Analysis of a photobioreactor scaling up for tertiary wastewater treatment: denitrification, phosphorus removal, and microalgae production

Abstract

The present work studies the removal of nutrients (nitrate and phosphate) from a synthetic wastewater simulating a secondary treatment effluent using the microalgae Chlorella vulgaris in autotrophic photobioreactors, together with an analysis of the critical points affecting the scaling-up process from laboratory to pilot scale. Laboratory experiments were done in open agitated 1-L photobioreactors under batch operation mode, while pilot-scale experiments were done using a 150-L closed tubular photobioreactor under continuous operation mode. In both scales, nitrate was the limiting substrate and the effect of its concentration on microalgae performance was studied. From laboratory experiments, an average microalgae productivity of 85 mg_VSS L⁻¹ day⁻¹ and approximate maximum N-NO₃⁻ and P-PO₄³⁻ removal rates of 8 mg N g_VSS⁻¹ day⁻¹, and 2.6 mg P g_VSS⁻¹ day⁻¹ were found. Regarding pilot scale, the average microalgae productivity slightly decreased (76 mg_VSS L⁻¹ day⁻¹) while the approximate maximum N-NO₃⁻ and P-PO₄³⁻ removal rates slightly were increased (11.7 mg N g_VSS⁻¹ day⁻¹ and 3.04 mg P g_VSS⁻¹ day⁻¹) with respect to the laboratory-scale results. The pilot-scale operation worked under lower levels of turbulence and higher dissolved oxygen concentration and light intensity than laboratory experiments; those parameters were difficult to control and they can be identified as the critical points in the differences found on both nutrient removal and microalgae production.