Liquid ferrate as a chloride-free alternative coagulant to ferric: Benchmarking water quality performance and post-coagulation treatment impacts

Residual chloride in coastal freshwater resources is particularly concerning the Netherlands, where stringent chloride limits (150 mg/L) exist for drinking water. FeCl 3 coagulant addition during treatment exacerbates the chloride concentration in the finished drinking water. This study presents the first systematic comparison of liquid FeCl 3 and chloride-free, liquid form ferrate (FerSol®, The Netherlands) and examines their treatment performance and potential impact on post-coagulation stages. Results confirmed no additional chloride in the water treated with FerSol. Under low G (454 s −1 ) and 20 mg Fe/L at pH 8, FerSol matched FeCl 3 coagulation performance in turbidity (96%), UVT (>93%), and residual iron (<0.1 mg/L), though showing 10% lower DOC removal. High G (761 s −1 ) compromised UVT and DOC reduction when elevating residual iron to ~0.3 mg/L. FerSol coagulation produced smaller, less settleable flocs with settling velocities 62% slower than FeCl 3 . Unlike FeCl 3 , FerSol reduced >7 log E. coli and 1 log B. subtilis at 30 μM after 60 min at pH 7.4. Furthermore, FerSol also decomposed high-MW biopolymer and humics fractions, resulting in the increases in low-MW fractions by 10% and NOX-AOC by 10× (~15 μg C/L). These resulted from the generation of • OH and 1 O 2 during FerSol decomposition. Additionally, in-situ formed Fe(III) by FerSol was characterized by colloidal Fe c ~72%, which was less effective in UVT and DOC removals. This study provides a benchmark for the treatment efficacy of FerSol compared to FeCl 3 . Tailored operational conditions are required to maximize its complementary oxidation and coagulation effects on water quality and biostability. • FerSol is a viable chloride-free alternative to FeCl 3 to eliminate secondary chloride residue in treated water. • FerSol achieves parity with FeCl 3 on turbidity, UVT, and residual iron (<0.1 mg/L), though lower DOC removal. • FerSol offers substantial oxidative benefits for biocidal effects against E. coli and B. subtilis. • FerSol dosing presents post-coagulation challenges: smaller, less settleable flocs and elevated AOC requiring downstream polishing.