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Cet article présente les résultats d'une étude sur le traitement par filtration sur charbon actif biologique (CAB), utilisé en usine de production d'eau potable en second étage de filtration, en aval d'une étape d'ozonation. L'objectif principal de cette étude était de caractériser la matière organique bioéliminée au cours de la filtration sur charbon actif afin d'obtenir une meilleure compréhension de l'abattement du carbone organique dissous (COD) et de la demande en chlore par ce procédé. Le carbone organique dissous biodégradable (CODB) éliminé au cours de ce traitement peut être corrélé à l'abattement de la demande en chlore et ce CODB présente une réactivité au chlore supérieure à celle du carbone organique réfractaire.Le couplage de techniques d'ultrafiltration au suivi des différentes fractions en cours d'incubation en batch pendant 35 jours avec un inoculum de bactéries indigènes libres, permet de préciser la nature des molécules susceptibles d'être bioéliminées et d'appréhender leur impact sur la demande en chlore de l'eau issue du traitement biologique. La nature de la matière organique présente dans l'eau en amont des filtres varie considérablement au cours de l'année; ainsi, en été, une augmentation importante de la fraction de molécules de haute masse molaire (> 10000 daltons) est observée. La filtration biologique n'affecte pas de manière significative la répartition des différentes tailles de molécules et de petites molécules (< 1000 dallons) peuvent être difficiles à bioéliminer sur un filtre biologique., The biodegradable organic fraction of the dissolved organic carbon (DOC) is mainly responsible for growth of microorganisms in water distribution network. Utilisation of high chlorine dosages is often used for controlling this regrowth, but the presence of high contents of DOC results in a large chlorine demand and in the formation of undesirable by-products. Biological activated carbon (BAC) filtration can reduce the regrowth potential by removing biodegradable dissolved organic carbon (BDOC) and some of the side effects of chlorination.This paper presents results of a study performed at the Ste Rose water treatment plant (110 000 m3/d) situated in the city of Laval (Quebec - Canada) in order to get better understanding of the elimination of DOC and chlorine demand by the processes involved in biological filtration (combination of ozonation and granular activated carbon filtration). The plant has used BAC filtration at full scale since 1984 and this research was part of a large research effort to evaluate and optimize biological treatment for North American conditions (PRFVOST, 1991). Ozonation is applied al a dosage of approximately 1.5 mg/l adjusted to maintain an ozone residual of 0.4 mg/l after 8 minutes of contact. The carbon filters are equipped with 2 m of media and are operated al a rate of 5-10 m/h; sampling ports are available on 10 different depths in the bed.The major objective of this study was to determine the parameters to optimize BDOC removal by BAC filtration in order to reduce the chlorine demand of the plant effluent. To achieve this, a number of water quality parameters were measured at different depths in the BAC filters : DOC, BDOC, short and long term chlorine demand, ammonia (fig. 1, 2, 3 and 4). A Dohrman DC 80 analyser was used for the determination of DOC. Chlorine demand is performed using a chlorination dosage adjusted by a ratio of 2 mg of chlorine per mg of DOC, incubation was done at pH 7 and 20 °C and residual chlorine is measured at different times by the DPD method. BDOC was measured at 20 °C by SERVAIS method (SERVAIS et al., 1987). Additional characterization of organic matter was performed by ultrafiltration (UF) in stirred cells (Amicon 8400) to identity the size of the molecules more likely to be eliminated.Molecular size distributions of the dissolved organic matter of raw and treated water, determined using ultrafiltration, show large seasonal differences resulting in significant changes of DOC (table 1) and chlorine demand reduction (table 2). The fractioning by UF was performed on ozonated water and in BAC filtered water for various empty bed contact times (EBCT); it shows that BAC filtration does not affect significantly the distribution of the different sizes of molecules as measured through DOC evolution (fig. 8).The removal of DOC in the BAC filters is associated with bacterial degradation as measured through BDOC (fig. 5). Monitoring of filters showed that BDOC removal is maintained (50 - 100 %) whatever the temperature (1 °C during 5 months in winter and sometimes as high as 28 °C in summer) and the organic composition of the ozonated water. The removal of BDOC is associated with a significant reduction at the short and long term chlorine demand (fig. 6). The results demonstrate that BDOC which is a part of DOC removed by biological filtration, is more reactive to chlorine than refractory carbon (fig. 7). However, if it is possible to correlate these two parameters for one type of water, no correlation could be established for the entire study because of the variations of chlorine reactivity of the refractory organic carbon.The use of UF techniques, linked to the monitoring of various parameters (DOC, Marine demand) during a batch incubation of the various size fractions, enables us to precise their biodegradable character (fig. 9 and 10). Ozonated water was ultrafiltered and the different ultrafiltrates were incubated for 35 days; BDOC and chlorine demand were measured on samples taken from the incubation bottles at different times of incubation (fig. 11). The data indicate Mal the degree of biodegradability differs amine different molecular weight fractions.The highest molecular weight fraction (> 10 000 daltons) is the least bioeliminable and the fraction in the low molecular weight ( 10 000 daltons), without any carbon elimination during incubation, suggests that these molecules undergo important structural changes under the action of biological treatment.

L'adsorption en phase liquide sur charbon actif est un sujet très travaillé sur le plan expérimental et de plus en plus dans le domaine de la modélisation. Les tentatives de description des courbes de percée ou de fuite des filtres montrant la saturation du matériau adsorbant remontent aux travaux de BOHART et ADAMS en 1920. D'autres équations avec d'autres approximations ont été proposées par la suite (THOMAS 1944 ; DOLE et KLOLZ 1946), HUTCHINS (1973) ; plus récemment, WOLBORSKA (1989) ou CLARK (1987) ont proposé d'autres modèles. Nous avons essayé de faire le point sur ces différents modèles, de montrer leurs origines communes, souvent à partir des équations de BOHART et ADAMS, les approximations apportées limitant leur domaine d'application, les grandeurs qu'ils permettent de déterminer : capacité maximum d'adsorption, constante cinétique d'adsorption, vitesse de déplacement du front d'adsorption. De tous ces modèles, un seul (CLARK 1987) permet une bonne représentation des courbes de percée. Nous en avons proposé une linéarisaüon qui facilite la détermination des paramètres nécessaires au calcul des courbes de fuite. Tous ces modèles ont été testés sur les résultats expérimentaux obtenus pour l'adsorption d'un tensioactif anionique : le décylsulfonate de sodium et ceci sur cinq petites colonnes de hauteurs différentes de charbon actif. Le modèle de CLARK a également été appliqué à des résultats obtenus au laboratoire (El HANI, 1987) sur l'adsorption et la dégradation biologique des acides humiques sur un filtre de charbon de 1m de haut, sur une période beaucoup plus longue (1 mois) et avec des lavages du filtre. Ce modèle permet de calculer la part qui n'est pas simplement de l'adsorption rapide (dégradation biologique et adsorption lente)., Low concentrations of organic contaminants are not easily removed by conventional treatment methods, but activated carbon bas a good affinity for various organics and is used in batch or column reactors.Much has been written concerning the prediction of the performance of powdered activated carbon (PAC) ; adsorptive capacity and equilibrium isotherms determined in « batch » reactor are proposed to simulate the performance of PAC for single or bisolute systems (DUSART et al. 1990, SMITH 1991). Some investigators have attempted to simulate column performance with mathematical models and the aim of this work is to present the principal models and verify how the different models are applied to break-through curves ; parameters which can be evaluated by the different equations will also be compared.As early as 1920 BOHART and ADAMS presented differential equations which govern the dynamics of the adsorption of vapours and gases on fixed beds and the final result, applied to the liquid-solid phase, yields the kinetic adsorption rate (k) and the maximum adsorption capacity (No) (eq. 3). By transposition to the liquid phase, we have calculated the concentration distribution in the bed (eq. 5) by using the kinetic constant k and the maximum adsorption capacity No obtained by equation 4; it was noted that only the low concentration range of the break-through curve can be used. Some approximations from DOLE and KLOTZ (1946) lead to the « Bed Depth/Service-Time (BDST) equation 7 proposed by HUTCHINS (1973) ; the service time of a column tb has a linear relationship with the bed depth Z (fig.3). The activated carbon efficiency No can be estimated and the adsorption rate constant calculated from the slope and the y-intercept.Recently, WOLBORSKA (1989) proposed a rectilinear equation InC/Co = At + B (eq. 10) for the initial segment of the break-through curve. The form of this equation is similar to equation (4) obtained tram the BOHART-ADAMS hypothesis. The mass transfer coefficient, ßa, the maximum adsorption capacity and the migration velocity v (eq.9) of the concentration fronts can be calculated from the constants A and B.The model developed by CLARK (1987) is based on the use of e mass-transfer concept in combination with the Freundlich isotherm (fig.4). The originality of this modal, in comparison to the others, consists in the existence of the equilibrium concentration Ce and the driving force equilibrium « C-Ce ». The general equation is equation (14). Two parameters A and r are determined by regression equations ; we proposed a simple method to calculate A and r by a linearization of the preceding equation (eq. 14). This is equation (16) In [(Co/C)n-1 -1] = In A -rt.Sodium decanesulfonate at a concentration of 20 mg ·l-1 was used as influent and activated powdered carton (200 ≤ ø ≤ 315 µm) as the fixed bed adsorbent layer to illustrate the comparison between the different models. The linear flow rates were 3.0 m . h-1 and the five columns tested were 3.1 ; 4.0 ; 7.5 ; 10.2 ; 12.5 cm high with a 1.45 cm2 cross section. The Freundlich isotherm equation (fig. 4) obtained in a batch system for an equilibrium time (t = 24 h for this activated carbon) gives a « n value » equal to 2.38.Figure 2 presents the experimental break-through curves obtained for the different bed heights ; by using equations (4 or 10) in the system (In C/Co, t) they are represented on the same figure by the dotted line. The agreement is only for the low values of C in the break-through curves.The coefficients A and B (table 1) are determined from the straight lines obtained with the low break-curve concentrations (fig. 1). The kinetic coefficient Sa, and the maximum capacity adsorption No are shown in table 1. The No value is similar to those obtained from the other equations. The migration velocity of the concentration fronts (r = 0,133 cm · h-1) is in good agreement with the experimental value (0,128 cm · h-1).The linearized Clark equation (16) gives a good representation of experimental results (fig. 6) alter the determination of A and r parameters (fig. 5 and table 2). With the use of the two parameters, the break-through curves have been recalculated (fig.6) and compared to experimental results. Their is good agreement. The A parameter is related to the depth Z of the adsorbant : A = Bez ·. B value can be determined with the different columns (fig.7).The Clark model can be applied to filers which have a biological activity ; the results obtained in the laboratory by EL HANI (1987) for the adsorption of humic acids (10 mg · l-1) on a 1m granular activated carton bed were analyzed by the Clark equation (fig. 8). The initial concentrations of humic acids are never obtained in the effluent because of biological degradation and/or slow adsorption in mesopores. From the difference in the area of the two curves, it is possible to calculate the supplementary biological degradation. For 95 cm of activated carbon in the column and after 800 h, the biological degradation represents 55 % of the total elimination. The percentage is constant alter 35 cm depth of the activated carton in agreement with the electon microscopy study that showed that the flora was only present in the 10 first centimeters.The use of this model is facilitated by our linearization and the case of particular phenomena : biological degradation or desorption. in the case of successive muld adsorbates fixation (REYDEMANEUF et al. to be published) can be studied and compared to the only adsorption phenomena.In conclusion, nome of the tested models lead to different parameters by using low break-through curve concentrations or others with the whole range of experimental points, but only one (CLARK) gives a good description of the break-through curves in our actual knowledge.

Cette étude de laboratoire a eu pour but d'examiner la réactivité du bioxyde de chlore sur un acide humique d'origine aquatique en solution aqueuse et en milieu neutre (pH = 7,5) et de préciser en particulier l'incidence d'une préoxydation chimique au CIO2 sur les potentiels de formation de composés organohalogénés (trihalométhanes, acides dicloroacétique et trichloroacétique, chlore organiquement lié) et sur l'adsorbabilité du carbone organique sur charbon actif.Les résultats obtenus montrent que radian du bioxyde de chlore sur racide humique Pinail à l'obscurité, conduit à des faibles abattements du carbone organique dissous (< 10 %) et de l'absorbance UV à 254 nm (de l'ordre de 30 %) et conduit à des productions potentiel es en composées organohalogénés très nettement inférieures à celles formées par chloration. De plus, une préoxydation chimique au bioxyde de chlore permet de diminuer d'une manière très significative la production de composés organohalogénés au cours d'une post-chloration et semble améliorer l'adsorbabllité du carbone organique sur charbon actif.L'oxydation de l'acide humique par le bioxyde de chlore s'accompagne, par ailleurs, de la formation de chlorites (0,65 mg/mg de CIO2 consommé) qui peuvent ensuite être oxydés en chlorates au cours d'une post-chloration ou réduits en chlorures par un traitement au charbon actif.Enfin, les résultats obtenus font apparaître que le mécanisme d'oxydation de composés organiques parle bioxyde de chlore en présence de la lumière ainsi que les interactions entre le bioxyde de chore, les chlorites, la matière organique et le charbon actif méritent d'être plus précisément étudiés., Chlorine dioxide has drawn much recent attention as an alternative disinfectant and oxidant for drinking water to replace chlorine because of its powerful disinfecting ability and its limited capacity to produce organohalogenated compounds. However, the use of chlorine dioxide leads to chlorite (ClO2-) and chlorate (ClO3-) as inorganic oxidation by-products which are reported to have toxic effects on humans. The reactions of ClO2 with simple organic compounds (phenols, aliphatic and aromatic amines...) produce polar compounds such as quinone, ketones, aldehydes and carboxylic acids while oxydation by-products of dissolved organic matter of surface waters (in particular humic substances) are largely unknown. Consequently, the aim of this work was to obtain a better understanding of the effects of the use of chlorine dioxide in drinking water treatment To this end, experiments were carried out with dilute aqueous solutions of an isolated aquatic humic acid (Pinail humic acid, PHA) and the objectives of this present study were :- To evaluate the ClO2 demand and to determine the productions of chlorite, chlorate and of organohalogenated compounds such as trihalomethanes (THMs), dichloroacetic and trichloroacetic acids (DCA, TCA) which are the main organohalogenated products formed by chlorination.- To show the effects of chlorine dioxide preoxidation on organic halide formation potentials (postchlorination) and on the removal of dissolved organic carbon (DOC) by activated carbon. In addition, reactions of chlorite with chlorine or with activated carbon were also examined.EXPERIMENTALPniail humic acid was dissolved in phosphate buffered ultra-pure water (pH = 7.5). Oxidation and adsorption experiments were carried out in headspace-free bottles, at 20 ± 1 °C and in the dark. Stock solutions of chlorine dioxide (4-6 g l-1) and of chlorine (6-10 g l-1) were prepared in the laboratory and titrated by iodometry. Residual chlorine dioxide concentration in PHA solutions was determined by spectrophotometric measurement at 360 nm and by two colorimetric methods : the chlorophenol red and the ACVK methods. Concentrations of DOC and of total organic chlorine or halogen (TOCI, TOX) were measured using a DOHRMANN DC 80 carbon analyser and a DOHRMANN DX 20 A TOX analyser equipped with a microcoulometric cell, respectively. THMs, DCA and TCA were determined by a gas chromatograph equipped with a 63 Ni electron capture detector after extraction by pentane for the THMs, and methylation in ether phase for DCA and TCA. Inorganic chlorine species were analysed by HPLC with a UV detector (ClO2-) or by chromatography (Cl-, ClO3-).RESULTS• Oxidation of PHA by ClO2The results showed that PHA consumed about 2 mg of ClO2/mg of DOC after a reaction time of 24 hours (fig. 1) and that there is a rapid consumption of ClO2 during the first 30 minutes of the reaction (fig. 2) Oxidation by ClO2 had no effect on DOC concentration (DOC removal : < 10 %) and led to a significant decrease (about 30 %) of the UV-absorbance at 254 or 270 nm (fig. 1 and 2), and to productions of ClO2- (0,65 mg of ClO2-/mg of ClO2 consumed) which were independant of the applied oxidant dose and of the reaction time.Furthermore, after a 72 hour reaction time in the dark, chlorine dioxide ([ClO2]0 = 5 mg l-1, [PHA]0 = 5 mg l-1, DOC = 2,6 mg l-1) produces very small amounts of chloroform (< 5 µg l-1), DCA (5 µg l-1) and TCA (5 µg l-1) and organochlorinated compounds (TOCl : 36 µg/mg DOC) compared to chlorine oxidation (tableau 1). However, in the presence of sunlight, ClO2 is rapidly photodecomposed (fig. 3) and the photodegradation products of ClO2 allow bromide oxidation (fig. 11) and lead to higher productions of organohalogenated compounds such as THMs (fig. 4).• Chlorine dioxide preoxidation followed by chlorinationAs shown in figure 5, chlorine dioxide preoxidation reduces the production of organohalogenated compounds and the chlorine demand during postchlorination. For a preoxidant dose corresponding to the ClO2 demand of PHA, the decrease in the formation potentials of CHCl3, DCA, TCA and TOCl was about 40-50 %. These results confirm the similarity of the action of chlorine dioxide and chlorine on aromatic structures which have high electron density carbons and which constitute probably the most reactive precursors of organohalogenated by-products.As far as chlorite concentration is concerned, the results showed that chlorite formed during the preoxidation step was completely oxidized to chlorate during postchlorination, under the experimental conditions used in this study (chlorine dose : 40 mg l-1; contact time : 24 or 72 hours). Because of the reactions of chlorine eh chlorine and with residual chlorine dioxide, a small increase in the chlorine demand was observed when PHA solutions were heavily preoxidized (fig. 5).• Chlorine dioxide preoxidation followed by activated carbon treatmentBatch experiments were carried out with a powdered activated carbon (PAC, granulometry : < 80 µm) which was obtained by crushing a commercial granular activated carbon (CECA 40,12 x 40 mesh). Once equilibrium was achieved (contact time : 10 days), adsorption isotherms indicated that chlorine dioxide preoxidation increases the absorbability of DOC on activated carbon (fig; 4tableau 2). Furthermore, chlorite in oxidized PHA solutions was reduced by PAC to chloride. The capacity of CECA 40 activated carbon for ClO2- reduction to Cl- was about 170 mg ClO2-/g of PAC (fig. 7). Other experiments showed that chlorite may react with specific surface groups on PAC to produce inorganic carbon (fig. 7) and with PHA only in the presence of PAC as shown the DOC and UV-absorbance curves in figure 8 and the increase of TOX concentration in the liquid phase in figure 9. Thus the observed increase in DOC absorbability on PAC after a chlorine dioxide preoxidation may be attributed to cheminal interactions between PAC, chlorite, residual chlorine dioxide and adsorbed organic matter and requires further study.