Dissolved organic carbon and trihalomethane formation potential removal during coagulation of a typical UK upland water with alum, PAX-18 and PIX-322

This paper considers the removal of dissolved organic carbon (DOC) from a typical UK upland reservoir water using three different coagulants; aluminium sulphate/alum (Al₂(SO₄)_3(aq)), polyaluminium chloride/PAX-18 (Al_n(OH)_mCl_3n-m(aq)) and ferric sulphate/PIX-322 (Fe₂(SO₄)_3(aq)). A comparison of DOC quality including fractional character, colour, specific ultraviolet absorption (SUVA) and molecular weight was made between the source water and the supernatants obtained following its coagulation in each case. Trihalomethane formation potential (THMFP) was compared before and after treatment to assess which coagulant performed best in terms of THM amelioration. The conditions (coagulant dose and pH) required to obtain optimal DOC removal were established using bench-scale jar tests. Statistically significant differences in DOC removal rates were observed between the different coagulants. THMFP removal was linked to a combination of high net DOC removal and the preferential removal of THM precursors. This selectivity may also be responsible for the increase in brominated THM species (BrTHMs) observed following treatment.