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.