Fotobiodepuración de aguas residualescinética de eliminación de nutrientes mediante microalgas

Laburpena

Microalgal culture is increasingly gaining importance, mainly due to its potentially viable biofuel production, however there still are many challenges ahead for the commercial production. Microalgal culture in wastewater could be a sensible step in the right direction. First, the Chapter II is an initial approach to the nutrient removal from wastewater by microalgae. This Chapter shows the effect of different concentrations of nutrients in the wastewater on the growth and nutrient removal of Chlorella vulgaris. This experiment was used to gain insight in the nutrient removal; however the results obtained when modelling the kinetic of nutrient consumption were not satisfactory enough. Luedeking-Piret-like equation was used, and despite the model fitted the experimental results, predicted nutrients contents of the biomass were not realistic, although this model has been used for lipids accumulation in microalgae in a satisfactory way. The nutrient uptake and biomass growth should not be considered independently each other due to the luxury uptake of microalgae (i.e. the ability to remove nutrients far exceeding the necessary for growth). Therefore, the Chapter III deals with the coupling in a single model (PhBT model) of two independent models that previously existed for nutrient consumption and biomass evolution respectively. Thus, thanks to this model, batch experiments of nutrient removal by microalgae can be successfully described considering the reserves of nutrients in the biomass. Chlorella vulgaris was used as a model organism in Chapters II and III since this species has been extensively studied in wastewater. To obtain insight in PhBT model, in Chapter IV the model was applied to experiments performed with different species of microalgae. Besides, experiments from Chapter III and IV have been useful as a screening to select, among the species tested, the most suitable species intended for wastewate treatment, as well as for comparison of the performance in wastewater and synthetic media. Since Scenedesmus obliquus demonstrated the best performance, this microalga was chosen as model microorganism for the following experiments (Chapters V and VI). Most systems at real scale are operated in continuous; however the productivities obtained from batch experiments are not valid to be extrapolated to continuous operation. The Chapter V shows, that based on batch kinetic growth parameters, it is possible to predict biomass concentrations and productivities in continuous reactors as a function of the hydraulic retention times. The optimum hydraulic retention times for the process depend on the objective of the culture; some guidelines for optimum retention times based on the specific growth rate from batch phase are given for different objectives. The systems aimed to wastewater treatment could potentially take advantage of the luxury uptake of microalgae observed on previous chapters, especially if it also takes place in darkness. This ability of microalgae observed on previous chapters has been further studied in Chapter VI. This chapter deals with the uptake of nutrients in dark and under illumination and the effect of different biomass concentrations. Finally, on Chapter VII (General Discussion) a new system of wastewater treatment that could arise from these findings is proposed.