Biogasification (or anaerobic digestion) is a biochemical process that converts organic matter to biogas (a mixture of methane, 50-70%, and balance carbon dioxide) under anaerobic con­ditions. Biogas can be used as a replacement for natural gas or it can be converted to electric­ity. The process is mediated by a mixed, undefined culture of microorganisms at near ambient conditions. Several terrestrial biomass feedstocks (agricultural residues, urban or­ganic wastes, animal wastes and biofuel crops) have been anaerobically digested and com­mercial scale digesters exist for the biogasification of such feedstocks.

Anaerobic digestion offers several advantages over other biofuel production processes like ethanol fermentation or thermochemical conversion. The microbial consortia in an anaerobic digester are able to naturally secrete hydrolytic enzymes for the solubilization of macromo­lecules like carbohydrates, proteins and fats. Therefore, unlike in ethanol fermentation proc­ess there is no need to incorporate a pretreatment step to solubilize the macromolecules prior to fermentation. In addition, since the process is mediated by a mixed undefined cul­ture, issues of maintaining inoculum (or culture) purity does not arise. Being a microbial process, there is no need to dewater the feedstock prior to processing unlike in thermochem­ical conversion where the feedstock is dried, to improve net energy yield. This is advanta­geous when it comes to processing aquatic biomass as these can be processed without dewatering. The anaerobic digestion process will also mineralize organic nitrogen and phos­phorous, and these nutrients can be recycled for algae growth [37].

The process primarily takes place in four steps. A mixed undefined culture of mciroorgan — isms mediates hydrolysis, fermentation, acetogenensis and methanogenesis of the organic substrates as shown in Figure 3. During hydrolysis, the complex organic compounds are broken down into simpler, soluble compounds like sugars, amino acids and fatty acids. These soluble compounds are fermented to a mixture of volatile organic acids (VOA). The higher chain VOAs like propionic, butyric, and valeric acids are then converted to acetic acid in the acetogenesis step. Acetic acid is converted to methane during methanogenesis. Hydrogen and carbon dioxide are also liberated during fermentation and acetogenesis. A different group of methanogens converts hydrogen and carbon dioxide to methane. This mixed microbial culture thrives in the pH range of 6-8. Digestion can be performed either at mesophilic conditions (30 — 38°C) or thermophilic conditions (49 — 57°C).

Aquatic biomass — macrophytes [38], micro and macro algae, have all been tested as feed­stock for biogasification. Microalgae have proportions of proteins (6-52%), lipids (7-23%)

and carbohydrates (5-23%) that are strongly dependent on the species and environmental conditions [39—41]. Compared with terrestrial plants microalgae have a higher proportion of proteins, which is characterized by a low carbon to nitrogen (C/N) ratio. The average C/N for freshwater microalgae is around 10.2 while it is 36 for terrestrial plants [40]. Usually the digestion of terrestrial plants is limited by nitrogen availability; however for microalgae this situation does not arise. Besides carbon, nitrogen and phosphorus, which are major compo­nents in microalgae composition, oligo nutrients such as iron, cobalt, zinc are also found [42]. These characteristics of microalgae make it a good feedstock for anaerobic digestion.

Previous studies have shown that macro algae like Ulva lactuca, Gracillaria vermiculophylla, Saccharina latissima etc. can be anaerobically digested producing methane at yields ranging from 0.1-0.3 LCH/g volatile solids (VS) [43]. Methane yields of microalgae like Spirulina pla — tensis (fresh water), and Scenedesmus spp. and Chlorella spp. (fresh water) ranged between 0.2 and 0.3 L CH4/g VS [44, 45] when these were codigested with other feedstocks like dairy manure and waste paper sludge, whereas other microalgae like Tetraselmis sp (marine), Chlorella vulgaris (fresh water), Scendesmus obliquess (fresh water) and Phaeodactylum tricornu — tum (fresh water) produced an average methane yield ranging from 0.17 to 0.28 L CHyg VS [45—47] when digested as sole feedstock. Table 3 summarizes microalgae digestion studies reported in the literature. The Table also lists the methane yield of cellulose powder as a benchmark to compare the methane potentials of microalgae feedstocks. Depending on the

type of microalgae, the methane potentials range from 5 to 78% of methane potential of cel­lulose. Choice of microalgae has an impact on the methane yield.

More recently when Nannochloropsis oculata was biogasified [48] in laboratory scale digesters at thermophilic temperature, the methane yield obtained was 0.20 L at STP/g VS. N. oculata was chosen because it can be grown easily in brackish or seawater, has a satisfactory growth rate and can tolerate a wide range of pH (7-10) and temperature (17 — 27° C). N. oculata is not rich in lipids but contains predominantly cellulose and other carbohydrates, which makes it a good feedstock for anaerobic digestion instead of biodiesel production. On a % (w/w dry matter) basis, the composition of N. oculata is: 7.8% carbohydrate, 35% protein and 18% lip­id. Rest of the components are amino acids, fatty acids, omega-3, unsaturated alcohols, as­corbic acid [49]. About 88% of the carbohydrate is polysaccharide. Of the polysaccharides, 68.2% is glucose, and the rest are fucose, galactose, mannose, rhamnose, ribose and xylose.

Based on N. oculata growth observed in the pilot raceways and the methane yield from di­gestion of this alga, an analysis was carried out to estimate energy production and land re­quirements. Currently the algae harvesting rate from the raceways are 9.64 g ash free dry weight (afdw)/m2/d. Note that afdw (ash free dry weight) is the same as volatile solids con­tent. An often cited study for algae growth has yielded a much higher productivity of 50 g afdw/m2/d for Platyomonas sp [50]. The algae biomass yield obtained in this study was only about 20% of the productivity potentially attainable. Optimization of growth conditions for N. oculata may improve its productivity. Using the methane yield value of 204 L/kg VS for anaerobic digestion of N. oculata, the annual energy output from a facility that grows the al­gae and subsequently digests it would be 27 MJ/m2/year. The area occupied (or footprint) of the digester(s) would be far less than the land area required for growing the algae. If the methane produced from this facility is converted to electricity, the electrical energy output would be 2.25 kWHe/m2/year assuming that the efficiency of converting thermal energy to electrical energy is 30%. The household electrical energy and natural gas consumption in the US for the year 2010 was 11,496 kWH/year and 2070 m3/year respectively. If the algae bioga­sification facility were to supply the entire electrical energy requirements for a household, the land area required would be 5108 m2 (1.26 acres). If in addition, the facility were to sup­ply the natural gas needs, then an additional 2900 m2 (0.77 acres) would be needed. In other words ~2 acres of land could supply all the energy needs of a household in US. If the algae productivities were improved then land requirement could be further reduced. At 50 g afdw/m2/d algae productivity, the land requirement would only be about 0.4 acres.

Despite useful methane production potential from biogasification and the ability to process dilute algal slurries in a digester, there are challenges to be overcome to commercialize this approach for producing bioenergy from microalgae. One bottleneck is that some feedstock characteristics can adversely affect anaerobic digestion. Unlike defined cultures used for production of biofuels like ethanol or butanol, the microbial consortia in an anaerobic di­gester is capable of secreting extracellular enzymes to hydrolyze and solubilize macromole­cules like cellulose, hemicellulose, proteins and fats. This characteristic has enabled several terrestrial biomass feedstocks like sugarbeets, sugarbeet tailings, napier grass, sorghum and aquatic biomass like water hyacinth and giant kelp to be successfully digested using practi­cal retention times. However, degradability of feedstocks containing high fraction of lignin (for example sugarcane bagasse, switchgrass, miscanthus and woody biomass like pine, eu­calyptus) is poor in an anaerobic digester. The refractoriness of these feedstocks has been at­tributed to low moisture, crystalline nature of the cellulose, and complex association of the component carbohydrates within lignin [51]. As seen from Table 3, the digestibility of micro­algae varies. Species with no cell wall or cell encapsulation composed of proteins like Chlor — ella vulgaris and Phaeodactylum tricomutum, has a higher yield of methane. Dunaliella tertiolecta has very low methane yield of 0.018 L/kg VS due to the presence of a cell wall con­sisting of cellulose fibers distributed within an organic matrix. So depending on the type of microalgae used it may be necessary to carry out some form of pretreatment of algae to im­prove methane yield and rate of methane production. The type of pretreatment may depend on algae type.

4. Conclusion

Aqueous and marine biomass can be processed into a variety of sources of energy. Due to the extreme dilution in water, non-thermal processes such as anaerobic digestion, fermenta­tion to bioalcohols, and lipid extraction are logical and useful methods to utilize key compo­nents of microorganisms to produce biofuels for the replacement or supplementing of traditional fossil fuels. However, thermal methods such as gasification of wet biomass may play a role in producing specialty fuels such as jet fuel that require a specific ratio of higher hydrocarbons that would prove otherwise difficult to manufacture, even given the require­ment of intense drying.

In order for biofuels sourced from aqueous and marine biomass to secure a market share in the world, research and development needs to further nature’s ability to produce higher concentrations of biomass with targeted characteristics and reduced footprints, while better utilizing available nutrients. This will allow for an ample supply of biomass to be produced without competition with the human food chain, that can be used renewably produce fuel that can power the world’s mobile fleet.

Author details

Robert Diltz1 and Pratap Pullammanappallil2 *Address all correspondence to: Robert. diltz@us. af. mil

1 Air Force Research Laboratory, Tyndall AFB, FL, USA

2 Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL, USA