Coagulation of upland coloured water with polyferric sulphate compared to conventional coagulants

Polyferric Sulphate (PFS) is a polymeric Fe(III) coagulant which consists of a range of preformed Fe(III) hydrolysis species with both high cationic potential and medium to high molecular weights. This study has investigated the potential advantage of using PFS in the treatment of upland coloured waters, compared with conventional metal coagulants. Laboratory experiments were carried out to compare the coagulation performance of the PFS, ferric sulphate (FS) and aluminium sulphate (AS), and to investigate the nature of the solid phase formed by the hydrolysis of the PFS and FS. The results indicate that the treatment efficiency of PFS for the removal of colour, UV-absorbance and DOC was significantly higher than that of the FS and AS, and was virtually unaffected at a low water temperature of 4°C, but slightly affected at 1.5°C. The nature of the natural coloured waters can be partially characterised by the value of SUVA and the E₄/E₆ ratio, and these can be used to predict the extent of coagulation. It is suggested that preformed PFS species with high cationic potential are able to persist during the flocculation process and this may be the reason for the superior treatment performance of PFS.