How to combine CO2 abatement and starch production in Chlorella vulgaris

Microalgae production has gained attention in recent years as promising systems for CO2 abatement as well as a source of proteins, pigments, vitamins, lipids, and carbohydrates. Particularly, starch can be used for bioethanol production in a well-established fermentative process. The aim of this work was to maximize and model biomass productivity and CO2 assimilation in continuous cultures of Chlorella vulgaris. The following culture parameters were studied: dilution rate, pH, temperature, light intensity, and nitrogen supply. The proposed model (r2 = 0.95) predicted a maximum biomass productivity of 0.7 g L−1 d−1 and CO2 assimilation of 1.3 g L−1 d−1. The experimental data agreed with these predictions, resulting in a maximum biomass productivity of 0.67 g L−1 d−1 (resulting in a CO2 assimilation of 1.23 g L−1 d−1). In addition, the starch content was determined, and the results were used as input into a second model, which aimed at predicting starch accumulation during CO2 abatement processes (r2 = 0.84). This second model predicted a daily and continuous production of biomass with a maximum starch content of 0.25 g g−1 d−1 (25% dcw), but under different culture conditions than those found for maximizing biomass productivity and CO2 assimilation. The maximum starch content experimentally determined was 0.2 g g−1 d−1 (20% dcw). Thus, to implement a biological system for CO2 abatement coupled to starch accumulation, it is necessary to find a compromise between these two processes. Hence, although yield in both processes would be reduced, a simultaneous process for CO2 mitigation and starch production would be feasible.This research was supported by the scientific Project CENIT SOST-CO2–New sustainable industrial uses of CO2, with financing by INABENSA S.A

Utilization of centrate for the outdoor production of marine microalgae at pilot-scale in flat-panel photobioreactors

The outdoor production of marine microalgae biomass in pilot scale flat panels photobioreactors, under not sterile conditions and using centrate as nutrients source, was studied. Experiments were performed modifying the centrate percentage, dilution rate and orientation of the photobioreactors. The strain Geitlerinema sp. was that one prevailing independently of the culture conditions. The higher productivity of 47.7 gbiomass·m-2·day-1 dry weight and photosynthetic efficiency of 2.8%, was achieved when using 20% centrate and a dilution rate of 0.3 day-1, whatever the orientation of the reactor, maximal nutrient removal capacities of 82%, 85% and 100% for carbon, nitrogen and phosphorus being obtained. Under non-optimal conditions up to 80% of the nitrogen and 60% of the phosphorus were lost by stripping and precipitation, respectively. Carbohydrates was the major component of the biomass followed by proteins and lipids. These results support the possibility to produce microalgae biomass below 0.59 €/kg, useful to produce biofertilizers and animal feed

Analysis of mass transfer capacity in raceway reactors

In the present work, a methodology is proposed to determine the mass transfer capacity in existing microalgae raceway reactors to minimize excessive dissolved oxygen accumulation that would otherwise reduce biomass productivity. The methodology has been validated using a 100 m2 raceway reactor operated in semi-continuous mode. The relevance of each raceway reactor section was evaluated as well as the oxygen transfer capacity in the sump to different air flow rates. The results confirm that dissolved oxygen accumulates in raceway reactors if no appropriate mass transfer systems are provided. Therefore, mass transfer in the sump is the main contributor to oxygen removal in these systems. The variation in the volumetric mass transfer coefficient in the sump as a function of the gas flow rate, and therefore the superficial gas velocity in the sump, has been studied and modelled. Moreover, the developed model has been used to estimate the mass transfer requirements in the sump as a function of the target dissolved oxygen concentration and the oxygen production rate. The proposed methodology allows us to determine and optimize the mass transfer capacity in the sump for any existing raceway reactor. Moreover, it is a powerful tool for the optimization of existing reactors as well as for the design optimization of new reactors