This project addressed the problem of the uncontrolled nature of the algal populations in wastewater treatment ponds. Because the dominant algal species found in a pond could range from small unicellular to large colonial or filamentous species, harvesting of the algae for biomass conversion would require a universally applicable harvesting technology, such as centrifugation or chemical flocculation, to enable the recovery of any algal type. However, these processes are very expensive (Golueke and Oswald 1965; see also Shelef 1984 for an ASP supported assessment, and Benemann and Oswald, 1996 for a recent review). If, however, algal species could be controlled in the ponds, then filamentous microalgae species might be grown that would be easier and cheaper to harvest using microstrainers. Microstrainers, which are rotating screens (typically 25 to 50 pm openings) with a backwash, are already widely used for removing filamentous algae, mainly filamentous cyanobacteria (blue-green algae) from potable water supplies. Thus, the first objective of this project, initiated in 1976, was to investigate how to selectively cultivate filamentous microalgal species in waste treatment ponds (Benemann et al. 1977).

The first issue was that conventional waste treatment ponds are generally deep (2 m), and unmixed. Such ponds do not maximize algal productivity, nor do they provide a uniform hydraulic flow or physical-chemical environment. Thus, this project focused on the use of shallow, mixed, raceway-type ponds, the “high rate pond” of Oswald (1963), for microalgae

roduction and demonstration of algal species control in wastewater treatment. Initially four small (approximately 3 cm2) and, four larger (12 m2) rectangular, paddle wheel-mixed ponds were built and used. The Richmond Field station also had a large pilot-scale (0.25 ha) shallow high rate pond that was fed settled (primary effluent) municipal wastewater, available for this research.

A fundamental theory of species control was developed based on selective recycle of harvestable biomass (Figure III. A. 1.). The concept is that harvesting filamentous algae and recycling part of the biomass back to the ponds (similar to the process of activated sludge), would favor the slower-growing filamentous algae over the faster-growing unicellular types (which would thus get washed out of the system). This theory was both mathematically proven and experimentally demonstrated in the laboratory in competition experiments with mixed cultures of Chlorella (a unicellular green alga) and Spirulina (a filamentous blue-green alga). Without biomass recycle Chlorella always out-competed Spirulina; with biomass recycle Spirulina could be made to dominate the culture (see also Weissman and Benemann 1978).

This process was also tested in outdoor ponds. Experiments with Spirulina grown on wastewater were not encouraging, as this species dominates only at high salinities and alkalinities. Thus, the first issue was the source of algal species for the experiments. A pond in the city of Woodland was found to have a dominant culture of the filamentous cyanobacterium Oscillatoria. It was decided to use this alga in the initial tests. First the alga was isolated from a small pond sample and grown in the laboratory. It was then inoculated into a small circular pond fed with settled Richmond algae. The results of the first experiment are shown in Figure III. A.2. The Oscillatoria culture grew, but the unicellular organisms grew faster. Only by completely harvesting the entire culture and recycling all of the biomass retained in the microstrainer (almost pure Oscillatoria), could the culture be maintained.

It was thought that perhaps the laboratory cultivation stage had selected for a laboratory-adapted strain that did not do well outdoors. A small microstrainer was taken to Woodland, and enough Oscillatoria biomass was collected and returned to the Richmond Field Station to inoculate one of the small circular ponds. The pond was diluted about one-third per day with wastewater, with about 40% of the algae harvest recycled. However, after a couple of weeks, a new algal species became dominant. This species, Micractinium, a colonial organism covered with large spines (thought to be a protection against zooplankton grazers), also was captured by the (26-pm opening) fabric of the microstrainers and recycled. This gave a Micractinium competitive advantage over the unicellular forms, but also over the Oscillatoria. Several more inoculations of Oscillatoria gave similar results, with Micractinium dominating.

However, even Micractinium could not be stably cultured with recycling, as during apparently unfavorable conditions it washed out (probably due to loss of spines) and then was replaced by Scenedesmus. On the other hand, when conditions were favorable for Micractinium, biomass recycle helped the culture dominate the ponds faster, but, regardless of recycling, this alga replaced Scenedesmus. Thus, the theory worked in principle, but in practice selective biomass

recycle could be only one, and not a sufficient, tool for controlling microalgal species in such ponds.

Biomass productivities equivalent to a total solar energy conversion efficiency of about 2% (about 15 g/m2/d) were achieved for about 1-week periods. Effluents from the more harvestable cultures were below the EPA wastewater treatment system discharge limits of 30 ppm suspended solids (SS), with reductions of over 80% for ammonia and 50% for total organic C. A preliminary economic analysis was also presented (Benemann et al. 1977).