At present little is known about the relationship between raw water characteristics, such as natural organic matter (NOM) content, and the universal applicability of coagulation optimisation through surface charge measurement. This research aims to investigate this issue by comparing case study sites in the US (Poudre River, Fort Collins, Colorado) and the UK (Albert Reservoir, Halifax) across periods of elevated organic levels. During the period of April to June 2004 in raw Poudre River water dissolved organic carbon (DOC) levels increased rapidly from 3.5 to 7.4 mg L−1 as a direct result of the spring snowmelt runoff, whereas at Albert reservoir, which is a moorland peat catchment, DOC concentrations varied between 7.8 and 10.1 mg L−1 during the period of January to March 2004. NOM is a highly heterogeneous mixture of organic compounds that vary with regards to acidity, molecular weight, hydrophobicity and charge density. XAD resin adsorption techniques were employed to fractionate the water into its hydrophobic and hydrophilic components. Results revealed that NOM composition and characteristics can vary both temporally and spatially, with increased DOC concentrations associated with both an increase in hydrophobic content and charge density. Optimising coagulation based on a zeta potential range (−10 to +5 mV) produced stable average DOC residuals for both locations, although the exact value is also dependent on the hydrophilic composition of the water and the coagulant used, with alum removing approximately 0.5 mg L−1 less DOC compared with ferric.

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