Optimization of a flat plate glass reactor for mass production of Nannochloropsis sp. outdoors

Abstract

The relationships between areal (g m −2 per day) and volumetric (g l −1 per day) productivity of Nannochloropsis sp. as affected by the light-path (ranging from 1.3 to 17.0 cm) of a vertical flat plate glass photobioreactor were elucidated. In general, the shorter the length of the light-path (LP), the smaller the areal volume and the higher the volumetric productivity. The areal productivity in relation to the light-path, in contrast, yielded an optimum curve, the highest areal productivity was obtained in a 10 cm LP reactor, which is regarded, therefore, optimal for mass production of Nannochloropsis. An attempt was made to identify criteria by which to assess the efficiency of a photobioreactor in utilizing strong incident energy. Two basic factors which relate to reactor efficiency and its cost-effectiveness have been defined as (a) the total illuminated surface required to produce a set quantity of product and (b) culture volume required to produce that quantity. As a general guide line, the lower these values are, the more efficient and cost-effective the reactor would be. An interesting feature of this analysis rests with the fact that an open raceways is as effective in productivity per illuminated area as a flat-plate reactor with an optimal light path, both cultivation systems requiring ca. 85 m 2 of illuminated surface to produce 1 kg dry cell mass of Nannochloropsis sp. per day. The difference in light utilization efficiency between the two very different production systems involves three aspects — first, the open raceway requires ca. 6 times greater volume than the 10 cm flat plate reactor to produce the same quantity of cell-mass. Second, the total ground area (i.e. including the ground area between reactors) for the vertical flat plate reactor is less than one half of that occupied by an open raceway, indicating the former is more efficient, photosynthetically, compared with the latter. Finally, the harvested cell density is close to one order of magnitude higher in the flat plate reactor, which carries economic significance. The advantage of vertical lamination of photoautotrophic cells provided by vertical plate reactors, is thereby clearly seen. The optimal population density (i.e. which results in the highest areal productivity) in the 10 cm plate reactor was obtained by a daily harvest of 10% of culture volume, yielding an annual average of ca. 12.1 g dry wt. m −2 per day (on the basis of the overall illuminated reactor surfaces, i.e. front and back) or 240 mg l −1 per day.