Modeling and Optimization of a Combined Ozonation and Microalgae Cultivation Process using Nutrients from Landfill Leachate

Abstract To couple an integrated system for the optimized cultivation of Chlorella vulgaris in a photobioreactor, using real-time monitoring through an RGB sensor, together with pretreatment by ozonation in a rotating packed bed (RPB) reactor, this study applied response surface methodology based on a Central Composite Design (CCD) and multiobjective optimization using the Normal Boundary Intersection (NBI) algorithm. Raw landfill leachate presented initial values of COD = 3540 mg L⁻1, BOD = 312 mg L⁻1, NH₃–N = 2965 mg L⁻1 and an initial BOD/COD ratio of 0.08. The ozonation pretreatment was carried out in an RPB reactor operating at 1000 rpm, with ozone concentrations ranging from 2.0 to 10.8 g O₃ m⁻3 and pH values between 4.3 and 9.2. Under optimized conditions (pH ≈ 7.0 and ozone concentration ≈ 2.7 g O₃ m⁻3), the BOD/COD ratio increased from 0.08 to approximately 0.28–0.30, representing an increase of more than 250%, while COD was reduced by up to 70%. Although ammoniacal nitrogen removal during ozonation was limited, NH₃–N concentrations decreased from 2965 mg L⁻1 to approximately 2024 mg L⁻1, enabling its subsequent use as a nutrient source for microalgal cultivation. Batch cultivation assays using treated leachate (80 mL) mixed with microalgal culture (120 mL) demonstrated effective nitrogen assimilation, achieving residual NH₃–N concentrations below 300 mg L⁻1 and biomass concentrations up to 299 mg L⁻1. The integration of automated monitoring based on RGB color stabilization allowed real-time identification of the stationary growth phase and controlled feeding of pretreated leachate. The combined ozonation–microalgae system demonstrated technical feasibility, reduced external nutrient demand by up to 75%, and enhanced biomass production using landfill leachate as a macronutrient source. This integrated and optimized approach represents a sustainable strategy for landfill leachate valorization, combining advanced oxidation, process automation, and microalgal biotechnology. Graphical Abstract