Simultaneous mitigation of multiple pollutants in urban stormwater systems illicitly connected with wastewater systems by an Fe(vi)-based process†

The illicit connection of wastewater pipes to stormwater pipes might result in the direct discharge of wastewater into natural water and even drinking water sources. The multiple pollutants in untreated wastewater effluent, including organic matter, nutrients, emerging contaminants (ECs) and disinfection by-product (DBP) precursors, posed risks to ecological safety. Herein, Fe(VI) and Fe(VI)/Fe(III)-based processes were found to be effective in treating overflow wastewater as a combination of coagulation and oxidation. In the presence of Fe(VI) below 200 μM, the addition of Fe(III) could further improve the removal of COD (43.1%), TP (87.9%), and turbidity (95.3%) compared to that by Fe(VI) alone. With respect to ECs, the highly detected paracetamol (PCT) of 10 μM in wastewater can be efficiently degraded by Fe(VI) exceeding 300 μM, which reached approximately 97.2% removal within 22 min. The rapid consumption of Fe(VI) by other organics present in wastewater necessitates the addition of Fe(III) at a low [Fe(III)] : [Fe(VI)] ratio to expedite the oxidation of ECs. For DBPs, the Fe(VI)-based process decreased DBP formation and DBP-associated cytotoxicity by about 50–80% at optimal dosage (300 μM) and prioritize the removal of haloacetaldehyde and haloacetonitrile precursors. This may be attributed to the efficient removal of aromatic protein-like components. However, the addition of Fe(III) may deteriorate the DBP formation control due to interaction between Fe(III) and Fe(VI) reducing the Fe(VI) oxidation capacity on organics. This study demonstrated that Fe(VI) and Fe(VI)/Fe(III)-based processes might be a promising treatment process for simultaneous removal of multiple pollutants in wastewater from illicitly connected urban stormwater systems.