Biogeochemically active vermiculite filters for decentralised greywater reuse: field-scale performance and nitrogen cycling mechanisms

Abstract

This study investigated the field-scale performance and mechanistic behaviour of exfoliated vermiculite as a biogeochemically active filter medium for decentralised greywater treatment in a low-income urban setting. A gravity-fed filtration system containing unmodified, thermally exfoliated vermiculite was operated for 12 months in Southlea Park, Harare, Zimbabwe, using real greywater from communal bathrooms. Physicochemical monitoring showed high early-stage removal of turbidity, total suspended solids, electrical conductivity and total dissolved solids, while phosphate was reduced to below detectable levels across sampling cycles. In contrast, total nitrogen increased in the effluent during warmer months, indicating that nitrogen behaviour was governed by transformation rather than simple removal. This response was attributed to ammonium desorption driven by cation exchange reversibility, biofilm-mediated nitrification and microbial mineralisation of organic nitrogen. The findings reposition exfoliated vermiculite from a passive sorbent to a reactive bio-mineral medium capable of supporting physical filtration, ion exchange and microbial transformation without chemical dosing or engineered aeration. A conceptual model is proposed to explain multiscale mineral-microbial interactions across structural zones within the filter column. The study provides field evidence for vermiculite-based filtration as a low-cost decentralised greywater treatment option with implications for water reuse, nutrient recovery and adaptive sanitation design in infrastructure-constrained settlements.