Membrane-assisted continuous cultivation enables stable and high-productivity microalgal single-cell protein production.

Growing demand for sustainable protein has increased interest in microalgal single-cell protein (SCP) production using waste-derived nutrients. However, batch cultivation is limited by progressive nitrogen depletion, which causes biomass protein content to decline below SCP specifications (>30% protein). This study evaluated membrane-assisted continuous cultivation for stable production of SCP-grade microalgal biomass from food waste digestate usingScenedesmus obliquus,Graesiella emersonii, andChlorella sorokinianain 3 L photobioreactors. In batch culture, protein-rich biomass was obtained only within a narrow 2-day window, with protein contents of 42-55%; after nitrogen depletion, protein content declined to 11-29% by day 8. In contrast, continuous cultivation sustained protein contents above 46% for up to 189 d, showing that continuous nitrogen supply can preserve SCP-grade protein content over extended operation. By decoupling solids retention time (SRT) from hydraulic retention time (HRT) using an immersed membrane, continuous operation was systematically optimized. An operating window of SRT 3 d and HRT 2-3 d achieved protein productivity up to 0.14 g/L/d while meeting the SCP protein-content criterion. These findings establish membrane-assisted continuous cultivation as a controllable strategy for stable, high-productivity production of SCP-grade microalgal biomass from food waste digestate.