A kinetic evaluation of nutrient and organic matter removal in greywater for green Walls: Assessing the performance of Mineral-Based, Organic, and Waste-Derived plant support media

Greywater recycling provides a means to address rising water demand with reduced environmental impact due to its abundance and lightly polluted nature. While many studies have examined high-tech treatment systems, these often require operator expertise and energy/chemical inputs that are not sustainable for a decentralised treatment process. Nature based systems such as green walls are emerging as alternatives. An effective plant support media is crucial to this treatment system and must be carefully chosen. This study compared the removal and kinetic performance of eight support media for nutrients and organic pollutants from synthetic greywater. The media were a mixture of organic and mineral materials, including waste-derived materials, and included: coco coir, spent coffee grounds (SCG), date seeds, SCG biochar, cattle bones, plastic carrier media, zeolite, and perlite. The best total organic carbon (TOC) removal was achieved by SCG biochar (50.7 %) and chemical oxygen demand removal (50.9 %) by zeolite. However, when accounting for leaching, which was significant in some organic media but expected to decrease with time, most organic media (SCG, date seeds, coco coir and bone) achieved around 80 % TOC removal. Of these materials SCG provided the most rapid removal. TOC and COD kinetics were best fitted by the pseudo-first-order model across all media except COD for SCG. The diffusion model plot indicates some diffusion-controlled process where sorbate concentration affects adsorption. COD elimination by SCG followed pseudo second order kinetics, suggesting chemisorption-dominated phenomena. The highest removal of phosphorus in greywater, after accounting for leaching, was achieved with cattle bone (88 ± 3.2 %), followed by SCG (55 ± 4.6 %). However, bone also had the highest phosphate leaching. Zeolite was the most effective in removing NH₄⁺, achieving 82 ± 3.7 % removal. The dynamics of phosphate removal followed a pseudo-first order model for most media. Ammonia kinetics were best fitted by intra- particle diffusion and pseudo second order models. In summary, different media targeted different pollutants and a synergistic blend of these media holds the potential for efficient pollutant removal from greywater through nature-based processes.