IV. B.2 a. Biodiesel Production and Algal Mass Culture for Wastewater Treatment

The laboratory work outlined earlier will be a relatively longterm effort. Even after the demonstration at the laboratory and small-scale (e. g., 1 m2) of the ability of genetically improved microalgae strains to exhibit high lipid productivities, many other factors and abilities are still required in algal mass culture. These, however, cannot be demonstrated in the laboratory. They include competitiveness, predation resistance, and harvestability. The issue thus arises of the need to carry out such algal mass culture research in parallel with the laboratory studies.

The outdoor projects of the ASP demonstrated the ability to mass culture microalgae under relatively unrestricted conditions (e. g., without a highly selective chemical environment), and to do so potentially at relatively low cost, as the inputs for power, nutrients, and water are rather modest. Considerable advances were also made in developing techniques for managing microalgae species in ponds, and demonstrating increasing biomass productivities. Of course, these subjects still require much more work. However, in the absence of improved microalgae strains, with the high total and lipid productivities required by the cost analyses, it would not be possible to make significant advances in this technology. Thus, a continuing emphasis on outdoor algal mass cultures, or on cost and resource analyses, is not recommended at this time. These accomplishments of the ASP now allow research to be focussed on the genetic work, as outlined above and in Section II, and to allow confident prediction of the ability to apply this research to mass culture systems. This is not to exclude some supporting outdoor studies, for example to verify the selection criteria used for the competitive strains for the genetic development. But, in general, the emphasis should be on genetics and strain improvements, not on outdoor culture technology development.

However, a strictly laboratory-based R&D program, may rapidly loose touch with the realities of the eventual applications. Thus, some outdoor mass culture R&D is recommended, specifically for near-term development and demonstration of a combined microalgae wastewater treatment— biodiesel production process. This recommendation is based on the potential for such systems to be developed and demonstrated rather quickly, and at relatively low cost. They would provide an early practical application of this technology, and justify the larger effort that would be required for the development of a significant microalgae-biodiesel industry. Benemann and Oswald (1996) present a detailed discussion of this approach.

IV. B.3. Conclusions

A microalgae biodiesel production system must be a solar conversion device, which operates at high efficiency and with minimal inputs at overall low cost. Cost constraints restrict consideration of such systems to the simplest possible devices, which are large unlined, open, mixed raceway ponds. Several decades of R&D in this field, in particular by the ASP, have revealed no plausible alternative to this basic design. Even some of the design details, such as the mixing devices (paddle wheels), depth (15-25 cm), mixing velocity (15-25 cm/s), CO2 transfer (countercurrent sumps), and others are fixed by the engineering and economic constraints. The commercial experience with open mass culture ponds suggests that such systems require relatively little further engineering development. Certainly, it would be of interest to determine the practical limits of such systems. Can single raceway pond scales be larger than 5 ha? What are the wind effects in such large systems? But overall, the engineering and hardware for the low-cost mass culture of microalgae cannot be considered a major R&D need in this field.

Any effort toward the development of closed photobioreactors is probably too high risk in the present context. Although such devices could have a role in the buildup and production of

inoculum (starter cultures), they are not likely to be an essential or crucial component of large-scale, low-cost microalgae culture processes for energy production.

Any future R&D program for microalgae CO2 capture and biofuels production must start with the development of the microalgae “biocatalysts.” The goal will be to construct strains via genetic engineering or other strain improvement methods that achieve very high solar conversion efficiencies and yield high lipid (oil) microalgal biomass, as required by the economic analyses. The central recommendation for a future R&D program is to emphasize such a biocatalyst development effort, building on the knowledge developed by the ASP. For a more near-term approach, there is a significant opportunity to develop and demonstrate microalgae biodiesel production as part of a wastewater treatment process; R&D in this area is also recommended. Finally, the international nature of the global warming problem now allows consideration of global impacts of such technologies. This could help justify a U. S., and an international, R&D effort, even if the impacts of microalgae biodiesel to future U. S. energy supplies were perceived to be modest.

[1] The Solar Energy Research Institute (SERI) became the National Renewable Energy Laboratory (NREL) in 1990. In this report, the laboratory may be referred to as either SERI or NREL, depending on the time period during which the work being described was performed.

[2] Diatoms. Diatoms are among the most common and widely distributed groups of algae in existence; about 100,000 species are known. This group tends to dominate the phytoplankton of the oceans, but is commonly found in fresh — and brackish-water habitats as well. The cells are golden-brown because of the presence of high levels of fucoxanthin, a photosynthetic accessory pigment.

Several other xanthophylls are present at lower levels, as well as P-carotene, chlorophyll a and chlorophyll c. The main storage compounds of diatoms are lipids (TAGs) and a P-1,3-linked carbohydrate known as chrysolaminarin. A distinguishing feature of diatoms is the presence of a cell wall that contains substantial quantities of polymerized Si. This has implications for media costs in a commercial production facility, because silicate is a relatively expensive chemical. On the other hand, Si deficiency is known to promote storage lipid

[3] Eustigmatophytes. This group represents an important component of the “picoplankton”, which are very small cells (2-4 pm in diameter). The genus Nannochloropsis is one of the few marine species in this class, and is common in the world’s oceans. Chlorophyll a is the only chlorophyll present in the cells, although several xanthophylls serve as accessory photosynthetic pigments.

2 T aken from the Proceedings of the April 1984 Aquatic Species Program Principal Investigators’ Meeting.

[4] hexane: acyclic hydrocarbons

[5]

NREL Microalgae Culture Collection strain designations are provided when relevant.

[6]The development of the SERI standard media is discussed in Chapter II. A. 1. The compositions of these media are given in T able II. A. 1.

[7] The viruses are highly infectious and grow rapidly within the algal cells. Algal growth is inhibited rapidly following viral infection. Synthesis of host DNA and RNA is shut down, and the organellar and genomic DNA is degraded. Viral gene expression entails transcription of early and late genes, and may include

[8] Imhoff Cone removals are 24-h laboratory settling tests, indicating percentage of algal biomass that spontaneously flocculate and settle (“bioflocculation”) under quiescent conditions after the culture is removed from the mixed ponds.

[9] Oocystis sp. (S/00CYS-1)

[10] Isolate and study strains suitable for mass cultures, then apply the productivity enhancement techniques developed under laboratory conditions to these strains.