A modular microbial–hydroponic system for circular urban plant cultivation

Urban environments need low-input cultivation systems that integrate plants into buildings while improving resource efficiency and circularity. This report presents the technical validation of the first operational Microbial–Hydroponic (Mi-Hy) prototype, known as SPIKA, which combines microbial fuel cells (MFCs), vertical hydroponics, and continuous sensing within an architectural-scale installation. The system was stress-tested for seven months (May–December 2025) in a demanding public setting at the Triennale Milano to evaluate its operational stability and robustness. A vertical hydroponic tower and an MFC stack were operated as parallel subsystems under a low-input, recirculating regime, supporting nine ornamental and aromatic plant species. Continuous sensing and imaging provided a longitudinal phenotypic record, enabling a semi-quantitative assessment of plant performance, including growth, flowering, and visual health. Results show stable physicochemical conditions (mean pH 7.1 ± 0.5; mean EC 510 ± 35 µS cm -1 ) and consistent plant growth without interruption. Over seven months, the system required 35 water refills (52.5 L) and 12 nutrient additions (190 mL). The 12-module MFC stack exhibited open-circuit voltages of 550–620 mV per module. These findings validate the operational reliability of the Mi-Hy framework and establish a baseline for future prototypes integrating nutrient recovery into hydroponic cultivation, advancing toward a fully coupled circular metabolism.