LIGHT SUPPLY COEFFICIENT - A NEW ENGINEERING PARAMETER FOR PHOTOBIOREACTOR DESIGN

The reliability of the incident light intensities, the average light i ntensities and the light energy supplied per unit photobioreactor volu me (E(t)/V) as indices of light conditions inside photobioreactors was investigated in cuboidal photobioreactors of various sizes. There was no good relationship between the linear growth rates of Chlorella pyr enoidosa and the incident or average Light intensities in the photobio reactors of various sizes. Although the linear growth rate increased w ith increase in E(t)/V, there was much scatter of data near the curve. At a given E(t)/V, the linear growth rates decreased with increase in the photobioreactor depth, indicating that the light distribution ins ide the photobioreactor must be considered for the rational design and scale-up of photobioreactors. A concept of light distribution coeffic ient (Kiv) defined as the cell concentration at which 50% of the photo bioreactor volume receives enough light for photosynthetic growth was therefore proposed. The linear growth rates increased with increase in Kiv but the data were scattered, At a constant Kiv, however,a linear relationship was observed between the linear growth rate and the E(t)/ V. Similarly, when the E(t)/V was held constant, there was a good corr elation between the Kiv and the linear growth rate. A light supply coe fficient, defined as E(t)/V . Kiv was then proposed as an index of the light supply efficiency of photobioreactors. There was a linear relat ionship between the light supply coefficient and the linear growth rat es of both C. pyrenoidosa and Spirulina platensis in cuboidal photobio reactors of various sizes. However, the slopes of the curves were diff erent for the two microorganisms. When various other types of both int ernally illuminated and externally illuminated cylindrical photobiorea ctors were used, good correlation was found between the linear growth rates of Chlorella and the light supply coefficient. This demonstrates that irrespective of the cell type, photobioreactor type and size, th e proposed Light supply coefficient can be used for quantitative evalu ation of light condition inside the photobioreactor. It is thus a usef ul engineering parameter for design and scale-up of photobioreactors.