Optimizing light/dark cycles and nutrient ratios for continuous microalgae application in nitrate removal and CO2 fixation

Nitrate pollution in well water poses significant health risks, leading the US EPA to recommend keeping nitrate-nitrogen (NO3--N) level below 10 mg/L. This study investigates Scenedesmus sp. and Chlorella vulgaris as green solutions for nitrate removal and CO2 capture, addressing challenges of traditional methods. It focuses on enhancing microalgal performance by optimizing operational conditions in batch mode before moving to continuous operation. In batch cultivation, Scenedesmus sp. outperformed Chlorella vulgaris, achieving a NO3--N removal rate (NRR) of 13.6 mg/L/d and a CO2 fixation rate (R-CO2) of 0.36 g/L/d within a 76-h hydraulic retention time (HRT). To optimize batch conditions, three light-dark cycles (24L:0D, 16L:8D, and 12L:12D) and nitrogen-to-phosphorus (N:P) ratios (5, 10, and 20) were evaluated. Scenedesmus sp., at an N:P ratio of 10 in both the 24L:0D and 16L:8D cycles, achieved NRRs of 13.6 and 13.4 mg/L/d, with HRTs of 76 and 78 h, respectively, while keeping PO43--P below 0.5 mg/L. Longer light durations positively influenced nitrate reduction and CO2 fixation, however the 24L:0D cycle resulted in a higher produced chemical oxygen demand (COD) of 35 mg/L. Reducing HRT from 4 to 3 days during continuous operation of Scenedesmus sp. under 16:8 and 24:0 conditions increased nutrient uptake and lowered NO3--N concentrations below 10 mg/L within 30 days. This study emphasizes the importance of optimizing light and nutrient conditions for effective nitrate removal and CO2 fixation by microalgae, as well as applying reduced HRT strategies for practical large-scale applications.