Natural organic matter fractionation along the treatment of water for human consumption

The main objective of this study was to characterize the organic matter present in raw water and
along the treatment process, as well as its seasonal variation. A natural organic matter fractionation
approach has been applied to Lever water treatment plant located in Douro River, in Oporto
(Portugal).
The process used was based on the sorption of dissolved organic matter in different types of ion
exchange resins, DAX-8, DAX-4 and IRA-958, allowing its separation into four fractions: very
hydrophobic acids (VHA), slightly hydrophobic acids (SHA), charged hydrophilic (CHA) and
hydrophilic neutral (NEU). The dissolved organic carbon (DOC) determination was used to quantify
dissolved organic matter. Samples were collected monthly, during approximately one year, from raw
water captured at the surface and under the bed of the river, and after each step of the treatment:
pre-filtration in sand/anthracite filters, ozonation, coagulation/flocculation, counter current dissolved
air flotation and filtration (CoCoDAFF) and chlorination.
The NEU fraction showed a seasonal variation, with maximum values in autumn for the sampling
points corresponding to raw water captured at the surface and under the bed of the river. It was
usually the predominating fraction and did not show a significant decrease throughout the treatment.
Nevertheless their low concentration, the same occurred for the CHA and VHA fractions. There was
an overall decrease in the SHA fraction throughout the water treatment (especially after CoCoDAFF
and ozonation) as well as in the DOC.
The TSUVA254 values obtained for raw water generally varied between 2.0 and 4.0 L mgC-1 m-1 and
between 0.75 and 1.78 L mgC-1 m-1 for treated water. It was observed a decrease of TSUVA values
along the treatment, especially after ozonation.
These results may contribute to a further optimization in the process of treating water for human
consumption.