The effect of the internal nutrient quota accumulation on algal-based wastewater treatment: Decoupling HRT and SRT to improve the process

The application of microalgae to wastewater treatment is an emerging technology following a circular economy approach. The optimization of such a process for large-scale applications in continuous reactors is not trivial, since both high nutrients removal and biomass productivity are desired. As microalgae are able to change their composition as a function of environmental variables, the accumulation of nutrients internal quota may have a role in the process. In this work, the kinetic parameters of microalgae growth and nutrients uptake/accumulation according to the Droop model were retrieved from experiments in continuous photobioreactors, and the model was used to investigate the behavior of the process. The analysis revealed that, by uncoupling hydraulic retention time (HRT) and solid retention time (SRT), both nutrients removal and biomass productivity could be optimized, which is not possible when operating a chemostat system. In wastewater with high nutrient concentration it was found that, by decoupling HRT and SRT, the same maximum biomass productivity of 470 g m−3 d−1 can be achieved as with a single chemostat reactor operating at SRT = 1.5 day, but reducing the outlet nutrient concentration by 15 %. In addition, by taking advantage of the Droop model's ability to predict the nutrient content in biomass, it is possible to work towards obtaining biomass with the desired internal amount of nutrients, varying, for example, the nitrogen quota from 11 % to 14 %, depending on the scope of the biomass produced.