Numerous microalgal strains were obtained from the SERI Culture Collection and tested in small-scale, 1.4-m2, ponds (Weissman and Goebel 1986; 1987). All strains could be grown quite successfully in these small units, although some, such as Amphora sp., did not survive more than 2 or 3 weeks before they were displaced by other algae. Cyclotella displaced Amphora under all conditions tested, even though Amphora was the most productive strain, producing 45 to 50 g/m2/d in short-term experiments. The green algae, e. g. Chlorella or Nannochloropsis, also could not be grown consistently. Their productivities were among the lowest, about 15 g/m2/d (similar to that in the prior year). Thus, one fundamental conclusion was that productivity is not necessarily correlated with dominance or persistence. However, these factors may be related to oxygen effects, as shown in later experiments.

Table III. B.2. summarizes the results of various experiments for the summer with small (3.5-m2) ponds and seven of the algal strains under different operating regimes, including controlling the oxygen tensions through degassing by air sparging.

A significant factor in pond operations was the oxygen level reached in the ponds, which influenced productivity and species survival. Ponds were operated with air sparging (and antifoam) to reduce DO levels, from typically 400% to 500% of saturation without air sparing, to 150% to 200% of saturation with sparging. Foaming, caused by air sparging, was still was a problem in some cases, as with the Cyclotella. However this alga exhibited approximately the same productivity with or without sparging despite the 20%-30% opaque foam cover, suggesting some positive effect of the lower pO2. For other algal species productivity differences of 10% to 20% were noted, and for some (e. g., C. gracilis), no specific effect of high versus low DO was noted.

These outdoor results were reproducible enough to detect differences of greater than about 10% between treatments. The major result of this project was that productivities were 50% to 100% higher than the previous year, with some species of diatoms producing 30 to 40 g/m2/d (AFDW, efficiency about 6% to 9% of PAR, or 3% to 4.5% total solar). The green algae were, as mentioned earlier, less productive than the diatoms.

A more detailed study of oxygen effects was also carried out in the laboratory, avoiding the confounding factors of CO2 supply, temperature, and light intensity. In general the diatoms were insensitive to high DO; most, but not all, of the green algal strains exhibited marked inhibition by high oxygen levels (Figure III. B. 8.). None of the oxygen-sensitive algae could be grown outdoors, suggesting this as a major factor in species dominance and productivity.

Laboratory studies were also carried out at both high light intensity and high DO, to determine the synergism between these factors. Both the apparent maximum growth rate and dense culture productivity were determined for comparisons. Higher levels of DO intensified the inhibitory effects of higher light observed in some cases. This was true in particular for productivity, with growth rates also affected. Of course, the actual density of the culture is a major factor

determining productivity, and dense cultures avoid most, if not all, the deleterious effects of high light intensity. High O2 and low CO2 are other factors influencing the response to high light, with O2 being more inhibitory at both low CO2 and high light levels. High oxygen also affects chlorophyll content, although this effect is most pronounced at low light intensities where chlorophyll levels are 50% higher compared to high light intensities.

Outdoor experiments were carried out to determine the effect of low CO2 (25 pM) and high (9-10) pH, which would be experienced in algal mass cultures, at least temporarily. Compared to the control cultures, one strain was not inhibited even at pH 10, two not at pH 9, and two were inhibited by about 33% at this pH, compared to the control at pH 8. Lowering pCO2 also resulted in similar levels of inhibition for the other strains. A role for bicarbonate in growth at high pH was established from the data, with metabolic costs estimated at about one-third of productivity, a major factor. This requires further investigation.

One strain, a Cyclotella species, exhibited an increase of lipid content of more than 40% of dry weight upon Si limitation. However, lipid productivity (9 g/m2/d), was not significantly different between Si-deficient and the Si-sufficient controls, because of the high productivity of the Si — sufficient culture. Optimizing for lipid productivity was considered possible, but requires more detailed study.

Perhaps most important, the data and simulations also suggest that maximizing productivity at an acceptable CO2/pH combination from the perspective of outgassing and CO2 loss from the ponds is possible, with operations above pH 8.0 required (for an alkalinity of 32 meq/L, higher for higher alkalinities) to avoid wasting of CO2.

Laboratory studies were also carried out during this project. These included a study of light conversion efficiencies that concluded that at low light intensities very high light conversion efficiencies can be achieved (near the theoretical maximum of about 10 photons/CO2 fixed). However, these and other laboratories studies carried out during this project would require a much longer review than possible here.

Finally, this project investigated harvesting of microalgae cultures with both polymers and FeCl3 (to enhance algal settling and sludge compaction) and cross-flow filtration. Organic flocculants at about 2 to 6 g/kg and FeCl3 at about 15 to 200 g/kg of algal biomass (AFDW) were required to remove 90% or more of the algal cells. Because of the high cost of the organic flocculants, costs were comparable for both flocculants tested. The organic polymers were also deemed to have significant potential for improvement and optimization. Cross-flow filtration, though effective, was estimated to be too expensive. A cost analysis of such a harvesting process was also presented.

In conclusion, this project significantly advanced the state-of-the art of low-cost microalgae biomass production, and provided the basis for the Outdoor Test Facility, discussed in Section III. B.5., following the review of the ASP-supported project in Israel.

Table III. B.2. Outdoor results summary for California pond operations.

Data from 3.5-m[9]ponds. (Source: Weissman and Goebel 1986.)

Kcal calculated fro* proximate composition, either 1) as aeasured or 2) deterained as SOX protein, lipid as aeasured, and CHO by difference.

Teaperature: Max 30-34*C, Hin 16-20 °С 1 Required re-inoculation 3 Required re-inoculation twice

3 Oxygen reaoval caused 20-302 coverage of surface with foam