From nutrient recovery to fertilizer product quality: A nitrogen-quality framework for manure-derived liquid fraction...

Manure-derived liquid fractions are increasingly considered as feedstocks for recycled fertilizer production, but their upgrading is still often evaluated through technology-centered indicators such as nitrogen removal, recovery efficiency, or total nitrogen concentration. These indicators are necessary for process assessment, yet they do not show whether a treated stream is a reliable fertilizer product. This review develops a nitrogen-quality framework for manure-derived liquid fractions and defines five product-relevant dimensions: nitrogen speciation, matrix cleanliness, nutrient density, compositional balance, and stability and consistency. Solid-liquid separation, biological nitrification, physicochemical recovery, membrane concentration, and hybrid treatment trains are assessed according to the quality changes they produce. The synthesis shows that no single process optimizes all fertilizer-relevant attributes. Nitrification is useful where nitrate-enriched liquids are desired; stripping and absorption are better aligned with concentrated ammonium products; membrane systems support mixed nutrient concentrates and high-cleanliness liquids; and lower-intensity stabilization can be appropriate for local organic-mineral fertilizer use. The proposed framework shifts the endpoint from nutrient recovery alone to fit-for-purpose fertilizer product quality. Future studies should report nitrogen species, matrix indicators, storage behavior, residual streams, and fertilizer functionality under realistic operating and use conditions.