After greywater is used to flush toilets, the resulting blackwater flows to a conventional septic tank (which provides primary treatment and solids settling) and then to an exterior botanical treatment cell. The exterior cell is constructed similarly to the interior greywater cell but is larger, typically buried or semi-buried, and planted with exterior landscape species rather than tropical food plants.

The treated effluent that exits the exterior botanical cell is clarified, low in pathogens, and high in plant-available nutrients. It is directed to the building’s exterior landscape through subsurface irrigation, completing the nutrient cycle: the nutrients from food consumed by the building’s occupants are returned to the soil that may eventually grow more food. The system meets or exceeds the effluent quality standards of conventional septic systems and does so without chemical additives.