Influence of nitrogen species and biomass retention time on nutrient removal and biomass productivity in a microalgae-based bioreactor

This study examined the effects of nitrogen species (NH₄⁺–N/NO₃⁻–N ratio) and biomass retention time (BRT) on nutrient uptake and biomass productivity in Chlorella sp. In batch photobioreactors (PBR), microalgae were cultivated with varied nitrogen species ratios (100% NH₄⁺–N, 50% NH₄⁺–N:50% NO₃⁻–N, and 100% NO₃⁻–N). It was observed that a medium containing 100% NO₃⁻–N boosted algae growth, with a maximum biomass concentration of 3188 mg/L. However, the lowest nutrient removal rates were obtained under the 100% NO₃⁻–N condition, according to the reverse logistics model. In contrast, the highest removal rates occurred for substrate containing 100% of NH₄⁺–N species based on first-order decay models, although yield biomass was lower (2940 mg/L). The presence of the ammonia nitrogen resulted in the flocculation of the microalgae into large flocs, suggesting that ammonia nitrogen was rapidly consumed to produce flocculation-related metabolic products rather than for biomass production. Using the 100% NH₄⁺–N medium, different BRT values (7, 5, 3, 2 days) were examined under continuous operation using a membrane photobioreactor (MPBR). At 3-day BRT, the maximum biomass productivity, nitrogen, and phosphorous removal rates were 214 ± 4, 63.1 ± 4.1, and 2.1 ± 0.6 mg/L d, respectively. The biomass yield increased as the BRT decreased, leading to an increase in pollutant removal rates. Finally, this study provides some essential information for improving the operating conditions of membrane photobioreactor system under different biomass retention times and various nitrogen sources in the feed. © 2022 The Author(s). Published by Elsevier B.V.