Evolution of Amino Acids and Dissolved Organic Matter in a Drinking Water Treatment Plant: Correlations with Biodegradable Dissolved Organic Carbon and Long-Term Chlorine Demand
Dissolved organic compounds (dissolved organic carbon (DOC), biodegradable dissolved organic carbon (BDOC), total amino acids and humic substances) and their chlorine demand were analyzed monthly at different steps of the water treatment plant of Méry-sur-Oise (Paris, France, 270 000 m3/d, Fig. 1). Total dissolved amino acids were determined by HPLC analysis with fluorimetric detection after hydrolysis of combined amino acids (proteins, polypeptides) by hydrochloric acid (DOSSIER BERNE et al., 1994 a); the separation of 17 amino acids was performed after orthophthaldialdehyde (OPA) pre-column derivatization. Humic substances were determined according to the method developed by THURMAN and MALCOLM (1981), by gravity feeding XAD-8 resins with acidified samples. The humic fraction was expressed as the difference between DOC before and after passage through the column. The method developped by JORET and LÉVI (1986) using a biologically active sand inoculum was used for the determination of BDOC. For the determination of chlorine consumption, the kinetic model described by JADAS- HÉCART et al. (1992) was used; it takes into account the long-term chlorine demand in terms of potential demand. This method was automated (DOSSIER BERNE et al., 1994 b) and computer-assisted. The chlorine dose was chosen depending on the DOC value and on the N-NH4+ content of the water; the applied dose was: 3 mg Cl2/mg DOC + 10 mg Cl2/mg N-NH4+. Chlorine determination was performed automatically by the spectrophotometric N, N-diethylphenylene-1,4-diamine (DPD) method. In raw water, DOC values may reach 5.6 to 6.5 mg C l-1 during the cold season, but the average yield of elimination was generally close to 40 % (Fig. 2). The biodegradable fraction of the organic carbon (BDOC), which represents 25 to 50 % of the DOC in the raw water, was partially removed in the plant and the residual concentration in treated water varied between 0.4 to 1.8 mg C l-1 (Fig. 2). A transitory increase in the BDOC values was generally observed during the ozonation step; it reached 0.2 to 0.5 mg C l-1 (Figs. 6 and 7). Whatever the period of the year, extracted humic substances constituted about 50 % of the DOC found in raw water (Fig. 3). This hydrophobic fraction was significantly removed in the treatment plant reducing the proportion of humic substances in the DOC of treated water to 16 - 23 % (Figs. 3, 6 and 7). The concentrations of total dissolved amino acids ranged from 100 to 260 μg l-1 C in raw water and from 50 to 150 μg l-1 C in produced water (Fig. 4); the main part of this elimination occurred during the clarification step (Figs. 6 and 7). No important seasonal variations could be observed for chlorine demand (Fig. 5); its removal occurred at each step of treatment and the average global elimination by the plant was of the order of 70 % (Figs. 6 and 7). On account of the low concentrations of amino acids, no direct relation could be shown between amino acid concentrations and the respective values of BDOC or of chlorine demand potential (Figs. 9 and 10). With regards to BDOC and chlorine demand potential, no correlation could be shown between these two parameters either (Fig. 8). However, it was possible to calculate the contribution of specific classes of dissolved organic compounds to chlorine demand and to BDOC. This calculation is based: i) on the concentrations of humic substances and amino acids measured during the monthly experiments at each point of the treatment plant, ii) on bibliographic data concerning the contribution to BDOC (biodegradability) and chlorine consumption of a large variety of model molecules (free or combined amino acids, aquatic humic substances). The results obtained in this way are reported in Table 3 for the treated water of the M?ry sur Oise plant. These results indicate that the small amounts of total dissolved amino acids present in treated water may account for 5 to 25 % of the BDOC value and for 5 to 23 % of the total chlorine demand potential. With regard to humic substances, their biodegradability was assumed to be very low but their contribution to the chlorine demand of the treated water was estimated between 10 to 40 %, whereas their DOC contribution ranges from 16 to 35 %. As a consequence, both amino acids and humic substances could account for 40 to 60 % to the chlorine demand of treated water. A special point should be noticed for total amino acids: their contribution to the DOC values of treated water was only 2 to 7 %, but they may account for a larger proportion of BDOC or chlorine demand potential (5 to 25 %).