Abstract:
Challenges to implement circular economy principles in the wastewater cycle are connected to the need of reducing nutrients (ammonium as NH₄⁺-N, and
phosphorus as PO₄-P) in treated effluent whilst enabling their recovery in an environmentally sustainable way. Conventional biology-based technologies fail
to address these challenges by having high greenhouse gases footprint and offering limited possibilities for nutrient recovery. The aim of this work was to
underline the mechanisms of removal and recovery of NH₄⁺-N and PO₄-P from wastewater through the ion exchange (IEX) process in order to optimise the
removal efficiency and maximise the recovery from IEX regenerant brines (sodium hydroxide and potassium chloride), when working at demonstration
scale over an extended period of time. The IEX process was tested in a 10 m³/day demonstration plant for 2.5 years using Zeolite-N and a hybrid anion exchanger (HAIX) for the removal of NH₄⁺-N and PO₄-P at empty bed contact times of 10 and 5 min, respectively. The operation at demonstration scale confirmed the resilience and consistency of the IEX process and the possibility to maintain high effluent quality (<0.3 mg PO₄-P/L and <1 mg NH₄⁺-N/L) despite changes in influent concentration (i.e. <0.006-26 mg NH₄⁺-N/L) and extended operational period (up to 63 consecutive adsorption/regeneration cycles with HAIX). Additionally, the regenerant brines were reused multiple times and nutrients could be recovered as high purity ammonium sulphate and hydroxyapatite using a hollow fibre membrane contactor for ammonium recovery and simple precipitation with calcium hydroxide and filtration for phosphorus recovery. The results obtained from this work additionally highlighted the need of an automated system to start the regeneration when the desired effluent quality is reached, the need of a nearly solids-free influent and high mechanical strength media to avoid media packing and losses. This work significantly moved the IEX process higher in the technology readiness level (from level 5 to level 7) for mainstream wastewater treatment with the advantages of simple operation, consistency, resilience and lower environmental impact (-25% cumulative energy demand, - 66% global warming potential, -62% marine eutrophication potential) compared to traditional biological processes.
