Using eco-friendly coagulants for biomass harvesting and water reuse in microalga cultivation

Abstract

The cultivation of microalgae is gaining prominence, but one of its main challenges lies in effectively harvesting the biomass and recovering the water used in the cultivation process. Addressing these issues is critical to minimizing environmental impacts and enhancing the sustainability of microalgae production systems. This study evaluates the efficiency of polyaluminum chloride (PAC) and a tannin-derived coagulant for harvesting Nannochloropsis oculata biomass, while also analyzing the potential for reusing the treated water. Microalgae were grown in a 5 L culture until a cell density of 10⁴ cells mL⁻¹ was achieved. After cultivation, samples were subjected to coagulation, flocculation, sedimentation, and filtration through a granular activated carbon system. An acute toxicity assessment was conducted using Daphnia magna as a bioindicator to determine the safety of the recovered water. Both coagulants demonstrated over 98% removal efficiency for suspended solids, turbidity, chemical oxygen demand (COD), and biochemical oxygen demand (BOD). PAC, at a concentration of 50 mg L⁻¹, yielded higher water quality than the tannin-based coagulant, which required 150 mg L⁻¹ for optimal performance. Furthermore, the treated water met international standards for irrigation in non-food crops. Toxicity testing confirmed that the treated effluent was non-toxic, making both coagulants suitable for microalgae harvesting, with PAC offering a more efficient option for water quality enhancement and lower chemical consumption.

Keywords: biomass harvesting, Nannochloropsis oculate., polyaluminum chloride, tannin-based coagulant, water reuse.