Seasonal assessment of experimental vertical-flow constructed wetlands treating domestic wastewater

The aim of this work was to compare the impact of different design (aggregate size) and operational (contact time, empty time and chemical oxygen demand (COD) loading) variables on the long-term and seasonal performance of vertical-flow constructed wetland filters operated in tidal flow mode. Compliance was achieved regarding ammonia–nitrogen, nitrate–nitrogen and suspended solids (SS), and non-compliance concerning biochemical oxygen demand (BOD) and ortho-phosphate–phosphorus. The filter with the highest COD loading performed the best regarding outflow COD concentration. Higher COD inflow concentrations had a significantly positive impact on the treatment performance for COD, ortho-phosphate–phosphorus and SS. The wetland with the largest aggregate size had the lowest mean nitrate–nitrogen outflow concentration. However, the results were similar regardless of aggregate size and resting time for most variables. Clear seasonal outflow concentration trends (low in summer) were recorded for COD, ammonia–nitrogen and nitrate–nitrogen. No filter clogging was observed.