Modeling algal productivity in large outdoor cultures and waste treatment systems

A deterministic mathematical model was used to describe the production of green microalgae (Scenedesmus obliquus and Coelastrum sphaericum) in outdoor mass cultures. The model was calibrated against 16 months of temperature and irradiance measurements, during which time productivity measurements were made in up to five ponds with surface areas of up to 263 m². During this period rates of algal dry matter production varied between 1·7 and 16·92 g m⁻² day⁻¹. The model predicted productivity to within 4·2% of the observed rates, for the same period. Negative productivity values (loss of biomass) were calculated for the months from November to January. It was concluded that appreciable amounts of biomass could be produced for 7 months per year in temperate areas.

Several assumptions were made during the construction of the model, especially with regard to loss factors, such as: respiration, release of exuded organic carbon and photo-inhibition. The latter was included as a separate factor in the model and is merely conceptual. Several applications of the model are discussed, one of which concerns the relation between areal density and productivity, where the optimal areal density for maximal productivity was calculated to be 38–41 g (dry wt) m⁻². A distinction was also made between cultures which were mainly autotrophic and waste systems. It was shown that the presence of gilvin and/or tripton would adversely influence productivities and that the contribution of these factors to vertical light attenuation would have to be measured in waste systems.