Le Traitement des Eaux de Consommation : La Nécessité d'une Recherche en Chimie de l'Eau Needs For Research in Water Chemistry in Drinking Water Treatment

Résumé

L'objectif principal de cet article est de montrer que les travaux de recherche en chimie de l'eau sont absolument nécessaires à la compréhension des phénomènes régissant la qualité des eaux de consommation produites et distribuées, ainsi qu'au développement de technologies innovantes dans le domaine du traitement de ces eaux. Pour l'atteindre, il a été choisi de présenter trois exemples des recherches menées à l'Université de Poitiers et à l'École Polytechnique de Montréal : deux exemples de recherche fondamentale, sur les matières organiques naturelles et sur les mécanismes d'oxydation par les radicaux hydroxyle et un exemple de la recherche d'application, sur l'évolution de la qualité de l'eau lors de son traitement biologique par ozonation couplée à la filtration sur charbon actif en grains.

Mots-clés:

• Recherche,
• chimie de l'eau,
• MON,
• chloration,
• ozonation,
• radicaux hydroxyle,
• CODB,
• CAG

Abstract

The main purpose of this paper is to demonstrate that a better understanding of chemical reactions and a better technologies development for the drinking water treatment absolutly needs research in water chemistry. In order to reach this objective, two examples of recent fundamental research works in water chemistry and one example of research applied to drinking water treatment have been selected.

The first one concerns a fundamental research on characterization and analysis of natural organic matter. Nowadays, the main analytical tools used for quantification of NOM are DOC (BDOC for the biodegradable fraction) and UV-absorbance. However, a better knowledge of NOM would be useful to predict water quality and determination of doses of some treatment chemicals, such as coagulants and oxidants. This paper presents some data about (i) NOM fractionning by XAD macroporous resins and relationship between UV-absorbance and 13C-NMR, and (ii) study of chlorination of the isolated fractions. Based on these results, one can suppose that aquatic NOM could be soon easily fractionnated and characterized by simples analytical tools. However, many research works are still necessary, particularly on the field of relationships between data of "heavy" analytical tools (13C-NMR, pyrolysis/GC/MS, etc.) and more simple others (UVA/DOC, BDOC, fluorescence etc.).

The second paragraph of the paper relates oxidation mechanisms by hydroxyl radicals. In ozonation of drinking water, currently applied in the drinking water treatment, OH radicals production is a necessary consequence, considered as the main action of ozone by some authors. So, disinfection quality, bromate production, BDOC formation, pesticides removal will never be entirely understood as long as hydroxyl radicals participation in ozonation processes will not be really cleared up. To illustrate this purpose, the example of ozonation of a free aminoacid (glycine) by molecular ozone and by OH radicals is shown. Oxidation by "pure" OH radicals, produced by radiolysis, as compared to oxidation by molecular ozone, (at acidic pH in the presence of radical scavengers) shows that two mechanisms are really different. So, some works published some years ago, mixed up the two pathways. Such mechanisms on aminoacids oxidation should allow to understand some more descriptive data on water disinfection and pesticides oxidation.

The third example concerns the changes of water quality during ozone/GAC treatment. It is well known that ozonation of naturel waters leads to an enhancement of water biodegradability, in terms of BDOC increase. So, if this BDOC is not removed in the plant, it can be the origin of bacteria regrowth in distribution system. The solutions are either to include, after ozonation step, a biological treatment step on activated carbon (BAC), or to increase chlorination doses applied in final disinfection. The effects of BAC treatment on BDOC and ozonation by-products removals, in the case of the drinking water treatment plant of Sainte-Rose (City of Laval, Quebec, Canada), are described. Finally, taking into account the cost of such a process, the last paragraph of this paper gives some recommandations for conception on BAC treatment.

As for general conclusion, some new research topics in water chemistry for the drinking water treatment are enumerated and some ideas about organization and development of applied and fundamental researches in water chermistry for the drinking water treatment, are given.

Keywords:

• Research,
• water chemistry,
• NOM,
• chlorination,
• ozonation,
• hydroxyl radicals,
• BDOC,
• GAC