Coagulation behaviour of eight natural water samples containing NOM is investigated to identify parameters influencing the process. A strong correlation between colour, DOC and UV-abs. was shown, regardless of the source and fractions. Coagulation of original water samples is discussed using analytical parameters in raw water and various molecular weight fractions. Over 40% of colour is given by NOM fractions with molecular weight cut-off (MWCO) < 10kD with one exception. About 13 of the colour is given by MWCO fractions > 50 kD for all waters. Larger NOM molecules required less coagulants per removed colour unit compared with smaller NOM molecules. Bulk NOM parameters and colloidal charges were identified to be the most descriptive parameters of the samples and the coagulability. The possibility to predict the optimum coagulant dosage for given process conditions is illustrated.

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