Sustainable Microalgal Biofuel Systems: Comparative Techno-Economic and Multi-Criteria Evaluation of Open Pond and Photobioreactor Cultivation

This study presents an integrated techno-economic analysis (TEA) and multi-criteria decision analysis (MCDA) framework for evaluating five microalgae cultivation strategies (open ponds and photobioreactors under continuous, semi-continuous, and fed-batch modes) within a wastewater-integrated biorefinery. Process simulations in SuperPro Designer, Monte Carlo uncertainty propagation, and AHP-based MCDA with six criteria under four weighting schemes were employed. The semi-continuous open pond (S2) achieved the most favorable economics (MFSP: −$2.63 ± 2.17 gal⁻¹), where the negative MFSP reflects a wastewater treatment cross subsidy rather than intrinsic fuel competitiveness, as treated water constituted ~85% of revenue in S1 (continuous open pond) and ~79% in S2. The continuous open pond yielded a marginally positive MFSP ($8.27 ± 3.42 gal⁻¹). PBR systems remained capital-intensive (MFSPs exceeding $165 gal⁻¹), although the fed-batch PBR achieved the highest biomass productivity (0.330 g L⁻¹ d⁻¹) and CO₂ fixation efficiency (~95%) within a 16-fold smaller reactor volume. Breakeven treated water tariffs of $0.9 m - ³ and $1.2 m - ³ were identified; without wastewater revenue, MFSPs rose to $23 gal⁻¹ and $49 gal⁻¹, confirming wastewater integration as a structural prerequisite for viability. A “Lipid Productivity Economic Paradox (wastewater-integrated open ponds only)” was identified, where increasing lipid from 30% to 36% increased MFSP by 9.73% (S1) and 107.19% (S2) due to reduced hydraulic throughput and treatment revenue. The MCDA revealed that the fed-batch PBR ranked first under three of four weighting schemes, while the semi-continuous open pond dominated only under economic priority. These findings provide a decision framework linking cultivation strategy to site-specific wastewater tariffs. • Wastewater-integrated open ponds achieve negative algal MFSP • Hydraulic throughput, not lipid content, drives biofuel economics in open pond • Semi-continuous open ponds outperform photobioreactors economically • Fed-batch photobioreactors maximize productivity within minimal cultivation volume • Techno-economic and MCDA reveal context-dependent optimal cultivation strategies