Chapter 36
Biogas Production from Algae and Cyanobacteria Through Anaerobic Digestion: A Review, Analysis, and Research Needs 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:
[email protected] J.W. Lee (ed.), Advanced Biofuels and Bioproducts, DOI 10.1007/978-1-4614-3348-4_36, © Springer Science+Business Media New York 2013
873
874
1
P. Bohutskyi and E. Bouwer
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,
36 Biogas Production from Algae and Cyanobacteria Through Anaerobic Digestion…
875
Table 1 Gross production of biogas in countries (2000 and 2007) Biogas—gross production (TJ) Country/area 2000 year 2007 year United States Germany United Kingdom France incl. Monaco Italy and San Marino Australia Republic of Korea Spain Total
123,966 23,341 33,912 6,133 5,480 5,780 1,380 5,492 205,484
183,674 100,628 66,657 16,896 16,240 11,643 7,912 7,693 411,343
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.
876
P. Bohutskyi and E. Bouwer
2 Algae as the Feedstock for the Anaerobic Digestion Process Algae are a large and very diverse group of organisms ranging from simple unicellular microalgae to giant macroalgae. The morphology of macroalgae or seaweeds resembles the terrestrial plants but the biochemical composition is significantly different. The major carbon storage products in terrestrial plants are starch and fructosan [13–15] that can be easily converted to biogas. However, the main components of terrestrial plant cell walls are cellulose/hemicellulose fibers embedded into a pectin matrix and cemented together by lignin [16–18]. This lignocellulosic complex is recalcitrant to biological degradation and requires intensive chemical (acid hydrolysis, alkaline wet oxidation, ammonia fiber expansion) or thermal pretreatment (steam explosion, hot water) before biological conversion [19–22]. The major components of macroalgae are polysaccharides, algal cell wall lack lignin. The main components of cell envelopes are ulvan and xylan in green algae; carrageen, agar, and xylose in red algae; alginate and fucoidan in brown algae. Cellulose is a structural component of the cell wall in many genera, but only in some green algae is the ratio on a level comparable to terrestrial plants. The main storage polysaccharides in macroalgae are floridean starch in red algae; chlorophycean in green macroalgae; laminarin; and mannitol in brown macroalgae. The biochemical composition of microalgae and cyanobacteria are significantly different from macroalgae. Often carbohydrates are a minor component of cell dry weight, whereas proteins and lipids account for the bulk of microalgal dry weight. One of the challenges in AD of algae is significant variation in biochemical composition not only among different phylum or genera, but also among similar species. Biochemical composition depends on many environmental factors, such as temperature, salinity, light intensity, and nutrient availability [23–27].
2.1
Cyanophyta (Blue-Green Algae)
The Cyanophyta is a unique group of prokaryotic microorganisms and a member of a large group of photosynthetic organisms [28]. In contrast to purple and green bacteria, the photosynthetic mechanism of cyanobacteria is oxygenic and similar to the photosynthesis mechanism in plants and algae. Several filamentous blue-green algae are able to form heterocysts, which contain the enzyme nitrogenase and fix atmospheric nitrogen [29]. Cyanobacteria possess chlorophyll a and phycobiliproteins as part of their light harvesting antennae [30]. But cyanobacteria lack membrane-bound cell organelles (nucleus, mitochondria, chloroplast), which are defining characteristics of the Eukaryotic Kingdom [31]. Cyanobacteria are found elsewhere in marine, brackish water, freshwater, and terrestrial habitats with a variety of morphological forms: unicellular and colonial non-motile, colonial, and filamentous [32, 33]. The characteristics of Cyanophyta are presented in Table 2.
36 Biogas Production from Algae and Cyanobacteria Through Anaerobic Digestion…
877
Table 2 Cyanophyta species major organic matter characteristics Characteristic Description
References
Nutrient reserves
[31, 492–496]
Cell wall organization
Cyanophycean starch (a-1,4-glucan) as carbon and energy; cyanophycin (arginine and asparagine polymer) as nitrogen storage; polyphosphate as phosphorus storage; poly(hydroxyalkanoate) Multiple-layered. Envelope consists of cytoplasmic membrane and cell wall. Optional outer membrane, s-layer, sheath, capsule, and slime. Four-layered peptidoglycan (murein) is principal component. Consists of glycan backbone with peptide cross linkages
Table 3 Biochemical and chemical composition of selected cyanobacteria Anabaenopsis Component Arthrospira maxima Arthrospira platensis sp. Ash Carbohydrates Protein Lipids
– 10–16 64–70 6
References
[505]
– 10–16 62–72 6–7
9.35 41.3 41.2 8.1
[497–504]
Oscillatoria deflexa 9.1 10 54.5 13.8
[506]
[507]
Table 4 Productivity of cyanobacteria Species
Reactor type
Arthrospira sp. Outdoor airlift tubular undulating row (11 L) A. platensis Outdoor tubular undulating row (11 L) A. platensis Dairy wastewater anaerobic lagoon effluent (1 L) A. platensis Indoor fermenter (4 L) A. maxima A. maxima Open pond
Parial Pvolume (g/m2-day) (g/L-day) References 25.4 47.7 70
1.15 2.7 0.07
[508] [509] [480]
– – –
0.17 0.16 0.21
[505] [510]
Cyanobacteria are used for a variety of purposes including as a food and feed supplement due to their high protein (Table 3) and vitamin content, as a good source of fiber, and for their good digestibility. Other current and prospective applications of cyanobacteria include the production of pharmaceuticals (antiviral, antibacterial, antifungal, and anticancer compounds), enzymes, wastewater treatment, and use as a biofertilizer [34, 35]. Cyanobacterial species are characterized by high productivity (Table 4).
878
P. Bohutskyi and E. Bouwer
Table 5 Rhodophyta species major organic matter characteristics Characteristic Description
References
Floridean starch (a-1,4-glucan) in cytoplasm [511–517] for long-term storage. Sugars and glycosides (trehalose, floridoside, maltose, sucrose) are the primary products of photosynthesis Cell wall organization Multiple-layered. Amorphous mucilage from [116, 512, 518–521] sulfated polysaccharides (agars and carrageenans) about 70% from dry weight Florideophyceae—rigid cellulose polysaccharides Bangiophycidae—rigid b-1,3xylan. Outer cuticle from protein or b-1,4mannan Corralinaceae and some Nemaliales calcified with CaCO3
Nutrient reserves
2.2
Rhodophyta (Red Algae)
The Rhodophyta is a relatively well-defined group of about 6,000 algal species with several features that differentiate them from other algal divisions, such as the presence of accessory phycobilin pigments, the absence of flagella and centrioles [36]. The vast majority of red algae are marine multicellular, macroscopic species, which account for the majority of the so-called seaweeds [37]. The main habitats are nearshore and offshore zones (down to 40–60 m) in tropical and temperate climate regions while the presence of accessory pigments allow algae to grow at depths down to 200–250 m. Species with calcified cell walls are important for the establishment and support of coral reef formation. Red algae are also found in brackish and fresh water, as well as in soil [38, 39]. Porphyra species are an important food source for humans in the Asia region [40]. Several Rhodophyta species (Gelidium, Gracilaria) are an important source of agar and agarose [41]. These polysaccharides are used in many laboratories for preparing culture media and separating nucleic acids [42]. Carrageenan is widely used in the food industry as a gel forming substance and stabilizer [43] (Tables 5–7). Structural, biochemical characteristics and productivity of selected red algae species are presented in Tables 5–7.
2.3
Chlorophyta (Green Algae)
Chlorophyll a and b are the dominant pigments in Chlorophyta and are the source of the second name of these organisms—Green algae. The secondary pigments are carotenoids (b-carotene, prasinoxanthin, siphonaxanthin, astaxanthin) which sometimes give algae their yellowish-green and red-green colors [44]. The major habitat for green algae is freshwater although they are also found in sea or brackish water, and in soil [38, 39, 45, 46]. Chlorophyta species are unicellular or colonial motile and non-motile, filamentous, coccoid, parenchimatous, and siphonous [37, 47].
G. tikvahiae (Taiwan) Palmaria palmata Gracilaria sp. (Florida) P. palmata Hypnea musciformis Chondrus crispus Rhodoglossum affine Iridaea cordata I. cordata
G. tikvahiae (Florida)
Porphyridium cruentum Porphyra Gracilaria chilensis G. chilensis G. tikvahiae (Florida) G. tikvahiae (Florida) G. tikvahiae
Airlift tubular (200 L) Natural population Outdoor tank Spray culture Outdoor tank, aerated, nutrients (50 L) Same, AD effluent (2.4 m3) Tank, aerated, nutrients (2.4–24 m3) Pond, non-aerated (9 m3) Pond (non-aerated) Pond (aerated) Pond (large scale) Cage culture Pond (
