion with other feedstocks, and integration of algae AD into other technological processes.

Acidic, alkali, and oxidative pretreatment approaches have been applied to enhance the biological degradation of biomass. Pretreatment by acid reagents is commonly used for solubilization of hemicellulose and lignin compounds that make other organic materials more available for enzymatic attacks. Also, during acidic pretreat­ment, the hemicellulose is hydrolyzed to sugar monomers, furfural, hydroxymeth- ylfurfural, and other products [240]. Acidic pretreatment has several possible drawbacks. First, some of the solubilized compounds can be toxic to methanogens. Second, the production of hydrogen sulfide or ammonia nitrogen instead of methane can be enhanced.

Addition of a strong alkali reagent causes solvation, saponification, alkali hydro­lysis, and degradation of polymeric organic compounds. Alkali pretreatment (62.0 mEq Ca(OH)2/L for 6.0 h) of the organic fraction of municipal solid waste increased the COD solubilization and enhanced methane yields up to 172% [241]. The influence of different alkaline reagents NaOH, KOH, Mg(OH)2, and Ca(OH)2 on solubilization of WAS has also been examined. The following levels of COD solubilization at pH 12 with the four alkaline reagents have been reported—39.8%, 36.6%, 10.8%, 15.3% [201] and 60.4%, 58.2%, 29.1%, 30.7% [225], respectively.

The effect of NaOH concentration was studied in the range from 0 to 26 g/L. COD solubilization rose significantly as the NaOH dose increased up to 5 g/L [225] or up to 7 g/L [201] . The largest biogas production or biodegradability was reported at NaOH concentrations between 4 and 10 g/L [225]. Additional amounts of NaOH led to decreasing amounts of biogas production. Based on the results from biotoxic­ity tests, the decrease in biodegradability was not caused by sodium toxicity, but by the formation of refractory compounds under extreme alkali conditions.

Several oxidative reagents have been tested for biomass pretreatment: ozone, oxygen, hydrogen peroxide, and peracetic acid. Hemicellulose and lignin are com­mon targets of oxidative pretreatment. The oxidative chemical reactions include electrophilic substitution; side chain displacement; radical reactions; and cleavage of alkyl, aryl, ether, and ester linkages [242]. Disadvantages of most oxidative meth­ods are losses of organic materials (e. g., sugars) due to the nonselective oxidation and formation of inhibitors.

The oxidative pretreatment of a wastewater sludge mixture with ozone resulted in COD solubilization from 1.3 to 29% for an ozone dose of 0.05 g/g COD and 40% at an ozone dose of 0.1 g/g COD [212]. The methane yield increased by a factor of

1.5 and 1.8, and the methane production rate increased by a factor of 1.7 and 2.2, respectively. Increasing the ozone dose (0.2 g/g COD) resulted in significant oxida­tion of the sludge organic fraction (about 30%), therefore decreasing the methane yield. The reactions of ozone with some chemicals, such as phenol and LCFA, can form products toxic to methanogens [243] .

Commercial-scale production of methane from algae requires the process to be eco­nomically feasible. Life-cycle assessment and calculation of net energy ratio (NER) are common methods to evaluate techno-economical parameters of biofuel produc­tion technology.

Although the available body of information on production from hydrates is still limited, there is sufficient information to begin identifying particular features, prop­erties, conditions, and production methods that are linked to a higher gas production potential and increase the desirability of hydrate deposits, and to use this informa­tion to develop a set of guidelines for the selection of promising production targets.

5.1 Desirable Features and Conditions

These include the following [125]:

• Large formation k and j, which are almost invariably associated with sandy and gravely formations that are characterized by low P, S and S, leading to

cap ’ wir’ gir

relatively high permeability to gas and aqueous flow.

• Medium SH (i. e.,30% < SH < 60%) corresponds to optimal QP in terms of magnitude and length of time to attain it. The effect of SH on production is not monotonic, but a complex function of SH and the timeframe of observation. A lower SH has the advantage of higher keff, leading to an earlier evolution of gas and a larger initial QP [131,132]. The disadvantages of a lower SH may include a larger water production and a lower total gas production because of early exhaustion of the resource. A high SH leads to slower evolution of gas and lower initial QP, but a higher maximum QP and total production.

• The most desirable targets can be easily identified from the inspection of the phase diagram (Fig. 14). The larger T provides a larger source of sensible heat to support the endothermic dissociation, and a larger initial P allows a larger pres­sure drop, leading to larger production rates. Thus, (a) hydrates that occur along the equilibrium line are very desirable, and (b) the desirability increases with an increasing equilibrium P (and, consequently, T). The production potential decreases as the stability of the hydrate deposit at its initial conditions increases (as quantified by the pressure differential DP = P-Pe at the prevailing reservoir T). In practical terms: we target the deepest, warmest reservoirs that are as close as possible to equilibrium conditions. In addition, the deeper reservoirs have larger overburdens and are therefore less prone to adverse geomechanical impacts.

• For reservoirs with the same hydraulic properties, SH, and P: the warmest possi­ble deposit is the most desirable. For reservoirs with the same hydraulic proper­ties, SH, and T: the reservoir with the lowest possible P is the most desirable.

• In terms of deposit classes: All other conditions and properties being equal, Class 1 deposits appear to be the most promising targets for gas production because of the thermodynamic proximity to the hydration equilibrium. Additionally, the existence of a free gas zone guarantees gas production even when the hydrate contribution is small.

• Class 2 and Class 3: Class 2 deposits can attain high production rates, but are also burdened by longer lead times of very little gas production; Class 3 deposits may yield gas earlier and can attain significant production rates, but there are indica­tions that these are lower than in Class 2. The relative merits of these two types will likely be determined by site-specific conditions.

• All classes: The difficulties of site access notwithstanding deeper and warmer oceanic accumulations appear to be more productive than permafrost ones they can have (a) a higher T (14°C is the maximum equilibrium temperature observed in permafrost-associated deposits) and a larger sensible heat available for disso­ciation and (b) a higher P, increasing the depressurization effectiveness, in addi­tion to (c) the beneficial dissociating effect of salt.

• All classes: The importance of impermeable or near-impermeable upper bound­aries cannot be overemphasized.

• In terms of production method: Depressurization appears to have a clear advantage in all three classes.

The growth of terrestrial plants is limited by a variety of factors, depending on growth location and conditions. What is seldom limiting, however, is reductant, i. e., solar photons. Rather, plant growth is typically limited by water, nutrients, or CO2. Likewise, an Electrofuels approach to the production of liquid fuels will be limited by a variety of factors, depending on growth conditions. Ultimately, the two factors most likely to provide insurmountable limits to production are reducing equivalents and inorganic carbon.

The approaches under consideration in the Electrofuels program utilize various means to assimilate reducing equivalents, and the limiting factor to uptake will vary as the approach. In the case of hydrogen-utilizing organisms, the aqueous solubility of hydrogen is likely to be limiting, at least under some growth conditions. Incorporated into the program are multiple efforts aimed at novel bioreactors that, at least to some extent, ameliorate these concerns. The growth of electrotrophs will ultimately be limited by the ability of organisms to assimilate electrons. The mechanisms by which these unique species take up electrons remain unclear, and the limits to growth mandated by such processes are similarly opaque. Still, if organisms must be physically anchored to an electrode through, for example, conductive pili, then production will be limited by the accessible surface area of a highly porous elec­trode. On the other hand, if conduction can proceed through at least some thickness of a biofilm, the effective surface area could be multiplied manyfold, greatly increas­ing the potential for growth. In any event, the ability to assimilate reducing equiva­lents will ultimately provide a limit to fuel production.

The provision of inorganic carbon likewise presents challenges for an Electrofuels approach to fuel synthesis. Plants autonomously assimilate CO2 from the atmo­sphere; while the very low concentrations of CO2 in air limit growth, the carbon is at least free and essentially limitless. In an Electrofuels approach, inorganic carbon will be furnished at concentrations beyond that available directly from air. Such sources might include the effluent of conventional power plants, cement kilns and the like, direct mining of geologic CO2, carbonate or bicarbonate, or CO2 produced as a coproduct during the production of conventional biofuels from biomass, which necessarily release significant quantities of the total carbon as CO2. The extraction of inorganic carbon from seawater may be feasible as well. While any of these approaches would decrease the Nation’s dependence on foreign sources of oil, only some are carbon-neutral, while others would contribute to anthropogenic atmo­spheric carbon loads during combustion.

Microalgal biomass requires pre-treatment before fermentation to enhance bioetha­nol production. Biomass pre-treatment process is a major contributing factor to bio­ethanol production cost. The carbohydrates of microalgae are stored inside the cell walls and between the intercellular matrices [60]. Thus, it is necessary to rupture the microalgal cell walls so that the entrapped carbohydrates can be released and, if

Fig. 12 Process flow diagram of bioethanol production from microalgae

necessary, broken down into simple sugars to be utilized in the fermentation process. Cell disruption methods can be classified as mechanical, chemical and biological [44,47, 58]. Each of these methods is able to liberate the sugar molecules.