Optimization of reverse osmosis operational conditions to maximize ammonia removal from the effluent of an anaerobic membrane bioreactor

Abstract

Anaerobic membrane bioreactors can now produce effluent that meets regulatory standards for BOD, enabling energy recovery and use of the treated effluent for irrigation. RO treatment of this effluent can potentially enable recovery of potable water and total ammonia nitrogen (TAN). In this study, we optimized the removal of TAN from the effluent of a staged anaerobic membrane bioreactor (SAF-MBR), a system that consists of an anaerobic fluidized bed reactor and anaerobic membrane bioreactor. The SAF-MBR effluent was treated using an ESPA RO membrane. The result was a high-quality RO permeate that meets the typical potable water guidelines (≤1 mg N L⁻¹). Hydraulic operating conditions (i.e., pressure and flux settings) did not affect TAN rejection efficiency, but pH had major impacts, due to changes in ammonium/ammonia speciation and membrane surface charge. At pH 6, the TAN rejection efficiency was optimal at 99.8%. For pH > 6, the passage of uncharged NH₃ increased, decreasing TAN removal. For pH < 6, the membrane retained a progressively less negative (carboxylate) charge as the pH decreased, decreasing ammonium removal from the optimum and allowing increased passage of ammonium into the permeate. Our results suggest that an RO membrane having a lower isoelectric point (IEP) can enable higher TAN rejection efficiencies. A more concentrated RO retentate enables more efficient recovery of ammonia for reuse, and the energy required is less than the energy needed for biological removal of NH₃ as N₂ followed by synthesis of NH₃ from N₂ by the Haber–Bosch process. Further systems level research is needed to assess the energy intensity of different options for recovery and reuse of concentrated ammonia.