Abstract
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.
Highlights
Disposal of wastewater without treatment often results in eutrophication, an excessive enrichment of the water body with nutrients which leads to algal blooms, and more long-term problems of heavy metals contamination [1,2]
The aim of this study is to develop a robust kinetic model that simulates microalgae growth in wastewater when limited by more than one substrate, incorporating the effects of NH3 volatilisation, oxidations of the ammonium nitrogen to nitrate by autotrophic bacteria, and the inherent changes of pH in the culture media
The From the model calibration, the following model parameters were determined for Desmodesmus sp. cultivation: specific microalgae growth rates based on ammonium nitrogen and based on NO3−, death rates, and the NH4+ to NO3− oxidation rate
Summary
Disposal of wastewater without treatment often results in eutrophication, an excessive enrichment of the water body with nutrients which leads to algal blooms, and more long-term problems of heavy metals contamination [1,2]. In Mexico only 57% and 32% of the total municipal and industrial wastewater generated in the country is treated [3], eutrophication has had a great impact in receiving water bodies because of the discharges of untreated wastewater. These deleterious environmental consequences can be avoided through carbon-neutral wastewater treatment using microalgae. The major challenge is an adequate understanding of the kinetics of microalgae growth in wastewater, to allow for optimal design and operation of ponds for wastewater treatment
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