Environmental assessment of microalgae valorization: Co-digestion with dairy industry wastewater vs. recovery of microalgae-based products

In this study, the harvesting of a mixed microalgae culture using chitosan as a flocculant was evaluated, and a Life Cycle Assessment (LCA) was conducted for various valorization scenarios of the harvested microalgae. The highest removal efficiency was found to be 98.1 ± 0.1 % for dosage of 20 mg chitosan/g dry microalgae biomass and a mixing time of 120 s. The harvested microalgae biomass was dried for the biochemical methane potential (BMP) test and for the characterization of carbohydrates, proteins, and lipids in the mixed microalgae biomass. For the BMP test, dairy industry wastewater (DIW) and microalgae biomass were selected in the following ratios based on volatile solids (VS): 100:0, 90:10, 80:20, 70:30, 60:40, and 0:100 (DIW:microalgae biomass). The highest methane production was observed at the mixing of DIW:microalgae in ratios of 100:0 and 90:10, with values of 282 and 280 mL CH₄/g VS, respectively. The carbohydrate, protein, and lipid contents of the dried mixed culture microalgae biomass (DMB) was found to be 30 ± 2.9 %, 43.3 ± 5.4 % and 22.8 ± 0.4 %, respectively. The data obtained in this study were used in the LCA, which was conducted for three different microalgae valorization scenarios; Scenario-1 for biogas production from DMB; Scenario-2 for feed production from DMB; Scenario-3 for biofuel production from DMB. According to the LCA analysis, Scenario-3 was found to be the most environmentally friendly alternative, due to the negative environmental impacts associated with the business-as-usual (BAU) products.