Comparison of performance between two 1,000-m2 and two 12-m2 ponds, operated under otherwise similar conditions except that one pond was screened to remove zooplankton and the other one was not so treated. Data averaged for July 1978. Note agreement in terms of productivity (grams VSS/m2/d, essentially some 90% AFDW algal biomass). (Source: Benemann et al. 1979.)

Table III. A.2. Productivity and settleability of microalgae in the 0.1-ha high-rate ponds.

Productivity (AFDW of suspended solids) averaged monthly. Varying hydraulic dilution rates and depths account for differences in productivities and harvestabilities between ponds. Algal biomass were estimated by microscopy at about 85% to more than 90% of AFDW productivity, the rest being waste derived solids. (Source: Benemann et al. 1979.)

This report (Benemann et al. 1978) originated with a Request for Proposals (RFP) by ERDA for a “Cost Analysis of Algal Biomass Systems” which included both micro — and macroalgae. The contract was awarded to the Dyantech R/D Co., who subcontracted for the analysis of the microalgal work with CSO International, Inc. Although the RFP specified a minimum scale for such systems of “100 square miles,” a single large unit was not feasible, and the analysis was carried out for individual modules of 800-ha. This system was to be independent of wastewater treatment and nutrients, which were deemed too small to provide “meaningful” energy supplies.

The first step in the analysis was to list 10 major sets of assumptions on which this process could be based (Table III. D.1.). These included

• essentially effortless species control,

• a yield of about 45 mt/ha/yr (20 t/ac/y),

• 4% N and 0.4% P in the algal biomass,

• 40 ha (100 ac) growth ponds with multiple channels, and

• harvesting by bioflocculation.

Month-by-month variations in biomass density, productivity, water and CO2 utilization, etc. were estimated based on a typical southwestern United States location, with productivities ranging from a minimum of 6 to a maximum of 18 g/m2/d.

Based on these assumptions, designs of the various system components were carried out, and supporting calculations made for the subsystems, including earthworks, pumps to move and lift the water, the supply channels and piping required, transfer structures, settling ponds, ducting for CO2, etc. The algal biomass would be digested to methane gas, but this was not included in the analysis. Based on estimates for various components, total capital costs were estimated (in 1978 dollars) at about $9,000/ha, without contingencies or engineering. Annualized costs, based on a 15% per annum capital charge, plus $700/ha operating costs for labor and nutrients, assuming free CO2, were about $2,000/ha, or about $45/t biomass. As pointed out in the report, “the basis for choosing many of the design features was low cost, or, actually, the highest cost allowable.” Thus, this report was primarily useful in identifying the major design assumptions and cost centers for such a process.

This report was the first truly detailed analysis of such systems, though it still was, in many aspects, highly conceptual. It was used by Regan (1980) for a similar analysis of a large-scale algal (B. braunii) hydrocarbon production process in Australia, and served as the basis for subsequent analysis by DOE and the ASP.

I Publications:

Benemann, J. R.; Persoff, P.; Oswald, W. J. (1978) “Cost analysis of microalgae biomass systems.” Final Report prepared for the U. S. Dept. of Energy, HCP/t1-605-01 Under Contract EX-78-X-01-1 605.

Regan, D. L. (1980) “Marine biotechnology and the use of arid zones.” Search 111:377-381.

Table III. D.1. Design assumptions for a microalgae production system.

(Source: Benemann et al. 1978.)

SUMHAEX LIST OF ASSUMPTIONS

Z ALGAE CULXTVAXION: Bio flocculating type* of microalgaa can

b« cultivated. № aigoificaat: effort at species control need be undertaken. Mixed populations cultivated. Mo pest control.

IX vTPT. n? yield is assuaed to be 20 tons/acre/year (ac 10,000 ВТО/lb higher heating value) corresponding to somewhat less than 22 solar conversion efficiency In southern U. S. Losses resulting as a consequence of predation, disease or excretion of phoeosynthedc products are already subscracted.

Ш CHEMICAL COMPOSХТХ0Н: Algaa contain 42 N, and 0.42 P.

Major nutrient losses are 102 of N and 10 2 of P (and other olcrountrlents) in the algal biomass produced per year (e. g.

160 lbs/acre/year M lose and 16 lbs/acre/year P lost.)

17 атяаі. DEHSiry: Tiald is not significantly affected by operating

ponds from —302 to +202 of algal densities specified by the yield and harvesting rate assumptions.

7 WAXES. USE: Water use is assumed to be 502 higher than

calculated rates from class A pan evaporation minus pre­cipitation data, to account for increased evaporation rates in growth ponds as wall as evaporation In harvesting ponds, conveyance channels and minor percolation. Can use brackish, saline waste or sea water. Surge and equalization basins must be provided.

71 CLIMATE ДМ0 SIZE: Avoid cold and low insolation regions.

Meed cheap level land. Land coecs are not considered.

711 GS0WTH PONDS: Channel width 200 ft?, baffle height IS",

earthwork height 24" with 2:1 sloping sides, growth ponds 100 acres (1,200 x >,630 ft). Assumes self or clay sealing or ponds (no significant percolation).

Till MIXING SYSTSI: Paddlewheels, three sets per 100 acre growth

pond. 0.2 ft/sec to 0.5 ft/sec mixing velocity. Assumes no erosion.

XI САВ20КДХЮН: Meed up to 5 M fc^/day of flue gas per 100

acre growth pond. Carbonatiou in covered paddlevheel stations and of return harvesting water.

2 HARVESTING STSTEM: Use a pond isolation process which operates

on a two day detention time (plus one day for fill and draw process). Assume up to ten-fold concentration and up to three successive stages of isolation.

III. B.3.a. HRP Design and Construction Phase, 1981

The project described in this section succeeded the projects reviewed in Section III. A. that took place at the University of California-Berkeley. That research group moved essentially intact (as Ecoenergetics, Inc., later renamed EnBio, Inc.) to Fairfield, California, some 30 miles north of Berkeley, with a pond system set up in nearby Vacaville.

The ASP funded this project starting in fall 1980. The objective was to demonstrate the HRP system using agricultural irrigation waters and fertilizers. The HRP was defined as a paddle wheel-mixed (approximately 10-20 cm/s), moderate depth (approximately 15-30 cm), algal production system. The R&D goal was to develop production technology for microalgae biomass with a high content of lipids. A detailed literature review concluded that the best option would be to use N limited (but not starved) batch cultures of green microalgae.

The plan view of the facility is shown in Figure III. B.5. The system consisted of four 200-m2 and three 100-m2 ponds, along with three deep harvesting ponds and four water and effluent storage ponds. This system thus provided considerable flexibility for the testing of a large number of variables and algal species, at a scale that would allow some confidence in the scale-up of the results. The units were lined with 20 mil PVC, to allow complete mass balances.

The report to the ASP describing this work (Benemann et al. 1981) provided considerable detail on the design of the system and the various considerations that went into selection of different design options and operating variables. For example, Table III. B.1. lists the calculations on which basis the carbonation requirements for the ponds were estimated.

After the facility was only partially constructed, the project was terminated by the ASP, as the Hawaii ARPS system, reviewed earlier, was deemed to have already demonstrated its superiority to the HRP design, even before any operations of either. However, after a hiatus of about 1 year, and with changes in the ASP management, funding for the California HRP project was reinstated in August 1982, and actual pond operations were initiated.

Figure III. B.5. Pond system design in California.

The schematic shows the four 200-m2 and three 100-m2 raceway ponds, three deep square algae harvesting (settling) ponds, and the mounds for location of the water supply and media recycle tanks. (Source: Benemann et al. 1981.)

Table III. B.1. Microalgae pond carbonation requirements. (Source: Benemann et al. 1981.)

The immediate issue arises of how to isolate, select, improve, and maintain the algal strains required for large-scale, low-cost microalgae cultivation. The ASP spent considerable effort in this area, with the isolation, screening, maintenance, laboratory studies, outdoor cultivation, and genetic improvement of microalgal strains (see Section II). In general, laboratory results were not predictive of outdoor performance. In addition, the strains most successfully maintained outdoors were those that spontaneously arose in and then dominated the ponds, often for considerable periods. Indeed, one conclusion from the outdoor culture work was that strains maintained in laboratory culture are, in general, not very competitive in open ponds.

What was attempted in this context, the mass culture of specific, selected and productive algal strains in large open ponds for long periods of time, has only been accomplished in algal mass cultures in a few cases, and is still rare in most industrial or environmental microbiology applications. In the case of microalgae mass cultures, only a few strains, Spirulina, Dunaliella, Scenedesmus, and Chlorella, have been successfully mass cultured at a commercial or large (>0.1 ha) scale. In the most successful cases, Spirulina and Dunaliella are maintained in open ponds through the use of chemically selective media, containing high bicarbonate and high salinity, respectively. Scenedesmus was mass cultured at the pilot scale in Germany and Chekoslovakia, and other countries, with the cultures obtained from isolates that invaded and dominated the ponds. Commercial Chlorella production, using selected strains, has suffered from culture instabilities, requiring frequent inoculation and short production runs, greatly increasing the costs of the process. Thus, commercial-scale production of microalgae does not provide a good guide for this problem.

In the case of industrial microbiology, only the traditional fermentations (e. g., ethanol, vinegar, cheese production) use selected strains that can be inoculated into and maintained in the production system, which must be relatively “clean” to avoid rapid contamination, but do not need to be sterilized. In environmental and agricultural microbiology it has not yet been possible to inoculate desired microbes (e. g., pollutant degraders, N fixers) into the open environments and demonstrate their survival and efficacy.

Within this context, the demonstration of the ability to mass culture at least some algal strains on a relatively long-term and reliable basis by ASP-supported projects in California, Hawaii and New Mexico, must be considered a significant advance and accomplishment. These results provide a fundamental basis for future developments and improvements in this technology. However, a basic issue still to be resolved is the source of the microalgal strains to be used in

outdoor cultures. The results of the ASP Program suggest that one choice would be to allow the production system to self-select the organisms. Strains that naturally invade potential production sites could be screened for subtle combinations of fast growth, competitiveness in high densities, and adaptation to prevailing environmental conditions. In this context, most of the critical parameters—temperature, light intensity, pH fluctuations—can be modeled rather easily at a modest scale. Thus, it should be possible to select such strains in downscaled models that would allow much better control than possible in large ponds over the selective conditions desired.

One factor essentially impossible to model or scale down is the biotic environment itself, that is, invasions by other microalgae, predation by grazers, infection by viruses, and other obvious or hidden biological effects that result in decreased productivities or even loss of culture. However, it appears from the experience with outdoor ponds, that these biotic effects are usually consequences of, not fundamental reasons for, loss of culture competitiveness. Further, some techniques have been developed to counteract such problems, for example rotifer grazing. In general, these problems will have to be dealt with when the technology has advanced to the point where large-scale culture efforts can be justified. That is, after high productivity cultures can be demonstrated at smaller scales, starting with laboratory simulations.

It is thus recommended that small-scale systems, mimicking as much as possible the outdoor environment, be used as selection devices for microalgae strains suitable for outdoor algal mass cultures. Suitability for mass culture can be established at a relatively small scales (<200 m[10]). Such selected “wild type” algal strains, would, of course, not necessarily exhibit the high biomass and lipid productivities required for the purposes of biodiesel production. Thus, considerable R&D will be required to genetically improve such strains. The techniques used to increase photosynthetic efficiencies or to optimize lipid quantity or quality, achieved with laboratory strains, must then be applied to the isolated strains suitable for algal mass culture.

Thus the recommendation for future R&D in this field is for a parallel track effort:

1. Demonstrate the feasibility to achieve with laboratory systems the high solar conversion efficiencies and lipid productivities required for biodiesel production.

A. (Top) — Maps of plasmids containing the neomycin phosphotranferase gene (nptH) flanked by regulatory regions from the acetyl-CoA carboxylase gene from C. cryptica. Both plasmids worked well as expression vectors in the diatoms C. cryptica and Navicula saprophila.

B. (Bottom) — Map of plasmid pACCl, containing the full-length genomic sequence of the acetyl-CoA carboxylase gene (accl) from C. cryptica.

Ш — nptII gene sequence; □ — acc1 coding sequence; □ — acc1 regulatory sequences (Source: Dunahay et al. 1995; Roessler and Ohlrogge 1993).

Replate cells on selective medium

(+ G418)

Pick G418-resistant colonies Test for npfll DNA via Southern Test for NPTII protein via Western

Figure II. B.9. Simplified schematic showing the protocol for transformation of diatoms by microprojectile bombardment (“gene gun”).

Figure II. B.10. Southern blot showing the presence of the nptH gene in transformed cells of

C. cryptica T13L.

Cells of C. cryptica T13L were transformed with pACCNPT5.1 via particle bombardment as described in the text. DNA from wild-type cells (wt) or G418-resistant strains (lanes a-f) were digested with HindIII and hybridized to a digoxigenin-labeled nptll sequence. The lane designated “5.1” contains HindIII-digested pACCNPT5.1as a control. The sizes of DNA fragments included as markers are indicated to the right. (Source: Dunahay et al. 1995).

Figure II. B.11. Western blot, showing the presence of the 30-kDa NPTII protein in transformed cells of C. cryptica T13L and N. saprophila.

In this example, C. cryptica and N. saprophila were transformed with pACCNPT10 and pACCNPT5.1, respectively. Crude cell extracts were separated on SDS-polyacrylamide gels, blotted onto a nitrocellulose filter, and NPTII protein was detected using anti-NPTII primary antibodies and alkaline phosphatase-conjugated goat anti-rabbit IgG secondary antibodies. The polyclonal antiNPTII antibody also recognizes a band of approximately 80 kDa in C. cryptica; however, the 30 kDa NPTII protein is seen only in the G418-resistant transformants. (Source: Dunahay et al. 1995).

The most fundamental assumption in microalgae biomass production for biodiesel fuels is that it will be possible to achieve near theoretical solar conversion efficiencies by overcoming the light saturation effect (see Section IV. A.2.c. for a brief discussion). The second most fundamental assumption is that it will be possible to achieve such very high productivities with microalgae cultures high in oils, approaching or even exceeding 50% of lipids by dry weight. This second assumption was tested by this project.

The concept of producing microalgae with a high oil content goes back almost 50 years, to work carried out, and even patented, by Sphoer and Milner (1949), who reported oil contents as much as 80% of the dry weight. Lipid content is affected by many parameters, but most particularly by N (and, for diatoms, Si) limitation, which can result in extraordinarily high lipid contents. However, it appeared from earlier work, and also from a survey of 30 species by Shiffrin and Chisholm (1981), that total productivity declined sharply upon nutrient limitation, resulting in a decline in total lipid productivity, although lipid content increased as a percentage of the cell mass. However, a re-analysis of the data suggested that the evidence for this was not clear-cut, as only rather widely spaced data points had been collected. In fact, an essential assumption in this field is that a “lipid trigger” activates lipid biosynthesis without necessarily reducing photosynthesis, at least for a transitory period (see Section II).

The experimental approach was to first grow the algal cultures under nutrient sufficiency and then to induce deficiency during batch cultivation, using light (single versus two-sided illumination of the 1 — L flasks) as a second variable. In continuous cultures, the growth rates and cellular N contents were dependent on illumination, and there was only a modest increase in lipid content with decreasing cellular N content, with lipid culture productivity maximal at about 5% N biomass. The key experiment was the up-shifting of the light received by the culture (e. g., from single — to double-sided illumination). The results are shown in Figure III. B.U., which demonstrate that cells shifted to a higher light intensity start growing (AFDW increases) at the rate of the higher light level cultures. However, lipid productivity shoots up to a much higher rate than with either of the steadily illuminated cultures. In practical terms, this could be exploited by diluting cultures for lipid induction. This experiment demonstrated the possibility of producing high lipids content by nutrient limitation while achieving a substantial increase in overall lipid productivity. Experiments with continuously diluted cultures, however, did not exhibit such responses, indicating the necessity to carefully control and modulate conditions to maximize lipid production.

Figure III. B.11. Maximizing lipid productivity light shift-up.

A. Top. Biomass yield during light shift.

B. Bottom. Lipid yield during light shift.

(Source: Benemann and Tillett 1987.)

During the 1975-1979 period, several other proj ects on microalgae fuels production were funded by ERDA/DOE, including the biophotolysis projects using heterocystous cyanobacteria, discussed earlier (see review by Benemann et al. 1980). Another biophotolysis project tested an optical fiber system for diffusing solar light into algal cultures, thereby overcoming the light saturation limitation to photosynthetic efficiencies (Manley 1979). This was shown to be impractical and was abandoned after only some very initial work. However, optical fiber photobioreactors are today the centerpiece of the 10-year, very large Japanese R&D program for microalgae CO2 utilization (Section IV. B.1.c.).

In 1976, Lawrence Livermore National Laboratory (LLNL) established a well funded in-house project that was very similar to the University of California-Berkeley project, including the use of microstrainers for harvesting filamentous microalgae, biomass recycle, and even a biophotolysis component using heterocystous cyanobacteria (Jeffries et al. 1977; Timourian et al. 1997). This project failed to receive support from ERDA, and was disbanded in 1977.

Professor Harry Gregor at Columbia University was funded for 2 years to develop membrane systems for cross-flow filtration harvesting of microalgae. However, the membranes available at the time, the pressure drops required, and the fouling problems encountered made this approach impractical (Gregor and Gregor 1978).

At Woods Hole Oceanographic Institutions, Drs. John Ryther and Joel Goldman carried out extensive research on microalgae cultivation in outdoor ponds on mixtures of seawater-secondary sewage effluent. When Dr. Ryther relocated to the Harbor Branch Oceanographic Foundation in Florida in the late 1970s, he was supported by DOE and later the ASP for the production of freshwater plants (water hyacinths, etc.) and seaweeds (Ryther 1981; 1982; 1983), as well as for microalgae culture collection work (See Part II. A.2.). Dr. Goldman also wrote a review on the theoretical and practical aspects of microalgae cultivation under contract with DOE (Goldman 1979a, b). One conclusion was that the productivity of microalgae systems would be limited, because of the light saturation effect and other factors, to below 50 mt/ha/yr. Although the analysis was correct, it was a very conservative conclusion, making no allowance for productivity improvements caused by fundamental and applied R&D advances, as discussed in the remainder of this report.

In 1978, Dynatech R/D Company prepared a report that analyzed the feasibility of using both macro — and microalgae systems, as well as other aquatic plants, for fuel production (Dynatech R/D Company 1978a). A major emphasis of this report was seaweed systems (“Ocean Farms”), as these were a major focus of the ERDA/DOE Fuels from Biomass Program at the time. This report concluded that macroalgae systems, based on open ocean giant floating seaweed farms, were technically and economically infeasible. The report also addressed the land-based microalgae systems, based on the report by Benemann et al. (1978) discussed in the previous section. The authors concluded that CO2 supply from power plant stack gases required “prohibitively expensive” duct work, distribution and transfer systems. It recommended further development only of emergent higher aquatic plants, such as water hyachinths and marsh plants, which can use CO2 from the air.

However, these conclusions were subjected to considerable critique. In response, a companion report was prepared that addressed “Reviewers’ Comments” (Dynatech R/D Co. 1978b). Although most of the comments related to the macroalgae systems, the conclusions regarding the microalgae process were also challenged, specifically with respect to CO2 transfer. It was pointed out that CO2 transport distances from the power plant to the ponds need not be longer than 10 km, as assumed in the Dynatech R/D Co. report and that CO2 transfer into the ponds could be both efficient and of low-cost. In other respects, including water and nutrients supply and use, the Dyantech R/D (1978a) report concluded that overall “it appears that there is a high probability that land-based aquatic biomass growth systems can be designed which are technically feasible and for which growth energetics are quite favorable.” This conclusion did “not necessarily imply economic feasibility.”

Actually, the Dynatech R/D Co. (1978a) conclusions were more positive than the opinions of some participants in this project. For example, Goldman and Ryther (1977) had earlier rejected the concept of microalgae fuel production, because, among other arguments, the water and fertilizer resources for microalgae ponds would be prohibitive. However, Oswald and Benemann (1977) countered arguments, pointing out, for example, that such a simplistic analysis failed to consider water and nutrient recycling. In this connection, Goldman (1979a, b) also reviewed the fundamental and practical aspects of microalgae biomass production, including the productivity data with outdoor pond systems. As part of the Dynatech Report, DOE published a “Topical Analysis” of aquatic biomass systems (Goldman et al. 1977), a good review of the scientific basis at the time.

I Publications:

Dynatech R/D Company, (1978a) “Cost analysis of aquatic biomass systems.” Report prepared for the U. S. Dept. of Energy, HCP/ET-4000-78-1, vol. 1.

Dynatech R/D Company, (1978b) “Reviewers comments on cost analysis of aquatic biomass systems.” Report prepared for the U. S. Dept. of Energy, HCP/ET-4000-78-2, vol. 2.

Goldman, J. C.; Ryther, J. H. (1977) “Mass production of algae: bioengineering aspects.” In Biological Energy Conversion (Mitsui, A., et al., eds.), Academic Press, New York, pp. 367-378.

Goldman, J. C.; Ryther, J. H.; Waaland, R.; Wilson, E. H. (1977) “Topical report on sources and systems for aquatic plant biomass as an energy source.” Report to the U. S. Dept. of Energy.

Oswald, W. J.; Benemann, J. R. (1977) “A critical analysis of bioconversion with microalgae.” In Biological Energy Conversion (Mitsui, A., et al., eds.), Academic Press, New York, pp. 379-394.

After this project was restarted, construction was completed and the first inoculation of algae into one of the 100-m2 ponds was made on August 13, 1982, using a mixed Micractinium- Scenedesmus culture obtained from the Richmond wastewater ponds (Section III. A.). However, these algae settled out due to lack of flow deflectors, and the culture was soon dominated by a Selenastrum sp. Both biomass concentration and productivity were quite low. Without flow deflectors at the far end of the ponds (away from the paddle wheel) the hydraulics were so poor that the ponds exhibited almost zero productivity. This was due to the formation of large countercurrent eddies resulting in “dead zones,” where algal cells settled. After flow deflectors were installed, the pond was inoculated on September 21 with an almost pure culture of Scenedesmus that had arisen spontaneously in one of the 12-m2 inoculum ponds. The culture remained well suspended and grew well (Benemann et al. 1983).

However, a similar inoculation into a 200-m2 pond resulted in almost complete settling of the culture, caused by poor pond hydraulics, even with similar flow deflectors installed. This indicated that the hydraulics of the ponds are critical to the success of the process and further, that the hydraulics are not predictable from one scale to another, even within a factor of two. After two flow deflectors were installed around the bends in the 100-m2 ponds, these ponds exhibited much improved hydraulics, with few eddies or settling of algal cells.

In contrast, similar deflectors did not improve hydraulics perceptibly in the 200-m2 ponds. Only after two more flow deflectors were installed at the end nearest the paddle wheels were satisfactory hydraulics observed in these larger ponds. A quantitative study of flow velocities was undertaken using a flow meter. The results were counterintuitive: flow velocities are higher on the inside than the outside of the channels. Clearly, pond hydraulics must be customized for each pond size and design to obtain even mixing.

As expected, productivities were rather low in the initial experiments carried out during October and November 1982. Maximum productivities (measured for 2 days) were only about 9 g/m2/d and average productivities less than 5 g/m2/d. These initial experiments included assessment of species dominance, N limitations, and mixing velocities. Pond operations ceased by the end of November 1982.

IV. B.1. Conclusions

IV. B. I.a. Cost and Productivity Goals

The overall conclusion from this review of 2 decades of DOE and ASP R&D in microalgal mass culture for biodiesel and other renewable fuels, is that this technology still requires relatively long-term R&D for practical realization. The initial, rather optimistic, cost and performance projections have not been met, or when met, the performance expectation (e. g., for productivity) have been raised. This was due, in large part, to the following factors:

1. The expectations for the future costs of fossil fuels have declined.

2. The value of by-product credits for waste treatment, greenhouse gas mitigation, or higher value coproducts are either uncertain or relatively low.

3. The recent engineering designs and economic analyses have projected higher costs than earlier estimated, partly because of greater detail and realism, thus requiring higher productivities to achieve cost goals.

4. The actual productivity results of the outdoor experimental work were well below the projections on which the economic analyses are based.

In this concluding section, these issues are briefly addressed, followed by a discussion of future R&D needs and recommendations.

The expectation for the economics of alternative fuels is a moving and uncertain target. Energy prices have been falling in real terms for more than 20 years, since the last oil-shock of the late 1970s. Competing within current market realities is not plausible for most renewable energy technologies. Indeed, electric industry deregulation is removing price supports for such technologies as wood, wind, and geothermal power. The price of fossil fuels will probably start to reflect at least some of their externalities costs, including air pollution and greenhouse gases, and plausibly even a cost penalty to account for their non-sustainable nature. However, any projection of the future price or costs of fossil fuels, with which renewable fuels such as microalgae biodiesel would need to compete in the marketplace, is rather uncertain and arbitrary.

For example, the use of a C-tax of some $50/t CO2 has been suggested, based on a current tax in Norway. However, if this were applied to all fossil fuels currently consumed, equivalent to some 20 billion tons of CO2 world wide, it would increase the energy sector of the word economy by $1 trillion, more than tripling current expenditures on fossil fuels, a highly unlikely possibility. Perhaps a more modest tax of $50/tC (approximately $ 14/t CO2), would be a more appropriate upper bound for greenhouse gas mitigation penalties (e. g., credits for renewable energy sources). At any rate, presently there is essentially no monetization of greenhouse gas mitigation, and any such figures are, at best, educated guesses.

However, greenhouse gas mitigation credits would likely be the overwhelming considerations in any future externalities cost accounting. Table III. D.7., summarized greenhouse gas credits required for microalgae systems, demonstrating the decisive effects of competitive fossil fuel costs on the necessary valuation of greenhouse gas mitigation. That table also demonstrates the major effect of productivity on the projected economics of such systems.

Another potential enhancement of microalgae biodiesel economics is in wastewater treatment. Here the technology and economics would be dominated by the competitive costs with an activated sludge plant, or other wastewater treatment processes, including conventional microalgae pond systems. The latter, known also as facultative or stabilization pond systems, naturally treat municipal wastewaters (sewage), liquid animal manures, food processing wastes, and even some industrial effluents. In current technology, with very few exceptions (e. g., the City of Sunnyvale, California) the algal biomass is not harvested, and thus it is discharged to the nearest body of water (river, lake, etc.), used for irrigation, groundwater recharge, or it settles to the bottom of the ponds. Such systems are not designed for maximizing biomass production. However, through conversion to high rate ponds, they provide a possible entry for introducing and demonstrating of microalgae biomass fuel production and CO2 utilization. Of course, their economics would not be dictated, except marginally, by their waste treatment functions, and their impacts on U. S. greenhouse gas emissions and fuel resources would be modest, at most a fraction of 1% of U. S. energy consumption and greenhouse gas emissions.

To expand the economic base and potential of such systems, other higher value coproducts or byproducts have been considered from such systems and processes. This is discussed in the following section.