The NER values for coupled production of biodiesel and biogas (residues) from

H. pluvialis and Nannochloropsis are 0.4 and 0.09, respectively, with approximately 58 and 76% of the output energy coming from biogas [488]. The sensitivity analysis showed that it is not possible to achieve an NER value larger than 1 even at most optimistic algal yield and oil content. Pumping, drying, and cell-disruption are the most energy consuming steps. Lardon et al. emphasize that integration of ADP into biodiesel production is a promising solution for external energy demand reduction and partial recycling of essential nutrients [489].

Energy output from T. suecica was reported for several conversion techniques (Fig. 22) [435]. Anaerobic digestion as a sole conversion technology has the largest energy output compared to biodiesel and bioethanol. Coupling of biogas production with biodiesel production gave slightly larger total energy output, and decreased the estimated biodiesel production cost from $72 to $47 per liter. But this cost is significantly larger than the current cost of petroleum-based biodiesel.

The life-cycle assessment of microalgal cultivation and biogas production shows that the NER value is equal to 1.51 for the following conditions: C. vulgaris is culti­vated in ponds with area of 100 ha, productivity of 25 g/m2-day, carbon dioxide sup­plied from biogas purification and methane combustion, and the supernatant liquid from the digester provides a portion of the fertilizers necessary for algal growth [490]. Technical parameters of the ADP include: CSTR digester, methane yield 0.292 L/ gVS, OLR 1 gVS/L-day, HRT 46 days, and algal biodegradability of 56%.

Zamalloa and colleagues reviewed three scenarios of growing algae in wastewa­ter effluent with productivity 20, 25, and 30 g/m2-day [491]. The algal production facility utilized carbon dioxide produced during electricity production. For all sce­narios, the NER values are larger than 1 and equal to 2.48, 2.67, and 3.34, respec­tively. This means that the energy output is larger than energy demand for algal production and anaerobic digestion. The major energy consuming processes are digester heating, mixing, algal pre-concentration, and pumping. The cost of bio­mass for three scenarios were $169, $138, and $117 per kg of dry weight and the levelized cost of energy were $0.232, $0.154, and $0.119 per kWh (given €1/$1 equal to 1.3652). A minimum algal productivity of 25 g/m2-day is required in order to achieve profitability with an OLR of 18 gVS/L-day and methane yield of 0.5 L/gVA. The fermentation of at least 75% of the VS is crucial for an economically feasible process. These assumptions are relatively optimistic but can be achieved in advanced anaerobic reactors.

6 Conclusions

Cyanobacteria and algae are feasible feedstocks for biogas production through ADP. Moreover, with current understanding and technology, the anaerobic digestion of algae has the promise to be the technology that can be applied for biofuel produc­tion in the nearest future. Despite more than 60 years of research and several advan­tages, the technology of methane production from algae is still far from wide application at a large scale. One of the reasons for that is the vast diversity of algae and cyanobacteria. They have different cell morphology, ecology, photosynthesis biochemistry, cell structure, and biochemical composition.

Further research directions that are critical for making ADP with algae

economically feasible for successful commercialization include:

• Engineering of efficient systems for algal cultivation and anaerobic digestion:

— Design of algal production units for better light illumination, penetration, car­bon dioxide dissolution, and oxygen off-take

— Design algal harvesting and dewatering units

— Develop methods of biomass pretreatment for greater VS reduction and higher methane yield

— Design of anaerobic reactors with lower HRT and high SRT

— Design of biogas upgrading systems

• Isolation and characterization of prospective natural organisms:

— Isolation of algal strains with high production potential, biochemical compo­sition with high colorific value, lower recalcitrance, robustness for cultivation in waste streams and to aeration by exhausted gases

— Isolation of anaerobic bacteria responsible for digesting algal biomass in nature

• Developing and application molecular biology and genetic tools, and “omics”

technologies for cyanobacteria, algae, and anaerobic organisms to target goals

such as:

— Decreasing algal photoinhibition, increasing photosynthesis efficiency and carbon dioxide uptake

— Algal simultaneous lipid accumulation and high growth rate

— High digestibility of algal biomass

— Algal persistence to bacterial and viral infections

— Development of stronger hydrolytic apparatus of anaerobic bacteria

— Robustness of methanogens for fluctuations in environmental and operational parameters

• Integration of algal production and AD with other technologies:

— Co-digestion with domestic, industrial, and agricultural wastes can improve the C:N:P balance

— Algal cultivation in wastewaters containing organic carbon and nutrients

— Co-location of algal production with carbon power plants that can be a source of carbon dioxide and waste heat

— Application of algal anaerobic digestion with other biofuel production pro­cesses or production of high-value products from algae

• Development of information technologies:

— For techno-economical analysis

— Dynamic modeling of systems

— Bioinformatics technologies

— Metabolic networks modeling

Acknowledgments This research was supported by The Bureau ofEducation and Cultural Affairs of United States Department of State though an International Fulbright Science and Technology Award to Pavlo Bohutskyi.