Elimination de l'arsenic : evaluation de différents procédés de traitement à l'echelle laboratoire et industrielle dans des conditions optimisées
Arsenic removal: assessment of different processes on bench scale and full-scale treatment conditions optimisation
SUEZ ENVIRONNEMENT - CIRSEE, 38 rue du Président Wilson, 78230 Le Pecq, France
L'arsenic est naturellement présent dans plus de 200 minéraux de l'écorce terrestre. Sa dissolution dans les ressources en eau dépend essentiellement du pH et du potentiel d'oxydoréduction du milieu. Dans le monde, le plus grand nombre de personnes exposées quotidiennement à des concentrations élevées en arsenic dans les eaux souterraines se trouve principalement au Bengladesh, en Indes et à Taiwan. De récentes études épidémiologiques faites sur ces populations montrent que l'ingestion journalière d'eau fortement concentrée en arsenic conduit à des maladies spécifiques comme la dépigmentation de la peau, des kératoses pouvant évoluer vers des cancers de la peau ainsi que des cancers des organes internes comme les poumons, le foie ou la vessie. Suite à ces études, les dernières réglementations sur l'eau potable aux Etats-Unis et en France ont abaissé la valeur paramétrique sur l'arsenic de 50 à 10 µg/l. Par conséquent, il est nécessaire d'améliorer les procédés de traitement existants et d'évaluer les performances des technologies émergentes pour satisfaire à la nouvelle réglementation sur l'arsenic. Plusieurs procédés de traitement efficaces existent. Le choix du procédé le plus performant pour un site donné dépend essentiellement des conditions suivantes : - Etat chimique de l'arsenic en relation avec les conditions de pH et de potentiel d'oxydoréduction de la ressource. - Présence éventuelle d'autres polluants à éliminer (fer, manganèse, dureté, etc.). - Mise en oeuvre d'un procédé d'élimination de l'arsenic dans une chaîne de traitement existante. - Dimensionnement d'un traitement industriel sur un nouveau site.
Abstract
Recent epidemiological studies shown the impact of arsenic on human health (Directives de qualité pour l'eau de boisson; 1994), (Kempster; 1981). Following these studies, Cancer International Research Center classified arsenic in group 1 with other compounds recognized as carcinogenic for humans. Consequently, European regulation on drinking water lowered the parametric value on arsenic from 50 µg/L to 10 µg/L, as in other countries like USA and Japan. In France, an overview of arsenic concentration in resources points out regions where natural arsenic concentrations in groundwater are higher than 100 µg/L (Charles; 1998). These regions are especially found in south-west France (Landes area) and north-east France (Vosges area). Out of France, countries where a large amount of the population is daily exposed to arsenic concentrations higher than 10 µg/L are mainly Bangladesh, India, China (Taiwan), Chile, Argentina and some states of the west coast of the USA. To face of this situation water treatment plants have to be implemented with appropriate processes to comply with the local arsenic regulation on drinking water. One of the main goals of this work was to achieve a complete bibliographic survey of the available treatments to decrease the arsenic concentration in drinking water below 10 µg/L. These processes are summarized in Tables 1 and 2. Some of these processes are illustrated by pilot scale and full-scale experiments conducted by CIRSEE in the following raw water conditions: - Removal of arsenic only at high concentration (400 µg/l) - Removal of arsenic only at low concentration (10 to 20 µg/l) - Joint removal of arsenic and iron. The complete bibliographic survey of the existing treatments to comply with the regulation on arsenic in drinking water, points out the wide variety of processes available at industrial scale and gives information about the expected performances. Depending on the specific conditions of each site (like resource quality parameters and already existing processes on the treatment line), arsenic could be removed with other pollutants in optimized processes such as enhanced coagulation, iron physicochemical oxidation or lime softening. If arsenic is the only pollutant to be removed from the raw water prior to its treatment on the plant, specific treatments may be implemented: ions exchange resin or catalytic adsorption on activated alumina and on granular ferric hydroxide. Performances are mainly depending on the pH and on the presence of competing substances. Among membrane filtration processes, nanofiltration is an interesting alternative due to its reliability as well as for its hardness removal capability. Hardness is one of the first reasons for consumers dissatisfaction. The best treatment choice for one site mainly depends on the following conditions: - Arsenic chemical state in relation to pH and oxidation-reduction potential values in the resource water. - Presence of other pollutants to be removed (iron, manganese, hardness, etc.). - Implementation of an arsenic removal process in already existing treatment plant. - Design of a full scale treatment on a new site.
© ASEES 2005