Operation of the two large ponds (Weissman et al. 1989; Weissman and Tillett 1990, 1992) was initiated in August 1988. Ponds were inoculated with T. suecica and operated at relatively high mixing velocities (30 cm/s in the lined pond, 22 cm/s in the unlined pond) to reduce sedimentation. Productivities were only 11 and 10 g/m2/d, respectively, lower than in the small ponds, but with an unknown amount of algal biomass settling out. After loss of this alga, M. minutum was inoculated, and productivities were, again, somewhat lower in the larger than the smaller ponds.

Experiments were also carried out in the small pond, primarily to determine the best operating pH and pCO2 range to help minimize CO2 outgassing while maximizing productivity. At reduced CO2 levels (higher pH) a decrease of 10% to 15% in productivity was observed with three algal species tested. Another variable tested was a 2- versus 3-day dilution routine, which had no significant effect. In addition, six cultures were examined for productivity in Si — or N — deficient media. Only one strain exhibited significantly higher total (AFDW) productivities with nutrient deficiency, but no lipid data were collected.

The conclusions from this work were, in brief:

1. Power for pond mixing is within the expected range, and quite low (< 1 kW/ha).

2. Pond mixing should be in the 15-25 cm/s range, and pond depth 15-25 cm.

3. CO2 utilization efficiencies of near 90% overall should be achievable with little compromise of productivity, through operation at an optimal pH/pCO2 range.

4. Only preliminary 1,000-m2 pond operations were carried out during this year, hampered by design and operational problems, which lowered expected productivities.

5. Large-scale pond productivities of 70 mt/ha/yr are realistic goals for this process, though probably not at this site because of low seasonal productivities.

6. Very high, 50 g/m2/d, single day, productivities were observed on some occasions.

7. The small-scale ponds can be used to screen strains and optimize conditions.

The final report (Weissman and Tillett 1992) in this series on the New Mexico OTF operations, reported on the demonstration of productivity for the two large ponds for 1 full year, continuation of the small-scale pond operations, and improvements in mixing and carbonation. One major improvement in the system was an automated data recording and operations system.

Mixing was improved by improving the flow deflectors and increasing operating depths from 15 to 22.5 cm, which is probably a better depth for large-scale systems. Culture instability was a problem, particularly in spring because of greater temperature fluctuations, and resulted in low average productivity of only 7 g/m2/d for March through May. In contrast, the average productivity was 18 g/m2/d for June through October, decreasing to 5-10 g/m2/d in November (depending on onset of cold weather), and only about 3 g/m2/d in the winter months.

Overall productivity, including 10%-15% down-time for the ponds for repairs and modifications, was 10 g/m2/d, only one-third of ASP goals (Table III. B.3.). Clearly the major limiting factor was temperature, as smaller systems in warm climates have achieved annual yields two to three

times as high. A major conclusion from this work is that scale-up is not a limitation with such systems. Climatic factors are the primary ones that must be considered in their siting.

A countercurrent flow injection system was installed in the sumps resulting in a carbonation system that was essentially 100% efficient in CO2 transfer. Overall CO2 utilization was higher than 90%. The unlined pond performed nearly as well as the lined pond, with minor decreases in productivity (10%-20%), CO2 utilization efficiency (5%-10%) and a small increase in mixing power. The unlined pond consumed only 0.04 w/m2, allowing the entire 1,000-m2pond to be powered by the equivalent of a 40-w light bulb. Species stability in the lined and unlined pond exhibited no significant difference. This work clearly established the feasibility of using unlined ponds in microalgae cultivation. This was a critical issue, as plastic lining of ponds is not economically feasible for low-cost production.

In the small 3-m2 systems, two variables were investigated: Si supply and pH. Both are major cost factors in pond operation, due to sodium silicate costs and CO2 outgassing. They affect overall productivity as well as lipid production. For Cyclotella, for example, productivity was about 20 g/m2/d at pH 7.2 or 8.3, but only 15 g/m2/d at pH 6.2. As the higher pH range is preferred, where CO2 outgassing is minimal, this demonstrates the feasibility of operating such cultures within the constraints of a large-scale production system. Si additions could be halved with only a modest decrease in productivity, suggesting that Si supply could be reduced, particularly if low Si-containing diatoms are cultivated. Also Si limitation can be used to induce lipid production, as was demonstrated during this project, with lipid biosynthesis increasing as soon as intracellular Si content dropped, with a 40% lipid content being achieved. However, overall, lipid productivity did not increase as CO2 fixation limitation also set in. This remains as a major issue for the future (See also Section III. B.5.d.).

Table III. B.3. Long Term OTF Results from 0.1-Ha Raceways.

gm/afdw/m2/d: grams of ash-free dry mass per square meter per day.

Pond liner: YES indicates a plastic lined pond; NO indicates an unlined (dirt bottom) pond.

III. B.5.d. Conclusions

The performance of the large-scale system improved considerably in all aspects during the 2 years of operations. The parallel use of the smaller-scale ponds helped guide this research, in particular in selecting algal strains and identifying operating characteristics. The high CO2 utilization efficiency demonstrated in the small and large-scale ponds was another major accomplishment of this project.

The major limitation of this project was the overall low productivity in the large-scale ponds. This was due in large part to the adverse climatic conditions at this location, and the initial suboptimal nature of the large-scale pond operations. Even so, productivities were lower than anticipated, with annual average productivities only about one-third the projected productivities by the ASP that would be required for minimal economics (see Section III. D.). This must be a major ongoing objective for future research, first in terms of overcoming the lower temperature limitations on productivity, and second by relocating this type of process development to more favorable climatic sites. (See Section III. B.6. for a discussion of temperature effects.)

But perhaps the major limitation of this project was that it did not carry out a longer-term process development effort. Although 2 years of data were collected for the large-scale ponds, the rapid advances made suggested that further research would have allowed continued improvements in performance and increased understanding of the overall process in specific critical areas of culture maintenance.

The engineering evaluation of the operation of the 0.1-ha raceway ponds showed these systems to be potentially very efficient in terms of energy, water, nutrient and CO2 utilization, and even basic construction cost inputs. Most important, the absence of liners did not significantly reduce pond performance (e. g., productivity). This was a major observation of this project, giving greater confidence in the engineering analysis and cost projections carried out by the ASP and DOE, discussed again in Section III. C.

A major uncertainty in this project was the nature of the species control achieved. A review of the data would suggest considerable success with species control, with several species cultivated successfully for relatively long periods. However, considerably more research will be required on this subject, as the tools were not available to allow a closer study of possible population dynamics (e. g., strain selection and even replacement) within the ponds. Thus, the subject of species control still requires considerable effort, as discussed further in the following section.