HRP Operations in California, Oct-Nov. 1982

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.

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