Nitrogen Recovery and CO2-Assisted Carbonate Formation from High-Ammonium Poultry Digestate via Gas-Driven Ammonia Stripping Coupled with Gypsum-Mediated Absorption

High-ammonium poultry digestate from thermophilic dry anaerobic digestion is often recycled, but excessive ammonia accumulation may inhibit anaerobic digestion and reduce process stability. This study developed a gas-driven ammonia stripping process coupled with gypsum-mediated absorption for digestate deammonification, nitrogen recovery, and CO2-assisted carbonate formation. Laboratory stripping experiments were conducted using simulated biogas to evaluate the effects of pH, temperature, and gas–liquid ratio. Under the selected condition of pH 11, 65 °C, and a gas–liquid ratio of 2, NH4+-N in 10 L digestate decreased from approximately 7980 to 1648 mg L−1 within 12 h, corresponding to about 80% removal. In the absorption step, the slightly soluble CaSO4 solution showed more stable NH3 capture than the CaSO4 suspension, and the corrected NH3-N recovery reached approximately 90–95%. XRD, SEM-EDS, precipitate mass estimation, and gas-phase CO2 variation supported the formation of CaCO3-containing precipitates. Pilot-scale operation using real biogas further reduced NH4+-N from approximately 8000 to 700–800 mg L−1 during 36 h of extended pilot-scale operation. Overall, the coupled process provides a preliminary resource-recovery route integrating ammonia burden reduction, nitrogen recovery, sulfate transfer, and CO2-assisted carbonate precipitation. However, full-scale sustainability still requires further long-term operation, complete nitrogen–carbon–calcium–sulfur mass balances, complete heat and energy-balance assessment, product-quality evaluation, and techno-economic or life-cycle assessment.