A Systematic Study of Ammonia Recovery from Anaerobic Digestate Using Membrane-Based Separation

Ammonia recovery from synthetic and real anaerobic digestates was accomplished using hydrophobic flat sheet membranes operated with H₂SO₄ solutions to convert ammonia into ammonium sulphate. The influence of the membrane material, flow rate (0.007, 0.015, 0.030 and 0.045 m³ h⁻¹) and pH (7.6, 8.9, 10 and 11) of the digestate on ammonia recovery was investigated. The process was carried out with a flat sheet configuration at a temperature of 35 °C and with a 1 M, or 0.005 M, H₂SO₄ solution on the other side of the membrane. Polytetrafluoroethylene membranes with a nominal pore radius of 0.22 µm provided ammonia recoveries from synthetic and real digestates of 84.6% ± 1.0% and 71.6% ± 0.3%, respectively, for a membrane area of 8.6 × 10⁻⁴ m² and a reservoir volume of 0.5 L, in 3.5 h with a 1 M H₂SO₄ solution and a recirculation flow on the feed side of the membrane of 0.030 m³ h⁻¹. NH₃ recovery followed first order kinetics and was faster at higher pHs of the H₂SO₄ solution and recirculation flow rate on the membrane feed side. Fouling resulted in changes in membrane surface morphology and pore size, which were confirmed by Atomic Force Microscopy and Air Displacement Porometry.