Activité biologique dans un filtre à charbon actif en grains

Cet article présente les résultats d'une étude, réalisée en usine de production d'eau potable, de la filtration biologique sur charbon actif en grains (CAG) utilisée en seconde filtration à la suite d'une étape d'ozonation. Les buts de cette étude étaient d'étudier la cinétique de colonisation bactérienne des filtres à CAG, d'analyser la qualité de l'eau en sortie de filtres durant la phase de colonisation biologique et de comparer le fonctionnement à l'équilibre biologique de deux filtres à temps de contact « eau-CAG » différents. Les résultats des mesures de biomasse bactérienne fixée, effectuées par une méthode de respiration potentielle de glucose, montrent qu'un volume d'eau filtrée de 30 - 50 103 m3/m2 de filtre est nécessaire pour coloniser le CAG. A ce moment, le biomasse fixée est d'environ 7 µgC/mf de CAG. La comparaison de la qualité de l'eau, en terme de carbone organique dissous biodégradable (CODB), entre l'entrée et la sortie du filtre étudié montre que durant la colonisation le processus de biodégradation, lié à l'installation d'une biomasse bactérienne suffisante, prend donc progressivement le pas sur le processus d'adsorption dominant au départ, mais qui diminue rapidement. La comparaison de filtres, fonctionnant dans des conditions différentes de hauteur de CAG et de vitesse de filtration, montre l'importance du paramètre temps de contact sur l'efficacité de la filtration biologique sur CAG. De ce point de vue, des temps de contact de 10 à 15 minutes offrent une bonne efficacité de la filtration avec un abattement de CODB pouvant dépasser 80 %.During the two last decades, granular activated carbon (GAC) filtration has been used in drinking water treatment plant in order to remove by adsorption a part of the organic pollution. The adsorption capacity of GAC is however rapidly saturated, requiring frequent regeneration or replacement, which are both expensive and fastidious.An interesting alternative, introduced in some French and Canadian treatment plants, consists in using GAC filtration without regeneration, taking advantage of the activity of microbial communities colonizing the GAC particles. In fact, this biological filtration, introduced in the treatment lines after land filtration and an ozonation stage, has the advantage of specifically removing biodegradable compounds which are the most undesirable fraction of dissolved organic-carbon (DOC).This paper presents results of a study carried out at the Choisy-le-Roi treatment plant to understand better of the processes involved in biological filtration and to derive guidelines for a rational management of these filters. The aim to the study was to follow the bacterial colonization of GAC filters, to test the quality of the filtered water during the colonization phase and to compare the functioning of two colonized inters working with different empty bed contact times.During the period of the study (from March to September 1989), GAC samples were collected at various depth in the filters in order to estimate the fixed bacterial biomass by a method of potential glucose respiration. Samples were also collected in the inlet and outlet waters for measuring the DOC and its biodegradable fraction (BDOC), and also the bacterial abundance estimated by epifluorescence microscopy.Fixed bacterial biomass measurements on GAC were made during the colonization of the GAC filter nr 56 (table 2). The data showed that a filtered water volume of about 30 to 50 103 cubic meters per square meter of GAC filter was required to reach a stable level of colonization (fig. 1). At this stage, the fixed bacterial biomass was around 7µgC/ml (fig. 1 and table 2).The quality of the water, in terms of dissolved organic carbon (DOC) and biodegradable dissolved organic carbon (BDOC), was tested during this colonization. The DOC in the inlet water was in the range of 1.71 to 2.28 mgC/l with a biodegradable fraction between 0.4 and 0.7 mgC/l. A removal of DOC was observed in all the sampled situations, it ranged between 0.49 mgC/l and 1.48 mgC/l; the BDOC removal ranged between 0.31 mgC/l and 0.50 mgC/l (table 3). At the beginning of the colonization, the removal was due to adsorption, practically equal fractions of BDOC and NBDOC were removed. The adsorption process rapidly decreased as indicated by the decrease of non biodegradable dissolved organic carbon removal (fig. 2). Later, biodegradation increased with the bacterial colonization of the GAC. So during the colonization, biological processes took turn with adsorption so that a significant removal of biodegradable dissolved organic carbon was maintained from beginning to end.Enumerations of bacteria were performed during colonization in the inlet and outlet waters of the nr 56 GAC filter by epifluorescence microscopy; they showed an increase of bacteria in the outlet water compared to the inlet water (table 4). This increase was nevertheless negligible in terms of carbon compared to the removed carbon; the average exportation was 0.0015 mgC/l white the removal was in the range 0.3 to 0.5 mgC/l.Two filters (filters nr 56 and 58) with various functioning conditions were compared after their biological colonization. A vertical profile of BDOC in the nr 56 filter is presented in figure 3; it shows that the must significant part of biodegradation took place in the first half of the filter, where the bacterial biomass was greater. A better BDOC removal was observed for filter 58 than for filter 56 (respectively 0.59 and 0.44 mgC/l) with empty bed contact time of respectively 15 and 10 minutes. In figure 4, the percentage of BDOC removal is plotted against empty bed contact time for filter nr 56 and 58. These data confirm previous results obtained with GAC pilot filters at the Neuilly-sur-Marne treatment plant (data also presented in figure 4) showing that the percentage of BDOC removal increases with increasing empty bed contact times. From this point of view, the actual contact times of 10 to 15 minutes applied in the treatment plant have shown a good efficiency of the filtration with BDOC removed higher than 80 %.GAC biological filtration is a very promising way of removing dissolved organic matter in drinking water treatment plants