Removal of natural organic matter (NOM) using different coagulants and lightweight expanded clay aggregate filters

Chitosan and lightweight expanded clay aggregates were investigated as alternatives to traditional metal coagulants and anthracite in pilot scale coagulation-filtration experiments for the removal of natural organic matter (NOM) from drinking water. The raw water tested covered a range of colour and organic carbon of 15–50 mg Pt/L and 2–5 mg NPOC, respectively. Aluminium sulphate, poly aluminium chlorides with different calcium content, iron chloride sulphate, and chitosan coagulants were tested. The dual media filter bed was built with 0.6 m of 0.8–1.6 mm lightweight expanded clay aggregates (Filtralite) above 0.35 m of 0.4–0.8 mm sand. A conventional anthracite-sand filter with the same layer depths and grain sizes was used as a reference. In general, the maximum permissible concentration level of 0.1-mg Me/L controlled the minimum coagulant dose requirements when metal-based coagulants were used (Norwegian water quality standard). Typical NOM removal efficiencies obtained with metal coagulantswere in the range of 75–90% and 40–70% with respect to colour and organic carbon, respectively. Chitosan was able to remove colour quite effectively, but this coagulant was less effective with respect to organic carbon removal. Lightweight expanded clay aggregate was a good alternative to anthracite, with lower rates of head loss build-up, and increased filter run length and filter storage capacity. Only small differences in effluent water quality were detected with the two filters.