Exhausted culture media reuse in autotrophic microalgae production: Optimization and modelling of Pseudococcomyxa simplex cultures

This study investigated strategies to reduce the water footprint in autotrophic microalgae cultivation by recycling exhausted culture medium under semi-continuous operation. Pseudococcomyxa simplex, a polyextremotolerant strain, was evaluated for medium reuse suitability. Experimental results demonstrated stable biomass productivity (∼0.2 g·L^-1·day^-1) over 30 days at a purge ratio of 30 % and dilution rate of 0.19 day^-1, reducing water consumption from 1000 to 320 kg·kg^-1 biomass and lowering medium costs to 0.05 €·g^-1. Full medium recycling, even with nitrogen supplementation, caused growth inhibition due to impaired photosystem II efficiency and chlorophyll synthesis. Partial recycling maintained the biochemical profiles, with stable proteins and lipids and a slight increase in carbohydrates at higher purge ratios. ASPEN PLUS® simulations provided mass and energy balances, CO₂ absorption dynamics, and water loss estimates, confirming experimental trends and identifying optimal operating conditions for maximizing biomass and metabolite yields while minimizing environmental impact. Limitations in modeling nitrogen uptake suggest the need for advanced kinetic-stoichiometric models to improve scalability. This integrated experimental-modeling approach demonstrates that controlled medium recycling significantly enhances process sustainability without compromising productivity, providing critical insights into sustainable bioprocess optimization.