Semi-continuous cultivation of Nannochloropsis oceanica enables robust brewery wastewater treatment under variable wastewater composition and improves biomass lipid accumulation

This study investigates the effects of time-dependent changes in brewery wastewater (BWW) composition on the treatment performance of Nannochloropsis spp. and evaluates a semi-continuous “fill-and-draw” feeding strategy to mitigate fluctuations in wastewater composition. BWW was collected at five distinct time points in the brewery's annual production cycle (referred to as Collections 1–5, C1-C5) and evaluated for microalgal treatment under batch and semi-continuous operation. In batch mode, three species ( N. oceanica , N. gaditana , N. limnetica ) grew on moderate-COD BWW (C1-C4, 3.05–6.45 g L −1 COD), producing 0.66–1.21 g L −1 biomass and achieving high nitrogen (84–100%), phosphorus (68–97%), and COD removal (55–81%). Nannochloropsis spp. displayed mixotrophic pathways, utilising up to 0.98 g C-sugar g −1 C-biomass and capturing up to 0.96 g C-CO 2 g −1 C-biomass (or 1.30 g CO 2 L −1 wastewater). A high-COD BWW (C5, 16.67 g L −1 COD) inhibited growth in batch mode. However, semi-continuous operation ( N. oceanica ) transitioning from C1 to C5 facilitated physiological acclimatisation and mitigated high-COD growth inhibition, sustaining 0.92–1.28 g L −1 biomass and achieving up to 100% removal of total applied nitrogen and 89% COD removal. Extended nitrogen limitation during semi-continuous operation increased lipid content versus batch cultivation (56.7 vs 44.4% DW) and yielded a comparatively stable biomass composition (53.1–56.7% DW lipids; 16.3–16.5% DW protein; 27.0–30.4% DW carbohydrate) despite using feedstocks with drastically different compositions. This work provides a practical operational strategy to ensure robustness of microalgae-based BWW treatment under dynamic variability in wastewater composition.