Hidrológiai Közlöny 1999 (79. évfolyam)
3. szám - Dombay Gábor: Bacterial regrowth phenomena in the drinking water distribution system. A bakteriális vízminőségromlás jelensége az ivóvízelosztó hálózatban
186 HIDROLÓGIAI K .Ö ZLÓNY 1999 . 79. ÉVF 3. SZ. ter. Applying the same low BDOC concentration with higher temperature (3) results in a 4 log increment in fixed active bacteria, consequently there is a significant interaction between substrate concentration and temperature. Applying high BDOC concentration the effect of the temperature changes m residence time: Although higher temperature in (4) initially results in more substantial fixed active biomass, in the course of residence time the Bib of (2) is higher. This virtual paradox can be explained by Figure 3. Higher temperature of (4) induces a very rapid BDOC consumption, resulting lower amount of active biofilm for higher residence time values. Figure 4. Active suspended bacteria Free active bacteria (XH) in the bulk is shown in Figure 4. In case of ideally stable water, (1), XH does not change in residence time. When the active biofilm is more substantial due to higher temperature, (3), the free active bacteria counts increase by 2 log. Applying high BDOC concentrations the increased biofilm activity results in a 4 log increment of XH. As it can be seen, the temperature difference between (2) and (4) does not induce a substantial difference in suspended active bacteria. In all cases residence time effects free active bactena only in the initial transient period, after XH stabilizes. This indicates, that under steady-state conditions, in the absence of chlorine residuals residence time does not correlate with active bactena in the bulk. Consequently residence time cannot be used as a single indicator parameter of bacterial water quality in the distnbution system. The asymptote of the XH curve is determined by water quality parameters (temperature and substrate concentration). 1 2 3 4 r«BU«nc* ima day Figure 5. Totál attached bacteria The evolution of totál attached bacteria (B, the totál amount of the biofilm) and free totál bacteria (X) can be studied by Figure 5-6. The curves have similar characteristics as of the active fractions, indicating that totál biomass alsó depends on growth. (O CTl CM CM r*«kíenc* time, day Figure 6. Totál suspended bacteria 7. Summary Bacterial water quality deterioration in the drinking water distribution system is due to biofilm activity. Biofilm kinetics is influenced by hydraulic and water quality parameters. The distribution system is a complex biofilm reactor of which behavior can be studied by the use of biofilm models. Model applications showed, that under steady-state conditions, in the absence of chlorine residuals, free active bacteria concentrations change in residence time only in the initial transitory period. The extent of bactenal water quality deterioration is determined by water quality parameters. Biofilm activity is primarily growth driven, depending on the actual BDOC concentration and temperature of the drinking water. Consequently, bacterial activity in the network can be reduced by substrate removal (ozone + activated carbon; nanofiltration). Maintaining an approximately constant chlorine concentration in the distribution system (by post-chlorination) can successfully mitigate active bacteria in the water, but does not prevent biofilm activity in the system. The application of biofilm-models in real distribution systems, under dynamic conditions can be considered the actual research goal of the fortheoming years. 8. References Anderson, W. A., Huck, P. M., Slawson, R. M. & Camper, A. K. (1997). BOM component evolution during drinking water treatment and distribution. AWWA Annual Conference, June 15-19, Atlanta, Georgia Bailey, I. W. & Thompson, P. (1995). Monitoring of water quality after disinfection in distribution systems. Water Suppty, 13(2), 35-48. Bishop, P L., Gibbs, J. T. & Cunmgham, B. E. (1997). Relationship between concentration and hydrodynamic boundary layer. Environmental Technology, 18, 375-386. Block, J. C., Bois, F., Reasoner, D. J., Dutang, M., Mathieu, L., PAquin, J. L. & Mailliard, J. (1995). Disinfection of a drinking water distribution system. Water Supply, 13(2), 1-11. Block, J. C., Mathieu, L., Servais, P„ Fontvieille, D. & Wemer, P. (1992). Indigenous bacterial inocula for measuring the biodegradable dissolved organic carbon (BDOC) in waters. Water Research, 26(4), 481-486. Bois, F. Y., Fahmy, T., Block, J.-C. & Gatel, D (1997). Dynamic modeling of bacteria in a pilot drinking-water distribution system. Water Research, 31(12), 3146-3156. Buffle, J., Delanoey, P„ Zumstein, J. & Haerdi, W. (1982). Analysis and characterisation of natural organic matters in freshwater-I : study of analytical techniques. Scheiz. Z. Hydrol, 44, 325-362. Burlingame, G. A & Anselme, C. (1995). Distribution system tastes and odors. In Advcmces in Taste-and-Odor Treatment and Control, eds. I. H. Suffet, J. Mallevialle & E. Kawczynski, AWWA. Denver, CO,pp. 281-319.
