Effects of Operating Conditions on Nitrogen Recovery from Post-Hydrothermal Carbonization Liquids Using Gas-Permeable Membranes

Hydrothermal carbonization (HTC) of digested dairy manure produces hydrochar and a nitrogen-rich post-liquid. This study evaluated a submerged tubular expanded polytetrafluoroethylene (ePTFE) gas-permeable membrane (GPM) system for ammonia recovery from post-HTC liquid derived from digested dairy manure, focusing on the effects of feed temperature, acid circulation rate, and feed volume-to-membrane surface area ratio (FV/MS). Compared with filtered digested manure, the post-HTC liquid had a similar total ammoniacal nitrogen (TAN) concentration (1151 vs. 1164 mg N L−1) but a slightly higher pH (8.38 vs. 7.94), which favored ammonia transfer. Increasing feed temperature from 20 to 60 °C raised 24 h TAN recovery from 63.5% to 99.6% and average TAN transfer flux from 11.1 to 17.7 g m−2 d−1, although it also increased water vapor crossover and diluted the acid trapping solution. Increasing acid circulation from 10 to 30 mL min−1 improved 48 h TAN recovery from 85.2% to 94.0%, while a further increase to 50 mL min−1 produced only a small additional gain. In contrast, elevating FV/MS from 0.015 to 0.045 m3 m−2 reduced 48 h TAN recovery from 94.0% to 59.6% while increasing average TAN transfer flux from 8.2 to 16.2 g m−2 d−1 because larger feed volumes sustained the concentration driving force for a longer period. Nitrogen mass balance showed that more than 94–99% of the initial TAN was accounted under most conditions, with only a small fraction lost to volatilization.