Closing the Nutrient Loop Through Multi-Cycle Phototrophic Reuse of Landfill Leachate in Cyanobacterial PHB Bioproduction

This study investigated a phototrophic approach to close nutrient loops by using landfill leachate as a culture medium to produce biomass and polyhydroxybutyrate (PHB) from a thermotolerant strain of Potamosiphon sp. A multi-cycle reuse scheme in which post-culture leachate was partially replenished with fresh leachate and reused in successive cultivation rounds to increase the biomass concentration (g/L) and the intracellular PHB content (% w/w) was tested. Three operational variables were optimized (leachate replenishment percentage, number of reuse cycles, and sanitation method (autoclaving, UV irradiation, or no treatment)) via the Box–Behnken response surface method. Both response variables were modeled with high predictive accuracy (R2 = 0.98 for biomass and R2 = 1.00 for PHB content). According to the experimental data, leachate replenishment emerged as the key factor influencing nutrient availability—particularly nitrogen and phosphorus—and thus PHB accumulation. The optimized conditions (2.17% v/v fresh leachate, three reuse cycles, and UV sanitation) yielded predicted values of 0.29 g/L biomass and 3.48% w/w PHB. These results demonstrate the feasibility of a controlled multicycle reuse process that integrates effluent treatment and biopolymer synthesis, offering a low-input, circular biotechnological approach for sustainable leachate valorization.