Enlèvement de la matière organique dans les filtres CAB Removal of organic malter in BAC filters

Résumé

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.

Mots-clés:

• Eau potable,
• charbon actif,
• matière organique,
• biodégradation,
• carbone organique dissous biodégradable (CODE),
• demande en chlore,
• ultrafiltration

Abstract

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 (<1000 daltons) represents more thon 50 % of BDOC and chlorine demand. However the smallest molecules (< 500 daltons) seem stable and they go through the filter without significant changes neither in DOC nor in chlorine demand. As for the medium fraction (between 1000 and 10 000 daltons), it is significantly bioeliminated but this BDOC reduction is net associated with a large chlorine demand removal. On the opposite, the removal of the chlorine demand of the largest molecules (> 10 000 daltons), without any carbon elimination during incubation, suggests that these molecules undergo important structural changes under the action of biological treatment.

Keywords:

• Drinking water,
• activated carbon,
• organic matter,
• biodegradation,
• biodegradable dissolved organic carbon (BDOC),
• chlorine demand,
• ultrafiltration