Enhanced greywater treatment in a two-stage green wall system by incorporating waste-derived adsorbents
Abstract
Greywater reuse offers a sustainable approach to alleviating freshwater scarcity, but concerns about treatment reliability limit its adoption. This study investigates a two-stage green wall system for greywater treatment, integrating vegetated units planted with Carex secta and vertical flow post-treatment columns (PTCs) packed with waste-derived adsorbents—calcinated kina (sea urchin) shell, ZnCl 2 -activated apricot stone biochar, and natural zeolite. The system’s performance was evaluated for removal of organic matter (COD, TOC), nutrients (ammonia, nitrate, nitrite, phosphate), and xenobiotics (anionic surfactants, aromatic compounds) under varying hydraulic and pollutant loads. Substrate characterization, plant health assessment, microbial analysis, and physicochemical water quality monitoring were used to elucidate plant–microbe–media interactions. PTCs effectively polished residual pollutants after partial removal in green wall units, achieving removal rates of 34% of phosphate, 99.8% of ammonia, 95% of surfactants, and 25%–37% of aromatic compounds (UV (294) ). While nitrate leached from green wall units (−34%), PTCs mitigated this by removing 64% of nitrate. TOC and COD were further reduced by 17% and 14%, resulting in overall removal efficiencies of 95% and 96%, respectively, across the two-stage system. Under stress conditions of 2-week drying followed by high pollutant loading, PTCs maintained pollutant removal, whereas green wall units showed reduced nutrient removal. Across the planted green wall, ammonia increased from 0.02 to 1.63 mg/L and phosphate from 3.42 to 4.44 mg/L. The non-planted control deteriorated more strongly, with ammonia increasing to 8.88 mg/L and phosphate to 8.32 mg/L. After stress recovery, vegetated units rapidly regained treatment efficiency, while controls released ammonia tenfold and phosphate twofold. This novel integrated treatment design demonstrates a scalable, low-maintenance solution for effective on-site greywater treatment and reuse.