L'élimination de l'arsenic dans les eaux à potabiliser

The removal of arsenic from drinking water

Véolia Water, Direction Technique, 1, rue GB.Pirelli - 94410 Saint-Maurice

Résumé

La nouvelle réglementation Européenne et Française (Décret 2001) qui limite notamment la valeur de l'arsenic à 10 µg.L^-1 dans les eaux de consommation implique d'ores et déjà la mise en conformité de tous les sites de production d'eau potable régis selon l'ancienne réglementation (50 µg.L^-1).

De ce fait, et selon la qualité de l'eau brute, il s'agira d'adapter les traitements existants ou d'en proposer de nouveaux afin de respecter la réglementation.

Le but de cet article est de présenter les performances de diverses technologies vis-à- vis de l'élimination de l'arsenic : Chaux, alumine activée, échange d'ions, membranes, matériaux adsorbants constitués essentiellement d'hydroxyde de fer ou de dioxyde de manganèse.

Abstract

European and French new stringent arsenic standard of 10 µg.L^-1 in drinking water will require many utilities to upgrade the present treatment systems or consider new treatment options. The objective of this work is to present the performances of different technologies to remove arsenic: lime, activated alumina, pressure membranes, coagulation - flocculation process, ion exchange, adsorbent media based on ferric hydroxide and manganese dioxide.

The addition of coagulants (Iron salts for example) is currently used because this technology allows also the removal of others contaminants (turbidity, colour, organic substances). The use of ion exchange resins or activated alumina media is performing but can be limited, due to the problem of the brine discharge, of the competition of the others ions present in the raw water to be treated and to the necessity to work under acid conditions.

The membrane technology is very little used when the objective is to remove only the arsenic, and when this element is the only one contaminant in the raw water. The membranes are justified when the total dissolved solids due to the presence of sulphates, nitrates, carbonates etc., is important and require a treatment. The discharge of the concentrate can be a problem.

The MnO2 media adsorbent is often used in industrial sites, even when the arsenic concentration in the raw water is very high. The frequency of the regeneration of the media is not a constraining parameter. This media does not loose neither its adsorption capacity after regeneration nor replaced with a new one, as we have observed with some types of activated alumina.

The ferric hydroxide media is actually the Worldwide experimented adsorbent due to its very easy implementation and its high adsorption capacity. But due to the fact that this media is not possible to be regenerated, the invest cost and the operating cost limit this technology to the small drinking water plants.

The lime softening technology is justified when a softened water is required. The produced sludge does not present any added value, and can limit the use of this technology. For this case, the suppliers prefer a treatment in two stages: removal arsenic following with a lime softening.

The in-line coagulation used before a membrane treatment (microfiltration or ultrafiltration) gives very good performances. The coagulant plays the role of adsorbent and the membrane plays the role of physic separator.