Abstract
This study integrated post-hydrothermal carbonization (HTC) liquids from raw dairy manure (RDM) and digested dairy manure (DDM) with an expanded polytetrafluoroethylene (ePTFE) gas permeable membrane (GPM) system. The effects of HTC temperature (180–240 °C) and residence time (1–4 h) on total ammoniacal nitrogen (TAN) concentration and pH in post-HTC liquids were first investigated. The post-HTC liquids, together with filtered manures, were subsequently applied in a tubular ePTFE GPM reactor to evaluate ammonia recovery performance. TAN concentrations in post-HTC liquids generally increased with increasing HTC temperatures and residence times. DDM-derived HTC-liquids had higher TAN concentrations (1150–1210 mg N L−1) and mildly alkaline pH (8.2–8.4), whereas RDM-derived liquids were acidic (pH 5.4–7.1) and had lower TAN concentrations (725–764 mg N L−1). All post-HTC liquids exhibited strong buffering capacity, indicating that bulk chemical pH adjustment would require substantial alkali inputs. The GPM system recovered TAN from all feed streams, but recovery was consistently higher for DDM-derived post-HTC liquids. Over the first 24 h, DDM-derived liquids achieved a mean TAN transfer flux of 11.05–14.46 g m−2 d−1, with the 210 °C HTC condition yielding the highest value, whereas RDM-derived post-HTC liquids achieved lower fluxes of 3.92–5.13 g m−2 d−1. Increasing feed temperature enhanced TAN recovery, although excessive heating also promoted water vapor transfer and diluted the recovered ammonium sulfate solution. Overall, the integrated anaerobic digestion-HTC-GPM configuration is a technically promising pathway for dairy manure nutrient recovery, with phosphorus concentrated in hydrochar and nitrogen recovered from the liquid phase as ammonium sulfate.