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
184 HIDROLÓGIA] KÖZLÖNY 1999. 79. ÉVF. 3. SZ. most widely used. Fransolet (1985) found, that even in dnnking water the time required for a proliferation which generally may result in 5 log-6 log bacteria/ml is relatively short. It indicates, that growth rate can be high even in an oligotrophic environment. In dnnking water BDOC is present in complex mixtures of carbon compounds for the growth of heterotrophic bacteria. A recently developed mixed substrate model (Lendenmann and Egli, 1998) imply that under carbon limited conditions HPC bacteria which are able to utilize a wide rangé of the carbon compounds can grow faster than those which exhibit a narrow substrate spectrum. At low BDOC concentrations, when growth is nutrient limited, bacteria utilize mixed substrates simultaneously. At high concentrations, when proliferation is growth limited, only the substrate supporting the highest growth rate is utilized. Taking into account that bacterial regrowth phenomena in the distribution system is due to biofilm activity, and that biofilm evolution is mainly because of growth, the amount of BDOC in the drinkmg water has a pnmary influence on the bacteriological water quality changes. In 1984 Rittman and Snoeyink introduced the concept of biologically stable drinking water According to the authors, a biologically stable drinking water does not support the growth of microorganisms to a significant extent Drinking water could be stabilized by the elimination of the limiting substrate components, the BDOC Nutrient levels in advanced high punty water systems are sufficient to permit microbial growth and reproduction to a troublesome extent (Husted et al., 1994). In highpurity industrial water systems, extremely low TOC levels (TOC<0.03 mg/l) could not eliminate biofilm formation and bacterial regrowth (Collentro, 1995; Gillis and Gillis, 1996). In dnnking water, the results of Sibille et al. (1997) alsó indicate, that there can be no measurable organic matter concentration that prohibits bacterial regrowth in the network. Consequently, the biological stability in drinking water does not mean the lack of bacterial activity, but refers to a relative stability of the system. Laurent et al. (1993) charactenze this stability with no measurable BDOC consumption and very limited bactenal growth. This stability could be achieved by producing dnnking water with very low BDOC concentration. Up to the present day no mformation can be found in the literature how exactly the biofilm reactor responds to the changes of BDOC concentration. The kinetics of biofilm under dynamic substrate conditions, the time required for its stabilization is not known. 5.2. Temperature In distribution systems significant temperature changes may occur due to seasonal variations. In addition, the distribution of water may cause change in water temperature in relation with stagnation (residence time) in main pipes and particularly in service lines (Prévost et al., 1997). Fransolet (1985) examined the effect of temperature on bactenal growth in drinking water, in batch conditions. He found, that temperature had an important influence on bactenal growth, bacterial activity increased significantly above 15°C. The effect of temperature can be mostly associated with an increase in the maximum growth rate (|i ma x), according to a sigmoidal relationship (Rosso et al., 1995). Temperature alsó effects lag phase of bactenal growth, from attachment to growth (Fransolet et al., 1985). Consequently, in low temperature water bacterial colonization may be very slow due to the wash down of attached bacteria. Using a pilot system, Lund et al. (1995) showed a close association with water temperature and attached biomass on pipe walls (expressed in dry weight). In winter penod with water temperature below 5°C bacterial proliferation significantly decreased. Using a pipe ng pilot system, Holdén et al. (1995) showed, that attached biomass and HPC counts in the bulk correlated with the temperature. AOC values in the water decreased by increasing temperature, showing that increased bactenal activity results in increased carbon uptake. Donlan et al. (1994) examined biofilm formation rates on cast irón substratum in drinking water distribution systems. They found that seasonal factors were the primary limiting factors for net cell accumulation, which was accounted to temperature. (It has to be mentioned although, that substrate concentration was not measured in the experiment.) The sampling campaign on the distribution system of Cholet (Heraud et al., 1997) showed, that a temperature rise from 16°C to 18°C induced a significant (1 log) increase of bactena at the sampling sites, at a low postchlonnation rate in the network. In the distribution system of Metz the higher bacterial activity at summer could be associated not only with the higher temperature, but the higher substrate concentrations of the treated surface water (Mathieu et al., 1995). Kerneis et al. (1995) alsó noticed seasonal temperature and BDOC changes in the distribution system They propose that BDOC threshold has to be associated with water temperature. For high water temperatures the threshold is much lower than for water in winter or early spring, when high enough levels of BDOC cannot be biodegraded due to the low bacterial activity in the network. Based on these results, a notable interaction can be assumed between drinking water temperature and BDOC concentration on bacterial activity in the network. 5.3. Inlet bacteria Piriou et al. (1997) examined the influence of inlet bacteria concentration on biofilm behavior using a pipe loop pilot The results indicate, that the inlet totál bactena has a positive influence on biofilm evolution, but the inlet active bacteria has no impact on biofilm activity. They conclude, that bactena deposition has little effect on biofilm evolution under steady state condition As described in Chapter 3., bactenal concentrations in the bulk has an effect on the adsorption process. Consequently, under dynamic conditions inlet totál and active
