Résumés
Résumé
Cette étude présente un suivi de l'élimination des acides aminés totaux par les différentes étapes de traitement de deux usines canadiennes de Ville de Laval (Québec, Canada). L'influence des saisons sur l'efficacité de l'abattement ainsi que le devenir des acides aminés dans les réseaux de distribution de ces usines ont aussi été étudiés.
D'une manière générale, l'étape de coagulation-floculation-décantation permet une élimination important d'acides aminés totaux (34-72%), l'effet de la filtration sur sable et anthracite est peu prononcé et varie avec les saisons, l'ozonation provoque une augmentation (20-100%) et la filtration sur charbon actif biologique (CAB) permet un abattement en eau tiède (25-34%), mais semble relarguer en eaux froides (35%). Finalement, la post-désinfection au chlore permet une élimination supplémentaire (47%) à cause de la réactivité de cet oxydant envers les acides aminés. La post-désinfection au bioxyde de chlore permet elle aussi d'abattre les teneurs en acides aminés (12-39%) et ce, malgré le fait que cet oxydant soit moins réactif envers les acides aminés. L'élimination des acides aminés totaux avant la chloration est préférable afin de limiter la formation de sous-produits de chloration.
Mots-clés:
- Acides aminés,
- carbone organique biodégradable (COB),
- traitement des eaux potables,
- réseaux de distribution
Abstract
Low concentrations of free and combined amino acids are found at every stage of water treatment (LE CLOIREC and RENAUD, 1984; SCULLY et al., 1988; BERNE et al., 1994). A decrease in amino acid concentrations has been observed after settling (LE CLOIREC et al., 1983; LE CLOIREC and RENAUD, 1984), while ozonation has been shown to increase free amino acid concentrations. Biological activated carbon (BAC) filtration may also decrease the concentration of total amino acids (JADAS-HÉCART, 1989; BERNE, 1994).
Total amino acids represent a small fraction of dissolved organic matter (1 to 3% of DOC), but account for an important part of the chlorine demand of treated water (JADAS-HÉCART, 1989; HUREIKI et al., 1994). Moreover, recent work by HUREIKI and GAUTHIER (1994) has suggested that some amino acids found in surface waters may represent a significant fraction of the precursors of some organohalogenated disinfection by- products (DBP). It is also possible that total amino acids amount to an important fraction of biodegradable organic carbon (BOC). These characteristics of amino acids suggest that removing them by treatment will improve water quality, both from a biological and a chemical (DBP) stability standpoint.
The objectives of the research described in this paper were to:
1. assess the seasonal variability of the removal of free and combined amino acids by different treatment processes in two Canadian water treatment plants located in the city of Laval (Québec, Canada);
2. monitor the fate of total amino acids in the distribution systems of these two plants with the objective of correlating total amino acids with biodegradable dissolved organic carbon (BDOC) and fixed bacterial density.
Sampling was conducted in two water treatment plants. The Ste-Rose water treatment plant (100,000 m3/d) uses conventional treatment (dynamic settling and dual-media filtration on sand and anthracite) followed by ozonation, pH adjustment, and post-chlorination using either chlorine of chlorine dioxide. The second plant studied is the Chomedey water treatment plant (180,000 m3/d) in which the following processes are used: conventional treatment (dynamic settling and dual-media filtration on sand and anthracite) followed by ozonation, second stage filtration on biological activated carbon (BAC), pH adjustment, and post-chlorination using chlorine dioxide. Samples were collected in the two distribution systems according to the residence time of the water calculated by a hydraulic model. Samples were taken directly from small diameter (15 cm internal diameter) ductile iron pipes.
Results of the monitoring of the treatment plants show a very strong decrease of total amino acids by coagulation-flocculation-settling (34-72%). First stage dual-media filtration may increase or decrease the concentrations of total amino acids present, depending on the time of the year. In all but one case ozonation increases the concentration of total amino acids (20-100%). To document the source of this increase, we verified the yields of hydrolysis of amino acids found in natural matrices under different hydrolysis conditions. The objective of this experiment was to verify if more drastic hydrolysis conditions would free some amino acids linked to more complex structures found in natural waters. If such were the case, this would explain why higher concentrations of total amino acids were obtained after ozonation. Results showed that the hydrolysis conditions used by BERNE (1994) are optimal for recovery of amino acids in the natural water studied (sand and anthracite filter effluent).
Total amino acids were decreased by BAC filtration in warm water (24 to 34%) but increased in cold water (+35%). This could either be related to the slower kinetics of the hydrolysis of combined amino acids by the fixed biomass or by the form in which amino acids were present in the winter matrix. The expected effect of chlorination on total amino acids was observed with a decrease of 47% at the post-chlorination step. Some impact of chlorine dioxide was noted although it is believed that chlorine dioxide will not readily react with free and combined amino acids. This effect was more pronounced in cold water and could be related to the fact that chlorine dioxide in full-scale plants is produced in the presence of excess chlorine (less than 10%).
The analysis of the composition of total amino acids present in the Mille-Îles River showed that the amino acids most commonly found were glycine, serine, alanine, leucine, lysine, aspartic acid and glutamic acid. We observed that the trends of major amino acids present followed the trends observed for total amino acids. This concordance of trends does not reflect the individual characteristics of each amino acid or the ability of a treatment process to remove or transform them. It most probably reflects the ability of each treatment process to remove or transform complex bound forms of amino acids.
In the Ste-Rose distribution system (DS), we observed stable concentrations of total amino acids regardless of the residence time. In the case of the Chomedey distribution system, concentrations decreased slightly with residence time, suggesting a greater stability of amino acids in the DS fed by biologically treated water. In both cases, levels of amino acids were very low close to the detection limit. In the case of the Ste-Rose DS, these low levels were attributed to biological removal, whereas in the case of the Chomedey DS this was associated with post-chlorination.
No direct correlation between biodegradable organic carbon (BDOC) and total amino acids was observed. This may reflect the fact that total amino acids represent a variable fraction of the total pool of biodegradable organic carbon depending on the source water composition and on the treatment process applied. In the case of the source water studied, total amino acids represented a major fraction of the biodegradable organic carbon pool: more than 42% in raw water and more than 45% in BAC filter effluent. These high proportions may partially be explained by the eutrophic state of our source water and by the method of BDOC measurement used.
In conclusion, amino acids represent an important fraction of biodegradable organic carbon which can be removed most efficiently by optimized coagulation-flocculation and settling, biological treatment and chlorination. It is preferable to remove total amino acids before chlorination in order to limit the formation of undesirable DBPs.
Keywords:
- Amino acids,
- biodegradable organic carbon (BDOC),
- drinking water treatment,
- distribution systems,
- Laval (Québec)