Ozonation of forward osmosis retentates: investigation of a novel approach to the quaternary treatment of urban wastewater

Graphical abstract showing the main three pillars of this paper: Forward Osmosis, Ozonation of FO retentates and Simulation Model for the calculation of the specific energy demand. This study evaluates a hybrid forward osmosis (FO)-ozonation process for the quaternary treatment of municipal wastewater. Laboratory-scale FO experiments with subsequent retentate ozonation were conducted to assess FO membrane rejection, organic micropollutant (OMP) removal, and bromate formation. Experiments were performed at volume reduction factors (VRF) of 2 and 8 using NaCl and MgCl2 as draw solutions (DS), and total power demand was modeled based on the geometry and hydraulics of a large-scale FO module. The FO membrane achieved high OMP rejection (85.9–100%), with MgCl2 enhancing retention of ammonium and ibuprofen. Ozonation efficiency depended on compound reactivity and applied ozone dose. Increased salinity at VRF 8 and with NaCl as DS reduced OMP removal due to enhanced ozone scavenging, while concurrently decreasing bromate formation. Power modeling indicated that the FO-ozonation process can reduce total power demand compared to standalone ozonation if DS regeneration is not required. Incorporating reverse osmosis for DS recovery substantially increases power demand, limiting applicability. The approach is energetically viable when the diluted DS can be directly applied, as in fertilizer-driven FO. Future work should examine recirculation of ozonated retentates to the biological treatment stage to evaluate impacts on activated sludge and OMP removal.