During FY1976-1977, the project described in Section III. A.2. continued with the same overall objective: to determine what pond operating factors could allow control over algal species, and thus permit cultivation of algal types that allow low-cost harvesting by microstrainers. The biomass recycling process described earlier continued to be studied, using the 12-m2 rectangular ponds and various pond operating strategies (mixing speeds, retention times, biomass recycle fraction), to test for their influence on microalgae species composition and productivity. Mainly the four 12-m2 ponds were used, with some initial experiments with the large 0.25-ha pilot pond.

An extensive series of experiments was carried out in the small ponds, with daily analysis of suspended solids (SS) concentrations (the best measure of algal biomass, although some, 10%- 15%, contributions from wastewater and bacterial solids could not be avoided). Other parameters measured, less frequently, were chlorophyll concentrations, BOD5, P, ammonia, and, microscopic algal counts, including cell dimensions. Experimental pond operating parameters tested included retention time (hydraulic dilution) and depth (though this was typically 25 cm), mixing speed, and biomass recycle ratios.

Both at short and long retention times the algal cultures invariably became unharvestable with microstrainers. Intermediate hydraulic retention times selected for larger colonial algal species that were more readily harvestable. However, long retention times also resulted in low productivities. There was an optimum residence time, which varied with depth of the culture and climatic variables that selected for harvestable cultures. However, biomass recycling was only marginally effective in improving biomass harvestability by microstraining. Mixing speeds >15 cm/s also improved harvestability by microstraining. Mixing speeds of 15 to 30 cm/s tended to induce algal flocculation.

Problems were encountered with zooplankton grazing of the algal cultures. Coarse (150-pm) screens did not effectively remove the grazers. Shorter retention times reduced grazer pressures, but also made the cultures less harvestable by microstrainers. In all the ponds, Scenedesmus dominated in the winter and spring, and then was replaced with Microactinium. Loss of dominance correlated with the breakup of the colonies, which may have been related to zooplankton grazing. The best productivity was 13.4 g/m2/d, during a 10-month period, irrespective of harvest efficiency. For the most harvestable pond, productivity was only 8.5 g/m2/d (of which only 7.2 g/m2/d was harvested by the microstrainers). Clearly, optimizing for productivity and harvestability required quite different operating conditions. It was concluded that the use of microstrainer harvesting and biomass recycling was unlikely to lead to both a high algal productivity and effective harvesting process.

After growing and harvesting an algal culture on sewage, enough nutrients remain to grow a second crop of microalgae. Such a second crop would then deplete available N. Due to excess inorganic and organic phosphates in sewage, sufficient P remains after harvest of the second algal crop to allow cultivation of additional batches of N-fixing microalgae. Of course, due to C

limitation, CO2 must be supplied to these cultures. Such a process, which would achieve so-called tertiary wastewater treatment (nutrient removal) is shown schematically in Figure

III. A.3., and was demonstrated during this project. Considerable problems were encountered with the secondary ponds (shown as a small box labeled “green algae” in Figure III. A.3.), due to culture instabilities, lack of sufficient algal removal in the first stage, grazers, etc. Figure III. A.4. shows a composite of the general results obtained, with a 7-day batch cultivation, at which point the culture settled quite well and, from its yellowish color was apparently N limited. The final, N-fixing, stage in this process was demonstrated under the following year’s project and a NSF-RANN funded project (Benemann et al. 1977).

In May 1977, cultures were started in the 0.25-ha high rate pond. That pond, mixed (poorly) with centrifugal pumps located in a 2.5-m deep sump, exhibited rather poor hydraulics. Sewage supply limitations resulted in longer retention times than desired. Despite these and other operational problems, the results were “reasonably consistent with the smaller 12-m2 ponds, both in productivity and harvestability responses to detention time” (Benemann et al. 1978). Productivities of 19 g/m2/d were observed over an 18-day campaign in summer, with a retention time of 3.5 days. Interestingly, zooplankton grazing was not as big a problem as with the smaller ponds.

These 0.25 ha pond algal effluents were also tested for settling, in a “pond isolation” experiment, in which cultures were removed from the pond and held for as long as 3 weeks in a settling pond. In one experiment, when the culture had been grown at a dilution rate at 0.22 day-1, it settled more than 90% in one day, while a culture grown at a dilution rate of 0.5 day-1 required 3 weeks to settle. This suggested another approach to algal harvesting, which became the focus of the project described in Section III. A.4.

Algal 8iomass