Integrating zeolite-based cation exchange and membrane contactors for ammonium recovery as ammonium sulphate: pilot experience from WalNUT project

<ns3:p> Background Nutrient recovery from wastewater treatment plants (WWTPs) can support circular economy strategies by converting nutrient-rich reject water into fertiliser products. This study presents the development and validation of a process integrating zeolite-based cation exchange for ammonium capture with a hollow-fibre membrane contactor to recover nitrogen as ammonium sulphate, and its use to formulate fertilisers, including a “smart” formulation with plant growth-promoting bacteria. Methods The work combined laboratory optimisation, pilot operation at TRL 5, and agronomic testing. The system was evaluated under optimised laboratory conditions and with real anaerobic digester reject water. Pilot-scale performance assessed nitrogen removal, recovery during zeolite regeneration, and conversion into ammonium sulphate. The recovered product was used in fertiliser formulations tested in pot and field trials. Results Under optimised laboratory conditions, the membrane contactor achieved concentrations up to 31.5 g N·L <ns3:sup>−1</ns3:sup> . Operation with real reject water removed ~86% of influent nitrogen, of which ~76% was recovered during regeneration, and ~ 81% of it converted into ammonium sulphate. The system treated up to 4 m <ns3:sup>3</ns3:sup> of reject water and produced 1,377 g N as ammonium sulphate, with regeneration requiring ~3.41 mol NaOH per mol NH <ns3:sub>4</ns3:sub> <ns3:sup>+</ns3:sup> . In pot trials, nutrient-balanced formulations performed best, while the bacterial inoculum showed limited benefits. Field trials showed higher maize forage dry weight (2.37 vs 1.78 kg plant <ns3:sup>−1</ns3:sup> ) and intermediate grain yield (9.6 vs 14.5 Mg ha <ns3:sup>−1</ns3:sup> ) compared to conventional fertilisation. Conclusions The study demonstrates the feasibility of recovering nitrogen as ammonium sulphate from reject water and producing effective fertilisers. Further work should optimise chemical consumption, improve operation, and validate performance across environments to support scale-up. </ns3:p>