Recycling microfiltration permeate for microalgal cultivation: Quantitative fouling analysis and phenotype-dependent growth performance

Biomass recovery represents up to 30% of the total energy demand in large-scale microalgae production. Crossflow microfiltration (MF, 0.22 μm PVDF) provides a gentle, scalable harvesting route, generating a concentrated retentate and a nutrient-rich permeate that is typically discarded. This study evaluates the reuse of MF permeate as a culture medium, comparing a cell-walled strain ( Chlorella sp.) and a wall-less, motile strain ( Dunaliella tertiolecta ). Permeate was tested under two formulations: nitrate only and nitrate plus micronutrients and benchmarked against the standard medium (control). Growth trials ( n = 2) revealed distinct phenotypic responses: Chlorella sp. reached 1.30 ± 0.05 g L −1 in the control, 0.80 ± 0.05 g L −1 in permeate enriched with micronutrients, and 0.20 ± 0.02 g L −1 in direct permeate, whereas D. tertiolecta achieved 1.43 ± 0.01 g L −1 in enriched permeate, exceeding the control (0.98 ± 0.02 g L −1 ) and direct permeate (1.02 ± 0.04 g L −1 ). Diafiltration (DF) improved permeate quality by reducing conductivity from 8.5 mS cm −1 to 5.0 mS cm −1 (38–42%), while steady-state permeate flux ranged from 14 to 25 L m −2 h −1 . Fouling analysis using the Hermia–Field model indicated cake-layer formation (R 2 = 0.98) for Chlorella sp. and mixed standard–cake mechanisms for D. tertiolecta . MF-permeate recycling thus emerges as a phenotype-dependent strategy to reduce freshwater and nutrient demand by approximately 40%, representing the first application of Hermia–Field fouling analysis to D. tertiolecta and the first phenotype-resolved comparison of recycled-medium performance. • Comparative microfiltration (MF) fouling analysis of two microalgal phenotypes • Hermia–Field model validated for predicting microfiltration fouling behavior • Permeate recycling demonstrated and efficiency influenced by algal phenotype • Integrated MF–DF strategy improves harvesting and water sustainability