Changes to Dissolved Organic Matter during Conventional Drinking Water Treatment Steps
Dissolved organic matter (DOM) in natural waters is a mixture of compounds. Some authors have fractionated this organic matter into large classes (humic substances, hydrophilic acids . . .). Humic substances have been defined as the fraction of organic matter retained on XAD8 resin at acidic pH (THURMAN and MALCOLM 1981); this isolation procedure is recommended by the International Humic Substances Society. Only a few investigators have dealt specifically with hydrophilic substances which are not adsorbed on XAD8 resin at acidic pH. The "hydrophobic/hydrophilic" distribution can be determined by a simple method of organic matter fractionation, using two superimposed XAD8 and XAD4 resin columns (CROUE et al. 1993). This procedures, carried out at pH 2, consists of first isolating hydrophobic substances (essentially humic acids) on an XAD8 resin and then isolating the hydrophilic acids from the XAD8 effluent on an XAD4 resin. The aim of this work was to study the evolution of the hydrophobic/hydrophilic distribution during water treatment steps as applied in waterworks. The analytical procedure was first applied to determine the change in the DOM distribution of ten surface waters after clarification. The results obtained after clarification were completed by the study of the effect of the coagulant nature (ferric chloride, aluminium sulphate, prehydrolyzed salt (WAC)). In the second phase, oxidation experiments using ozone, chlorine and chlorine dioxide were conducted on raw and clarified waters to determine their effect on the DOM distribution. Finally the fractionation procedures were carried out on two water treatment plants to compare our laboratory data with results obtained in a working plant and to observe the change in DOM distribution during granular activated carbon (GAC) filtration. Dissolved organic carbon (DOC) concentrations were analysed using a Dohrman DC 80 apparatus. UV absorbance was measured with a one or five centimeter cell using an Uvikon spectrophotometer. Oxidation experiments were carried out in a batch procedure. The study of different coagulants was made in the laboratory with a Jar Test procedure described elsewhere (LEFEBVRE 1990). Biodegradable dissolved organic carbon (BDOC) was analyzed according to the suspended bacteria method (SERVAIS et al. 1987, 1989). The hydrophobic/hydrophilic distributions of raw and clarified waters were found to be unchanged by the clarification step in seven of ten test waters. These results indicate that for a studied water, the relative reduction in DOC of one fraction was of the same order of magnitude as the other fraction. The distributions of R1.2, R4 and R6.1 were significantly modified by the clarification treatment. In these three cases, the hydrophobic substances showed the greater DOC reduction. The "non-retained" hydrophilic substances became predominant in the R6.1 clarified water, whereas in the cases of R4 and R1.2 the total hydrophilic fraction and the hydrophilic acids increased, respectively. The results obtained in the laboratory on three different raw waters, clarified by different coagulants (ferric chloride or aluminium salts), showed that the nature of the coagulant (iron or alum) can influence the hydrophobic/hydrophilic distribution. A marked influence was found in the case of R6 and less significant results were obtained for R1.2. Powdered activated carbon had no real effect on the DOC distribution, under our experimental conditions (applied dosage 25 mg/L). If the removal of each fraction is considered, and if it is assumed that each removal is independent from the others, ferric chloride appears to be the best coagulant. It removes humic substances efficiently as well as hydrophilic acids (> 72%), but is less efficient for the "non-retained" hydrophilic fraction. The DOC distribution of a clarified water depends on the distribution in the original raw water and the nature of the coagulant. Under our experimental conditions (applied dosage: 1.5 mg oxidant / mg DOC), ozone, chlorine and chlorine dioxide significantly affected the DOM distribution of the R1.2 water. The hydrophobic substances showed the higher relative DOC reduction, which can be correlated with an increase in the hydrophilic fraction. The greatest change was obtained for treatment with ozone. In the case of chlorine and chlorine dioxide, an increase of the "non-retained" hydrophilic fraction was observed whereas for ozone the two hydrophilic fractions increased. These results are in agreement with current knowledge about the action of ozone, chlorine and chlorine dioxide. An increase in the BDOC fraction was observed with the applied ozone dosage. In the case of R1, ozonation of raw and clarified waters appeared to shift the dissolved organic carbon distribution towards the "non-retained" hydrophilics. A good correlation exists between the BDOC increase (BDOC/BDOCo) and the decrease of hydrophobic acids or the increase of "non-retained" hydrophilics (slopes are respectively 12.3 ; - 16.5 and 15.5). In the case of another sample of R1.2, the comparison of BDOC in the XAD4 effluents of raw and ozonated waters indicated that at least 62% of BDOC produced by ozonation was in this fraction. This result indicates that the "non-retained" hydrophilics of this raw water are not biodegradable compared with those induced by ozonation. Results obtained on samples taken on two water treatment plants indicate that ozonation and GAC filtration have a small effect on the hydrophobic/hydrophilic DOM distribution. Intermediate ozonation at the industrially applied dose slightly modifies the distribution; a slight decrease of hydrophobic substances is observed. GAC filtration induces an increase of the hydrophobic fraction. The water treatment process includes clarification (EFS), inter-ozonation (EFSO3), and GAC filtration (EFCAG). GAC filters A and B had been respectively running for one and three years; filter C has been regenerated one month before. Good organic matter removal is obtained during clarification (removal of DOC and UV-absorbance: 75% and 88%). This treatment step changes the DOM distribution: increase of the hydrophobic fraction and decrease of the hydrophilic fraction. BDOC was completely removed. Intermediate ozonation (0.7 mg O3/mg DOC) modifies the DOC distribution and creates BDOC. This BDOC, in absolute value (0.36 mg/l), is equal to the increase of hydrophilic fraction. On the oldest filters (A and B) BDOC was completely removed, but this was not the case for filter C, probably because the bacterial biomass was insufficient. On GAC filters hydrophilic substances were found to be better removed than humic substances. The age of the GAC bed seems to have an effect on DOC distribution: the effluent from the oldest GAC filter contained less of the hydrophobic fraction than did the effluent from the youngest filter.