ABSTRACT The exploitation of microalgae in a wastewater treatment process is currently an open is... more ABSTRACT The exploitation of microalgae in a wastewater treatment process is currently an open issue, and its actual applicability is still under investigation. In this work the effects of temperature, day/night irradiation and bacterial competition were studied on growth and nutrient removal of Chlorella protothecoides cultivated in real and unsterilized primary urban wastewater. C. protothecoides showed a linear dependence of growth rate on temperature under continuous irradiation with a maximum at 30 °C. Continuous flow experiment under day–night irradiation condition showed that a cyclic steady state was achieved, with significant differences in biomass concentration and nutrient removal after dark and light periods. The presence of a native microflora in wastewater did not affect the microalgal growth, both in batch and continuous flow experiments. N and P were efficiently removed in all conditions tested, while the COD was not consumed by microalgal biomass. Thus, in view of a large scale application, a two step depuration process would be required, where the photobioreactor will remove nutrients as N and P and an activated sludge reactor downstream will reduce the organic matter. The experimental data obtained were used to design a possible process of this type.
The use of bicarbonate as carbon supply in a large scale photobioreactor for industrial microalga... more The use of bicarbonate as carbon supply in a large scale photobioreactor for industrial microalgae production could reduce the compression and transportation costs associated with the direct use of CO 2 from flue gas. Some microalgae species are able to directly exploit bicarbonate as a carbon source, but the actual yield in view of large scale production is not clear, in particular for eukaryotic and lipid-rich strains. In this work, the exploitation of bicarbonate as carbon source by Chlorella protothecoides was critically assessed. Both batch and continuous experiments were carried out with the addition of NaHCO 3 to the culture medium. The growth of this species was found to be strongly affected by the increased pH, resulting in a reduced yield if compared with that from an air−CO 2 bubbling supply. To overcome this issue, a pH control system was applied, so that the pH value was maintained below 8. In these conditions a specific growth rate was measured close to that obtained under CO 2 bubbling, probably due to the equilibrium of carbonate, which is shifted to H 2 CO 3 and free CO 2 . Thus, it appears that C. protothecoides is not able to directly use bicarbonate, but exploits the CO 2 made available from the shifting of the carbonate equilibrium. In addition, the presence of bicarbonate also caused a lipid accumulation in C. protothecoides, up to about 40% dry weight, compared to the constitutive value of 10−12% dry weight. On the basis of the results obtained, a process scheme was proposed, integrating microalgae cultivation with CO 2 capture by absorption with a recirculating carbonate solution. Process simulation was carried out to show the technical feasibility of this process.
ABSTRACT The exploitation of microalgae in a wastewater treatment process is currently an open is... more ABSTRACT The exploitation of microalgae in a wastewater treatment process is currently an open issue, and its actual applicability is still under investigation. In this work the effects of temperature, day/night irradiation and bacterial competition were studied on growth and nutrient removal of Chlorella protothecoides cultivated in real and unsterilized primary urban wastewater. C. protothecoides showed a linear dependence of growth rate on temperature under continuous irradiation with a maximum at 30 °C. Continuous flow experiment under day–night irradiation condition showed that a cyclic steady state was achieved, with significant differences in biomass concentration and nutrient removal after dark and light periods. The presence of a native microflora in wastewater did not affect the microalgal growth, both in batch and continuous flow experiments. N and P were efficiently removed in all conditions tested, while the COD was not consumed by microalgal biomass. Thus, in view of a large scale application, a two step depuration process would be required, where the photobioreactor will remove nutrients as N and P and an activated sludge reactor downstream will reduce the organic matter. The experimental data obtained were used to design a possible process of this type.
The use of bicarbonate as carbon supply in a large scale photobioreactor for industrial microalga... more The use of bicarbonate as carbon supply in a large scale photobioreactor for industrial microalgae production could reduce the compression and transportation costs associated with the direct use of CO 2 from flue gas. Some microalgae species are able to directly exploit bicarbonate as a carbon source, but the actual yield in view of large scale production is not clear, in particular for eukaryotic and lipid-rich strains. In this work, the exploitation of bicarbonate as carbon source by Chlorella protothecoides was critically assessed. Both batch and continuous experiments were carried out with the addition of NaHCO 3 to the culture medium. The growth of this species was found to be strongly affected by the increased pH, resulting in a reduced yield if compared with that from an air−CO 2 bubbling supply. To overcome this issue, a pH control system was applied, so that the pH value was maintained below 8. In these conditions a specific growth rate was measured close to that obtained under CO 2 bubbling, probably due to the equilibrium of carbonate, which is shifted to H 2 CO 3 and free CO 2 . Thus, it appears that C. protothecoides is not able to directly use bicarbonate, but exploits the CO 2 made available from the shifting of the carbonate equilibrium. In addition, the presence of bicarbonate also caused a lipid accumulation in C. protothecoides, up to about 40% dry weight, compared to the constitutive value of 10−12% dry weight. On the basis of the results obtained, a process scheme was proposed, integrating microalgae cultivation with CO 2 capture by absorption with a recirculating carbonate solution. Process simulation was carried out to show the technical feasibility of this process.
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Papers by Barbara Gris