Effect of organic loading rates on olive pomace biochar-enhanced vertical flow constructed wetlands for wastewater treatment

This study investigates the performance of vertical flow constructed wetlands (VF-CWs) using olive pomace biochar (CW-B) versus a sand-only control (CW-C) under two organic loading rates (OLR): low (20 g COD/m².d) and high (70 g COD/m².d). At low OLR, CW-B achieved higher removal efficiencies than CW-C, with 60% vs. 50% for COD, 85% vs. 71% for NH₄⁺-N, 74% vs. 52% for TKN, 81% vs. 67% for TSS, and over 3-log unit reductions in faecal indicators. Increasing OLR led to an overall decrease in treatment efficiency, although CW-B remained superior. Biochar had a limited effect on phosphorus, sulphate, and hardness removal. Release of NO₂ and NO₃ confirmed active nitrification, which decreased at high OLR. These results highlight the potential of olive pomace biochar as a sustainable and effective VF-CW substrate, especially under moderate organic loads.