River and lake waters were separated into four fractions to study the removal of nitrogen and carbon organic matter using chitosan (CH) and aluminium sulphate (AS). The fractions were very hydrophobic acid, slightly hydrophobic acid, charged hydrophilic acid and neutral hydrophilic. The results showed that the whole and fractionated water from both sources have a markedly hydrophobic character. However, lake water had a lower NOM concentration than river water. The ratio of dissolved organic carbon and dissolved organic nitrogen (DOC/DON) and dissolved inorganic nitrogen to total dissolved nitrogen (DIN/TDN) were higher in the hydrophobic fraction from both sources. Similarly DOC, colour and ultraviolet absorbance at 254 nm (UV254) also presented higher values in the same fraction. Chitosan achieved the better results in the removal of NOM from Boaco water, whole and fractionated, whereas aluminium sulphate achieved better removal from Juigalpa water. DON and DIN were removed by aluminium to about 30%. The DOC/DON and DIN/TDN ratios decreased with both coagulants in whole waters from both sources. The hydrophobic fraction contributed most to the formation of trihalomethanes, slightly hydrophobic acid being the biggest contributor. Lake water led to less THM formation than river water.

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