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.

This study focused on the optimization of hydrothermal carbonization (HTC) of nutrient-rich dairy manure digestate using response surface methodology based on a face-centered central composite design (FCCD-RSM), aiming to simultaneously maximize hydrochar (HC) yield and nutrient recovery. FCCD-RSM was employed to systematically evaluate the impact of HTC reaction temperature (180–240 °C) and reaction time (20–90 min) on HC yield, phosphorus (P) content in HC, and total ammonia-nitrogen (TAN) concentration in the HTC liquid. Results showed that the P content in HC and the TAN concentration in the HTC liquid increased with increasing HTC temperature and reaction time, whereas HC yield decreased. Based on experimental data, quadratic polynomial models were developed. Using RSM with equal weighting assigned to the three responses, the optimized conditions were determined to be 216.5 °C and 84.1 min, resulting in an HC yield of 79.03 ± 2.61%, P content in HC of 1.76 ± 0.04%, and TAN concentration in the HTC liquid of 891.67 ± 16.67 mg/L. These values were close to the predicted values and fell within the 95% confidence intervals. Therefore, the optimized HTC parameters can simultaneously and effectively produce P-rich HC and TAN-rich liquid, providing insights into a more sustainable approach to circular waste management and nutrient recycling on dairy farms.

This study integrated post-hydrothermal carbonization (HTC) liquids from raw and digested dairy manure with a tubular 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 higher HTC temperatures and longer residence times. HTC of digested manure yielded liquids with higher TAN concentrations (1150-1210 mg N L-1) and mildly alkaline pH (8.2-8.4), whereas post-HTC liquids derived from raw manure were acidic (pH 5.4-7.1) and exhibited 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 ammonia from all feed streams, but recovery was consistently higher for post‑HTC liquids from digested manure. Residual heat from the HTC process can benefit the subsequent GPM process, as increasing post-HTC liquid temperature enhances ammonia recovery. Over 24 h, post‑HTC liquids from digested manure achieved a daily average TAN transfer flux of 11.05-14.46 g m-2 d-1, compared with 3.92-5.13 g m-2 d-1for liquids derived from raw manure. Overall, the results indicate that an integrated anaerobic digestion-HTC-GPM configuration, using digestate as the HTC feedstock and relying on aeration-based pH adjustment is a technically promising pathway for nitrogen and phosphorus recovery from dairy manure.