Chemical and Structural Characterisation of Bio-Earth as a Sustainable Substrate for Wastewater Treatment in Constructed Wetlands

Constructed wetlands (CWs) are growing as sustainable, nature-based solutions for wastewater treatment, esteemed for their cost-effectiveness, ecological benefits and nutrient removal capabilities. This study introduces bio-earth, a newly engineered substrate recovered from biomined legacy waste, for its first application as a sustainable medium in CWs. Using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDAX), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR) and hydraulic and microbial analyses, the bio-earth was found to possess optimal properties for wetland applications including high porosity (40%), good permeability (3.19 × 10–3 cm/s) and a mineral composition (e.g. quartz, calcite, hematite) that enhances structural integrity, pH buffering capacity and contaminant adsorption. Moreover, bio-earth effectively immobilised heavy metals and supported diverse microbial populations (e.g. Bacillus and Pseudomonas), which are critical for pollutant degradation. Domestic wastewater treatment using vertical-flow CWs with the bio-earth-based system achieved significant removal efficiencies for BOD5 (68%), COD (72%), nitrate (60%), TSS (78%) and a 2.1-log reduction in coliforms, showing better results than the unplanted control during the 24-week study. According to these results, bio-earth is a promising, eco-friendly substrate for circular waste management. Further field-scale studies are recommended to evaluate its long-term performance in full-scale wetland systems.