Formation et évolution de quelques cétoacides et aldéhydes dans les usines de traitement d'eau potable comportant une étape d'ozonation Occurrence and Behavior of Some Ketoacids and Aldehydes during Drinking Water Treatment Processes Including an Ozonation Step

Résumé

Les étapes de désinfection de l'eau, telles que l'ozonation et la chloration, génèrent des sous-produits d'oxydation de nature variée. Ces composés sont soupçonnés d'être toxiques à plus ou moins long terme. Certains d'entre eux sont, de plus, facilement biodégradables et favorisent donc une reviviscence bactérienne dans le réseau de distribution. Enfin, à cause de leurs caractéristiques organoleptiques, ils peuvent conduire à la détérioration de la qualité sensorielle de l'eau distribuée. La recherche de plusieurs sous-produits d'oxydation dans l'eau potable a pu être effectuée grâce à de nouvelles techniques d'analyse quantitative par chromatographie en phase gazeuse: il s'agit des aldéhydes et des cétones de faible poids moléculaire, des acides haloacétiques et de certains céto-acides. Ces composés ont été recherchés dans des usines comportant une étape d'ozonation. L'influence de ce traitement sur la formation des aldéhydes et des céto-acides est démontrée dans cette étude. L'ozonation multiplie la concentration totale d'aldéhydes par un facteur variant de 2 à 4 suivant les usines et les trois céto-acides recherchés ont été trouvés en quantités importantes dans des eaux ozonées. La filtration sur charbon, lorsqu'elle existe, s'avère efficace pour l'élimination de ces composés. Les trois acides chloroacétiques sont présents dans des eaux chlorées, en sortie d'usines appliquant des taux de chloration assez importants. Enfin, I'évolution de ces sous-produits d'oxydation tout au long d'un réseau de distribution a pu être expliquée par leur biodégradabilité.

Mots-clés:

• Sous-produits d'oxydation,
• ozonation,
• aldéhydes,
• biodégradabilité,
• acides haloacétiques,
• céto-acides

Abstract

New regulations are being considered by the US Environmental Protection Agency (US EPA) concerning a variety of disinfection by-products formed during chlorination and ozonation (halonitriles, haloketones, haloacids, low molecular weight aldehydes...) and many surveys are underway to assess the presence of such products in drinking waters. This renewed interest for disinfection byproducts (DBPs) arises from their suspected carcinogenic or mutagenic properties. In addition to possible long term health effects, specific disinfection by-products may also induce immediate water quality deterioration due to their objectionable organoleptic properties. Biodegradable DBP's also probably contribute a substantial proportion of the biodegradable dissolved organic carbon (BDOC).

As far as oxidation disinfection practices in France are concerned, the use of ozone is frequent. Also, most major French treatment plants include an activated carbon filtration process which is likely to remove some of the DBPs as well as some of their precursors. This paper summarizes the results obtained along various treatment plants in the Paris area, concerning three major families of DBPs considered for regulation: the DBPs investigated include 30 aldehydes and ketones, chloroacetic acids and 3 ketoacids. The aldehydes and ketones were measured by GC-ECD or GC-MS after derivatization with PFBHA; the chloroacetic acids were measured using a micro extraction method with methyltertiobutylether followed by diazomethane methylation and GC-ECD or GC-MS; the six plants investigated in the Paris area treat surface water from the river Seine upstream of Paris (Morsang, Vigneux, Orly, Ivry) or groundwater which is artificially recharged with Seine river water downstream of Paris (Le Pecq, Aubergenville). The Alençon plant which is located outside the Paris area treats raw water from the Sarthe river plus groundwater from various wells. All the treatment lines studied include an ozonation step which is followed at some plants (Morsang, Ivry, Vigneux, Le Pecq-Minor) by a granular activated carbon (GAC) filtration. The treatment line investigated at Vigneux used an ozone/hydrogen peroxide combination as an oxidation step. Three of these treatment lines (Morsang, Le Pecq-Minor, Alençon) comprise a prechlorination step; the other plants only use chlorination as a final disinfection step.

The study compares the total concentration of aldehydes detected before and after ozonation as well as after GAC filtration. Approximately half of this total is usually due to formaldehyde while acetaldehyde, glyoxal and methylglyoxal represent most of the remainder. These concentrations which initially range from 1 to 25 µg/l show a drastic increase after ozonation. Depending on the water DOC and ozonation conditions, the total level of aldehydes is multiplied by a factor of 2 to 4. The final chlorine disinfection step used at all these plants does not significantly influence the total concentration of the aldehydes, therefore the level of these DBPs at the outlet of the plants is mainly determined by the ozonation or ozone/GAC filtration steps.

The three aldo and ketoacids analysed were glyoxylic acid, pyruvic acid and ketomalonic acid. They were detected in ozonated water with total concentrations which range from 65 to 80 µg/l. Glyoxylic acid alone, represents half of these quantities.

Chloroacetic acids were not detected at the outlet of the plants which are supplied by groundwater (Le Pecq-Minor, Aubergenville) and which apply a low chlorine dose (0.1- 0.2 ppm) as a final disinfection stop. Although the Morsang treatment line investigated applies a low prechlorination dose ([smaller or equal] 1 ppm) in addition to the low postchlorination dose, no haloacids were detected at the outlet, which is agreement with the low levels of trihalomethanes usually detected at this plant. The absence of haloacids at the outlet of this plant can be attributed to the efficiency of the ozone/GAC combination as well as to the low prechlorination dose applied. Haloacids were only detected at the outlet of the Orly, Ivry and Alençon plants which apply a rather high chlorine dose during the final disinfection stop (between 0.8 and 2.2 ppm) in order to maintain a residual in the distribution system. Typical levels of haloacids are found between 10 and 35 µg/l, mainly under the form of dichloro and trichloroacetic acid.

To summarize, the levels of the specific DBPs investigated remain well below their individual WHO recommendations (respectively 50, 100 and 900 µg/1 for dichloroacetic acid, trichloroacetic acid and formaldehyde). Unless more drastic national regulations are implemented, the interest in the fate of these DBP's mainly lies in their possible secondary effects such as enhancement of bacterial regrowth in distribution systems or degradation of drinking water organoleptic properties.

Keywords:

• Disinfection by-products,
• ozonation,
• aldehydes,
• biodegradability,
• haloacetic acids,
• ketoacids,
• drinking water,
• Paris