Operation and clogging mechanisms of a refurbished vertical flow filter treating urine-derived wastewater

This study evaluates clogging mechanisms and treatment performance of a full-scale vertical flow filter (VFF) treating nitrogen-rich supernatant from urine downstream of a struvite precipitation unit in a decentralized sanitation system. Following six years of operation under high seasonal nitrogen and high salt loads, the original lava sand filter clogged and was replaced. Clogging processes were investigated through physical and chemical characterization of filter media, including particle size distribution, hydraulic conductivity, BET surface area, and elemental composition. System performance was assessed over a one-year monitoring campaign (July 2024–July 2025), analyzing nitrogen species and key physicochemical parameters. Clogging resulted in a reduction of effective grain size (d10) from 0.11 to 0.008 mm and hydraulic conductivity from 10−4 to 10−7 m·s−1, associated with precipitation of calcium phosphate compounds (up to 395 mg P·100 g−1). The VFF achieved an average NH4–N removal of 83.5% after refurbishment. Despite lower temperatures, higher nitrification performance was observed during winter, with a significant seasonal difference (p = 0.0069) and a strong negative correlation between temperature and NH4–N removal (ρ = −0.895, p < 0.001), indicating nitrogen loading as the dominant control factor. The results demonstrate that clogging is driven by coupled physical and chemical processes and highlight the importance of load management for long-term system stability.