Optimization of Phytoremediation in Horizontal Subsurface Flow Constructed Wetlands: Evaluating Plant Diversity Effects on Tannery Wastewater Treatment

Abstract

Leather tannery wastewater contains hazardous chromium, sulfides, and high organic loads that threaten aquatic ecosystems. This study compared phytoremediation performance of horizontal subsurface flow constructed wetlands (HSF-CW) planted with monocultures and polycultures of macrophytes. Five pilot-scale systems (7.0 m² each) were operated for 120 days at Banthala Leather Complex, West Bengal: (1) Phragmites australis, (2) Typha domingensis, (3) Vetiveria zizanioides, (4) binary combination (Phragmites + Typha), and (5) ternary combination (Phragmites + Typha + Vetiveria). At an optimal hydraulic retention time (HRT) of 8 days, the ternary system achieved maximum removal efficiencies: 86.8% chemical oxygen demand (COD), 74.2% total nitrogen (TN), and 89.6% chromium (Cr). Polyculture systems demonstrated significant synergistic effects, with the ternary combination outperforming monoculture averages by 6.2–8.4% for COD and 7.8–9.2% for chromium removal. Phragmites exhibited the highest chromium bioaccumulation (421 ± 52 mg/kg), while Typha showed superior nitrogen uptake (48.2 ± 6.1 g/kg). Plant-mediated chromium removal accounted for 18.4% of total removal in the ternary system versus 8.6–14.2% in monocultures. These findings demonstrate that polyculture HSF-CW systems with spatially optimized species distribution offer a sustainable, low-cost (USD 1.20/m³) pre-treatment solution for chromium-laden industrial wastewater.