Flexibly connectable combined filters (co-filters) for decentralized treatment of greywater

Affordable and sustainable water purification and reuse remain a global challenge for decentralized systems. The release of untreated or insufficiently treated grey and wastewater into surface waters contributes to pollution and increases downstream treatment costs due to nutrients, organic matter, heavy metals, and microbial contamination. Hybrid approaches combining biological and physicochemical processes are emerging, yet modular systems enabling direct integration of living plant roots with functional biopolymer-based filters remain underexplored. Here we demonstrate a modular co-filter system integrating plant roots with biopolymer filters enabling effective decentralized greywater treatment within flexibly connectable pot units. The combination of Sparganium erectum with cellulose-chitosan (CC) filters, including copper-functionalized variants (CCC) was evaluated using synthetic greywater under laboratory conditions. The co-filter achieved substantial reductions in organic matter, nutrients, turbidity, and microbial indicators. CCC filters exhibited enhanced antibacterial activity against Escherichia coli via copper release, whereas CC filters had limited antimicrobial effects. The plants tolerated metal-containing greywater and contributed to nutrient and organic matter removal without adverse effects on plant biomass. Treatment performance improved with increasing system complexity. These results demonstrate the feasibility of a modular, nature-based co-filter concept as an approach for decentralized greywater treatment and water reuse.