Influence of climate and potabilization treatments upon the oxydability of waters at the polishing stage
Oxidizability by KMnO4 in a hot acidic medium was used for 8 months to estimate organic matter in clarified-ozonized then GAC-filtered waters, treated on a pilot plant (Figure 1). Flowrate of the River Seine (DEB), temperature (TEM), pH (PHE) and oxidizability of the waters before GAC-filtration (OXE) are shown to vary with time (Figures 2 to 5). The first two parameters are generally used to classify raw waters but here they are not enough (Figure 6). Then, multivariate statistical analyses - principal component analysis and discriminant analysis - are carried out on 129 samples : they show (Figure 7) that two parameters, one in connection with climate only - temperature - and the other in connection with both climate and treatments - pH - are able to classify these waters into three groups equivalent to three seasons; the error coefficient is of 6 % only. Seasonal characteristics of these waters are given in Table 1. Average values of oxidizability (OXE) do not vary with the seasons, but standard-deviation is the greatest in Spring. There is no apparent connection between the different treatments - preozonation level (PO3), quantity of flocculant added (WAC), land-filtration velocity (VFS), ozonation level (O3F), residual ozone level (O3R), GAC-filtration flowrate (DAL) - except between the two ozonation treatments (Figure 3). Their seasonal variations are given in Table 2. Variance analyses show that the 3 factors, quantity of floccutant (WAC), sand-filtration velocity (VFS) and absence of preozonation (PO3), have no effect upon the oxidizability of the water supptying the GAC-filter (OXE). This parameter is a function of preozonation (PO3), interozonation (O3F) and residual ozone levels (O3R) and sequency (SEQ) : the models achieved, determined by linear multiple regression, account for less than 30 % of the variance (Figure 9). The GAC-filtration at 5 m.h-1 diminishes the oxidizability values of zero to 0.8 mg O2l-1 and their variability; however, some releases appear at the end of Spring, during the re-equilibration phase of the filter (Figure 10). By linear multiple regression, we can express the oxidizability of the filtered waters (OXS) as a function of (OXE) and temperature (TEM). The models achieved differ from one season to another (Figure 11) and account for 32 to 73 % of the variance; thus the working of the GAC-filter is dependent on the seasons. Many authors assume that the risk of bacterial regrowth in the distribution network is minimal when the oxidizability of the treated waters is minimal. Here, the oxidizability of the filtered waters (OXS) is at its lowest when temperature (TEM) and (OXE) are lowest (Figures 12 and 13). These conditions are often met in the middle of the seasons, when the quantity and the quality of the organic matter vary little; the fitter is then balanced. The quality of a treated water therefore varies with the seasons, despite polishing obtained through treatments applied upstream, which tend to improve it and diminish its variability.