Kinetic modelling of microalgae cultivation for wastewater treatment and carbon dioxide sequestration

A simple and robust microalgae kinetic model has been developed for application in the prediction and control of algae cultivations in wastewater. The microalgae kinetic model was calibrated using experimental cultivation data from Desmodesmus sp. to determine specific microalgae growth rates (μₘₐₓ and μₘₐₓNO₃), microalgae death rates (μd), and the NH₄⁺ to NO₃⁻ oxidation rate (μB). Model parameters obtained were: μₘₐₓ = 0.17 day⁻¹, μd = 0.004 day⁻¹, and μB = 0.14 day⁻¹. Microalgae specific growth rate based on NO₃⁻ alone (μₘₐₓNO₃ = 0.1 day⁻¹) was lower than the overall growth rate (μₘₐₓ). The kinetic model was validated using additional experimental data for the Desmodesmus sp. and Scenedesmus obliquus cultivation in wastewater containing 0% and 7% landfill leachate, with accuracy above 98% in all cases. These results demonstrated the kinetic model was accurate in predicting microalgae growth, wastewater nutrient removal, and changes in the culture media pH. Biomass productivity of the algae culture was associated with an exponential increase in the media pH, which led to ammonia volatilisation and decreased carbon intake. Between 28.8 and 29.7% of the initial NH₄⁺ was lost to ammonia volatilisation in wastewater containing 7% landfill leachate. Hence, loss of ammonium nitrogen contained in domestic wastewater must be avoided to ensure steady and efficient inorganic carbon utilisation which inherently maximises biomass production efficiency. The optimal pH for the microalgae culture was 8.1, at which point microalgae could achieve about 99% carbon fixation efficiency. To ensure constant pH in the microalgae growing system, immediate removal of the OH⁻ generated is needed, which could be facilitated by injections of 1.14 g CO₂ and 0.067 g OH⁻ per gram of produced algae when using NH₄⁺ nutrient, and 1.54 g of CO₂ per gram of produced algae when using NO₃⁻ nutrient. This could be done in a wastewater pond by using an optical density-controlled smart CO₂ injection system.