Integrated digestate concentration, ammonia recovery, and water reclamation via a novel two-stage submerged gas-permeable membrane system

This research introduces an innovative series-connected two-stage submerged gas permeable membrane (STSGPM) system for anaerobic digestate treatment, integrating ammonia recovery, digestate concentration, and water reclamation. The system achieved over 99 % ammonia removal across various temperature gradients, with the 65/55/25 °C gradient showing optimal water flux, where the average water flux of the first-stage SGPM (SGPM1) was recorded at 3.43 L/m 2 /h, while the second-stage SGPM (SGPM2) maintained a consistent flux of 4.25 L/m 2 /h. STSGPM achieved interception rates of 99.8 % and 99.41 % for inorganic ions and organic matter, respectively. After 10 cycles, SGPM2 remained stable, but SGPM1 experienced fouling from gel-like organic deposits (proteins, microbial products, fulvic acid), which were fully removed post-cleaning. The high-purity (NH 4 ) 2 SO 4 , the concentrated digestate meeting agricultural standards, and the reclaimed water compliant with urban miscellaneous water use standards highlight the effective reutilization of the three by-products. This aligns with the zero-discharge objective, underscoring the potential of STSGPM to advance resource recovery from anaerobic digestate. Additionally, the recovery of (NH 4 ) 2 SO 4 and high-quality water generates potential profits, reducing the net operating cost of the STSGPM system to $0.155 per kilogram of NH 3 recovery. Overall, the STSGPM process represents a cost-effective and efficient ammonia recovery strategy with significant potential for widespread application.