Optimisation of equine manure as fertiliser: combined use of glass wool and activated carbon

The valorization of anaerobic digestate as a liquid biofertilizer is central to circular bioeconomy strategies but is constrained by heavy metals, inorganic salts, and pathogenic microorganisms. This study evaluates a dual-material filtration approach combining glass wool and granular activated carbon (GAC) to improve the physicochemical and microbiological quality of horse-manure digestate. Three column configurations were tested (C1–C3) with increasing media loads. The balanced configuration, C2 (0.3 g glass wool, 4 g GAC), achieved substantial reductions in key parameters: hardness (–52.9%), sodium (–80%), zinc (–66.6%), copper (–50%), and hydrogen sulfite (–74.9%); alongside a 22.2% increase in total alkalinity and the highest filtrate recovery (96%). C3 (0.5 g glass wool, 6 g GAC) completely removed nitrate and nitrite (100%) and reduced fluoride by 33%, but with lower hydraulic performance (76% recovery). C1 was the least effective overall. Microbiological analyses showed consistent dominance of Proteus spp. across conditions, while glass wool–containing setups (C2–C3) favored the emergence of capsulated Klebsiella spp. colonies; no E. coli was detected. Overall, the integrated glass wool–GAC filtration offers a low-cost, scalable route to safer, more stable liquid biofertilizers, supporting sustainable reuse pathways in the circular bioeconomy.