Coupling microalgal cultivation with forward osmosis dewatering for ammonium-rich digestate treatment: Operando visualization of thick yet reversible biofouling

Ammonium-rich anaerobic digestion effluent (ADE) offers a potential nutrient source for microalgal cultivation, but biomass concentration and fouling management remain challenges. This study combined Chlorella vulgaris cultivation in pretreated real ADE with forward osmosis (FO) dewatering and operando optical coherence tomography (OCT) to relate deposit development to cleaning response. Among the conditions tested, 30 % ADE yielded the highest measured dry cell weight (DCW; 2.00 ± 0.09 g/L) and DCW-based productivity (65.6 ± 2.1 mg/(L·d)), whereas lower DCW and apparent nutrient removal were observed at 40 % ADE. DCW represented retained dry solids, and modified BG11 (mBG11) served as a nitrogen-deficient reference. After 12 h, normalized flux decreased to 0.19 for mBG11 and 0.16 for 30 % ADE, while volumetric concentration factors reached 1.30 and 1.43, respectively. The individual contributions of changing osmotic conditions, concentration polarization, and fouling were not resolved. OCT revealed a thinner but more persistent deposit under mBG11 and a thicker, more readily removable deposit under 30 % ADE. The latter reached an apparent thickness of 86.0 ± 1.9 μm after 12 h, with post-cleaning residual signals near or below the nominal axial resolution. Across five consecutive 12-h cycles, residual deposits accumulated and operational flux recovery declined under both conditions. These observations demonstrate the value of combining structural and operational measurements to assess deposit persistence. The modest concentration achieved and repeated draw-solution replacement leave long-term performance and practical feasibility unresolved.