Results from laboratory tests, conducted to study different aspects regarding the removal of humic substances by the environmentally friendly cationic biopolymer chitosan, are presented. Chitosan was shown to be an efficient remover of the high molecular weight humic substances, significantly reducing the colour and UV absorbance in typical humus-rich Norwegian surface waters with low particle content. By adding small amounts of Fe3+ together with a reduced dose of chitosan, it was possible to significantly improve the removal of the medium molecular weight humic substances. The highest charged (low Fa) chitosans were the most efficient coagulants, indicating that charge neutralisation was the triggering mechanism for the coagulation. Surface charge measurements of coagulated raw water gave strong indications in the same direction. However, net positive surface potentials of flocs after neutralisation indicated that excess chitosan participated in the build-up of flocs, suggesting that bridging or electrostatic patch flocculation could be important mechanisms during flocculation. However, the molecular weight of chitosan had no significant influence on the maximum removal of the humic substances.

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