Chemical washing of anthracite-sand filter media: A laboratory-scale evaluation

Aged anthracite–sand dual-media filters can retain coagulant-based floc and may develop mudballs over time, even when routine hydraulic backwashing maintains normal operation. Targeted chemical assisted cleaning approaches may help reduce this floc accumulation. In this research, a bench-scale protocol was developed to determine an effective chemical wash for aged media collected from a full-scale filter that had been in service for approximately four decades and showed high post-backwash floc retention turbidity (>120 NTU). The research was conducted in two phases: Phase 1 screened common chemical agents at equal concentrations 2% (w/v), and Phase 2 evaluated the best-performing chemical over a concentration range of 1% to 7% (w/v). Performance was assessed by monitoring turbidity trends across rinses, recovered floc mass, retained-mass ratio, and surface elemental analysis. In Phase 1, NaOH outperformed acidic (HCl, citric acid, acetic acid), oxidizing (NaOCl), and surfactant-enhanced options. In Phase 2, removal increased with concentration until 5% (w/v); NaOH achieved the highest floc recovery and substantially reduced aluminium associated surface deposits from alum coagulation. These findings suggest that chemical washing may be a useful approach for cleaning aged granular filter media when hydraulic backwashing alone is insufficient to remove accumulated deposits.