Sequencing sedimentation, sorption, and biofiltration to optimize year-round phosphorus removal in a constructed...

Phosphorus (P) pollution in agricultural catchments remains a major challenge for water quality protection. Constructed wetlands are increasingly applied, yet their efficiency under variable hydrology and seasonality is not fully recognized. We assessed a Sequential Sedimentation-Biofiltration System (SSBS; 3700 m²) treating an agricultural stream in central Poland during its 3rd-4th operational year. System performance was evaluated for total phosphorus (TP), orthophosphate (PO₄), and total suspended solids (TSS) across three treatment units: sedimentation (SED), permeable reactive barrier (PRB), and biofiltration (BIO). The SSBS operated at an average hydraulic loading of 2.1 m/d, with inflows ranging 0.046-0.181 m³/s and average P loading of 0.285 g P/m²/d. Overall removal was greater in the growing season (TP: 15 %, PO₄: 20 %) than in the non-growing season (TP: 1 %, PO₄: 7 %). Unit-specific patterns differed: SED ensured stable TP retention (17 % in growing, 43 % in non-growing), BIO achieved robust PO₄ (27 % in growing, 36 % in non-growing) and TSS removal (70 % in growing; 87 % in non-growing), while PRB underperformed and often resuspended solids. Inter-unit differences in TP removal were significant in the growing season (p = 0.0062). Partial Least Squares Regression (PLSR) identified inflow concentration as the main driver of TP and PO₄ removal, with temperature more influential for TP and loading rate for PO₄. Sediment fractionation revealed dominance of redox-sensitive and NaOH-non-reactive P, and absence of Ca-bound P, indicating high release potential. Overall, the sequential SED-PRB-BIO configuration demonstrated complementary, season-dependent functions that sustain year-round pollutant control, though long-term performance requires proactive sediment management and regular maintenance.