Evolution of the humic/non humic NOM distribution and its biodegradable fraction during drinking water treatments
The goal of our study was to evaluate the impact of water treatment processes (i.e. coagulation- flocculation, ozonation and GAC filtration) on the natural organic matter (NOM) of various river and reservoir waters based on DOC and BDOC analyses and the determination of the humic/non humic NOM distribution (fractionation of the DOC at acidic pH using two successive XAD-8 and XAD-4 resin columns). Analyses carried out on ten French raw surface waters have shown that the BDOC fraction accounted for 11 to 38 % of the DOC. The humic/non humic distribution of the NOM varied slightly with the origin of the studied water. About 50 to 60 % of the DOC was found to be incorporated into the humic fraction (NOM adsorbed on the XAD-8 resin), the hydrophilic acids (adsorbed on the XAD-4 resin) accounted for 10 to 25 % of the DOC while the non adsorbed hydrophilic solutes (hydrophilic neutrals that constitute the XAD-8/XAD-4 effluent) represented 15 to 30 % of the DOC. Clarified water samples were collected from water treatment plants after coagulation/flocculation/sand filtration when no preoxidation was used. For water utilities which included a preoxidation step in their treatment process, raw water samples were coagulated and flocculated at a laboratory scale with Jar Test equipment using the same conditions (nature of the coagulant, pH, dose) as those used in the corresponding treatment plant. Globally, 38 to 70 % of the DOC and 38 to 88 % of the BDOC were removed during coagulation-flocculation, depending on the water site. In general, the humic/non humic NOM distribution of clarified waters showed a slight increase in the proportion of the non humic organic fraction as compared to raw waters, which indicates that humic substances (higher molecular weight organics) are preferentially removed during coagulation-flocculation. Ozonation experiments were carried out on a raw water and a clarified water sampled from the same water site using a semi batch reactor (ozone was generated from high purity oxygen). For both waters, very little variation of the humic/non humic NOM distribution was observed for applied ozone doses around 0.5 mg O3/mg C or below. For higher ozone doses, the NOM distribution was dramatically changed despite only a small reduction of the DOC. The large reduction of the humic fraction was followed by a proportional increase of the non adsorbed hydrophilic solute fraction (small reduction of the DOC). As the applied ozone dose was increased from 1 to 3 mg O3/mg DOC, the non-adsorbed hydrophilic solute fraction also increased. The shift from high molecular weight organics such as humic materials to more hydrophilic organics (high polarity and low molecular weight organic solutes) during ozonation has often been mentioned in the literature. The increase of the non humic substances was followed by an increase of the BDOC. Results have also shown that higher ozone doses yield higher BDOC. Similar observations could be made with the raw and the clarified water. As a conclusion of this work, samples were collected at the different steps of a water treatment plant (raw water, clarified water, intermediate ozonated water, GAC filtered water). The impact of coagulation/flocculation, and intermediate ozonation on BDOC and the humic/non humic NOM distribution confirmed the previous observations. The GAC filtration had a large impact on the DOC distribution while the BDOC was only slightly reduced. The NOM of the treated water was found to be more hydrophobic in nature with compounds that showed apparent molecular weights below 1 000 daltons (more than 80 % of the DOC).