Optimisation constructed wetlands through mixed macrophyte planting: functional synergy between Iris pseudacorus and  Pontederia cordata for polluted river water treatment  and agricultural reuse

The ongoing deterioration of surface water quality in Mediterranean coastal regions calls for resilient and efficient Nature-based Solutions (NbS). This study investigates the role of macrophyte diversity in enhancing the purification performance of horizontal subsurface flow constructed wetlands (HSF-CWs). Four vegetation configurations were evaluated: an unplanted control, two monocultures (Iris pseudacorus and Pontederia cordata), and a mixed culture (MC), for the treatment of highly polluted water from the Oued Seybouse (north-eastern Algeria).
The experimental design utilized two-way analysis of variance (ANOVA) and principal component analysis (PCA) to evaluate the influence of vegetation configuration, hydraulic retention time (HRT), and their interaction on treatment efficiency. The results demonstrated a statistically significant superiority of the mixed culture over all other systems, as confirmed by Tukey’s post-hoc test (p < 0.001). After 14 days of treatment, the MC system achieved removal efficiencies of 99.9% for faecal coliforms, 81.9% for total nitrogen, and 69.1% for biochemical oxygen demand (BOD₅). PCA further revealed a distinct functional response of the mixed culture, characterised by a 65.4% reduction in electrical conductivity and complete removal of total coliforms (100%).
These enhanced performances are attributed to complementary ecological functions between the two macrophyte species. The superior radial oxygen loss (ROL) of I. pseudacorus promoted aerobic nitrification, whereas the rapid nutrient uptake and diverse root exudates of P. cordata enhanced denitrification and pathogen suppression. The treated effluents complied with the most stringent irrigation reuse standards established by the WHO (2006) and EU Regulation 2020/741 (Class A), demonstrating the suitability of the mixed culture system for safe agricultural water reuse. Overall, this study provides evidence that macrophyte diversity significantly improves the efficiency of constructed wetlands compared with monocultures, offering an effective and sustainable Nature-based Solution for decentralised wastewater treatment and water reuse in Mediterranean regions facing chronic water scarcity.