Résumés
Résumé
Incluse dans une étude plus générale de caractérisation de la Matière Organique des eaux naturelles superficicielles, cette partie présente les résultats obtenus par ultrafiltration. Les différents paramètres analytiques suivis concernent le COT, COD, CODB, UV à 254 nm et le potentiel de formation des trihalométhanes (pTHM). Les méthodes expérimentales, mises en oeuvre sur les eaux brutes et clarifiées (notées eb et ec respectivement), comprennent l'ultrafiltration (seuils de coupure: 10, 1 et 0,5 kilodaltons), les demandes en chlore et en bioxyde de chlore ainsi que l'étape de clarification, si sur les usines une préoxydation existe.
Le seuil de coupure de 10 kd ou 1 kd caractérise l'origine d'une eau brute. Les eaux de barrage, contrairement aux eaux « courantes », ont un classement logique des différentes fractions des masses moléculaires apparentes et ceci quel que soit le paramètre analytique suivi (COT ou absorbance UV). Sur toutes les eaux clarifiées ultrafiltrées, la fraction supérieure à 1 kd reste majoritaire. Dans nos conditions experimentales, ce seuil de coupure induit le changement le plus significatif entre une eau brute et son eau clarifiée. La valeur de l'absorbance UVec permet dans un premier temps d'approximer la valeur du COTec.
Les autres paramètres analytiques (CODB, pTHM et demandes en oxydants) aussi bien sur les eaux brutes que clarifiées n'ont pas pû être corrélées avec un ou plusieurs autres paramètres physico-chimiques de ces mêmes eau.
Mots-clés:
- Eaux brutes,
- eaux clarifiées,
- ultrafiltration,
- COT,
- COD,
- CODB,
- UV à 254 nm,
- chlore,
- bioxyde de chlore
Abstract
The following paper presents the work being carried out in a general study of natural organic matter. The three lines of work chosen by SAUR concern:
- The fluorescence of natural waters or humic substances (SlMON et al., 1993);
- The determination of dissolved hydrophobic and hydrophilic substances on each step of a water treatment plant (CROUE et al, 1992 a and b);
- The molecular weight fractionation of dissolved organic matter, subject of this paper.
The different raw or clarified waters (respectively rw and cw) were sampled in SAUR water plants. Some inorganic analytical characteristics of some raw waters are given in table 1. All raw and clarified water samples were characterized according to total organic carbon (TOC), UV absorbance (254 nm) and trihalomethane formation potential (THMFP) under the following conditions: about 20°C, 4 mg Cl2/mg TOC, and a 72-hour contact time in the dark. A Dohrmann DC80 and a Uvikon 930 were used for the determination of TOC (DOC) and UV-absorbance at 254 nm, respectively. When a preoxydation step was employed at the water plant, the clarification treatment was performed with a laboratory apparatus described in another paper (LEFEBVRE and LEGUBE, 1990). Bioeliminable Organic Dissolved Carbon in water was determined by the method described by JORET and LEVI (1986). Cl2 and ClO2 demands of raw and clarified waters were conducted as batch operations with oxidant doses of 1, 2 and 4 mg oxidant per mg TOC. Residual chlorine and chlorine dioxide in solutions were determined respectively by spectrophotometric measurements by two colorimetric methods: the DPD and ACVK methods.
Ultrafiltration was conducted on raw and clarified waters to determine apparent molecular weight distribution (AMW). Organic matter distribution was determined with an Amicon system (stirred cell) with membranes characterized by nominal cutoff limits of 10, 1 and 0.5 kilodaltons. For each cutoff membrane, we took the initial water and a new membrane was used for each filtration. All permeates were characterized according to TOC and UV absorbance.
In the case of clarified waters, the results included all the data, whatever the coagulant salt and its applied dosage.
As shown in figures 1 and 2, no relation exists between UVrw absorbance or the ratio UVrw/TOCrw and TOCrw. For the same raw water, the ratio UVrw/TOCrw fluctuates according to a change between fulvic, humic and hydrophilic acids during the seasons (fg. 3). A better relation between UVcw and TOCcw was obtained: UVcw (254 nm, 5 cm)=0.1091 TOCrw - 0.0231 (r2=0 758), demonstrating that organic matter, which resists coagulation, has quite the same aromaticity whatever the water source (fig. 9). An approximation of TOCcw can be done by a simple measurement of UVcw at 254 nm. Figure 4 shows a correlation between DOCrw and TOCrw (DOCrw (mg/l)=0.888 TOCrw + 0.094; r2: 0.945).
Except for 2 waters, UV absorbance removal was greater than TOC removal (fig. 13).
The AMW distribution of all the studied waters was influenced in the same degree by clarification, with a shift in DOC and UV absorbance to the < 1 kd fraction (fig. 14). Organic matter with AMW above 1 kd was essentially removed during clarification, since TOC concentrations were found slightly to be lower or unchanged in the < 1 kd fraction. But in some cases (e.g.: Charente, table 4) a significant removal of the < 1 kd fraction was obtained. Decrease in the relative UV/TOC ratio between raw and clarified waters was also observed with the different fractions. Neverthless, in the case of 4 clarified waters (table 2), the > 10 kd fraction can represent at most 49.7% and 57.3% respectively of TOCcw and UVcw absorbance. All of the coagulants (ferric chloride, aluminium sulfate or prepolymerised alum salts) were completely in effective in removing dissolved organic matter with an AMW of legs than 500 daltons. The significant removal of high molecular weight compounds is followed by a small reduction in the THMFP expressed in µg per mg of TOC.
The THM formation potential (THMFP) of raw waters is affected by the water source. Chloroform accounted for 85.4% of the THMFPrw and its average value was 63 µg per mg TOCrw. The clarified waters showed a marked difference in their THMFP distribution. Chloroform percentage was very different, ranging between 30 and 93% of THMFPcw.
Clarification treatment slightly reduced the Cl2 and CIO2 demands of the waters: 2.7 mg Cl2/mg TOCrw and 23 mg Cl2/mg TOCcw; 25 mg ClO2/mg TOCrw amd 2.3 mg ClO2/mg TOCcw.
An estimation of TOC removal by coagulation with aluminium salts can be done by ultrafiltration of this unknown water with a nominal cutoff of 10 kd (fig. 15).
The value of the fraction above 10 kd or below 1 kd could be a useful parameter for the determination of the type of an unknown water, whatever the analytical parameters (tables 3a and 3b). In the case of reservoir raw waters, the order of fractions distribution was logical, whatever the analytical parameters (TOC or UV absorbance): the > 10 kd fraction; the 10 kd >> 1 kd fraction and the < 1 kd fraction (fig. 6 and 7). Figures 6 and 7 do not show the same sequence in the case of river water (e.g.: Mayenne, Garonne, Thames,.. .).
No relation exists between BDOCrw and TOCrw or UVrw/TOCrw ratio (Fis. 5a and 5b). BDOCrw represents 0 to 36.4 percent of TOCrw. BDOC removal by coagulation is very different from one water to another (table 5), varying from 35% to 100%.
Keywords:
- Raw waters,
- clarified waters,
- ultrafiltration,
- TOC,
- DOC,
- BDOC,
- UV at 254 nm,
- chlorine,
- chlorine dioxide