Techno-economic estimation of wastewater phycoremediation and environmental benefits using Scenedesmus obliquus microalgae

This study investigated the dual application of Scenedesmus obliquus for wastewater phycoremediation and biochemical component accumulation in microalgal cells. The microalgae grown in wastewater showed micro-elements uptake and removal efficiencies of 71.2 ± 3.5% COD, 81.9 ± 3.8% NH₄⁺, ∼100.0% NO₃^-, and 94.1 ± 4.7% PO₄^3-. The growth profile of Scenedesmus obliquus indicated a specific growth rate of 0.42 ± 0.02 1·d^-1 and carrying capacity of 0.88 ± 0.04 g L^-1. The lipid, protein, and carbohydrate yields (w·w^-1 of dry weight) were 26.5 ± 1.5%, 28.5 ± 1.5%, and 27.5 ± 1.6%, respectively. The de-oiled biomass was subjected to biochemical extraction, achieving protein and carbohydrate yields of 25.3 ± 1.4% and 21.4 ± 1.2%, respectively. Fourier transform infrared spectroscopy showed several functional groups (e.g., NH, CH₃, CH₂, CO, CN, PO, and SiO) on the biomass surface, confirming the accumulation of biochemical elements in microalgae. The thermal analysis of microalgal biomass depicted sequential stages of dehydration (60-190 °C), devolatilization (200-490 °C), and solid residue decomposition (490-600 °C). The cost-benefit analysis of microalgae cultivated in wastewater was derived regarding amortization and operating costs and energy and environmental benefits. The net profit of phycoremediation was 16885 US$·y^-1, resulting in a payback period of 14.8 years (i.e., shorter than the project lifetime). Accordingly, the proposed phycoremediation process was economically viable.