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
182 HIDROLÓGIAI K .ÖZL ÓNY 1999. 79. ÉVF 3. SZ. zed rapidly, within one day (Haudidier-Boraert, 1989; Block et al., 1995). Biofilm accumulation is the net result of the following physical, chemical, and biological processes (Characklis and Marshall, 1990): - Substratum conditioning by the adsorption of organic molecules; - The transport of bacteria (active and dead) to the substratum by convective and diffusive transport; - Adsorption of bacteria, which can be - reversible physicochemical adsorption, and - írreversible adsorption, which is considered a permanent bonding to the substratum, frequently mediated by extracellular polymenc substances (EPS); Transport could be the rate limiting step of bactenal colonization in oligotrophic environment. Mueller (1996) found that under very low nutrient conditions where starvation occurs, the adsorption characteristics of a cell can be sigmficantly different from healthy cells of the same species. Cellular motility increased for starved bacteria, resulting in an increased transport efficiency to the substratum. Theoretically, if the time required for cell adhesion to the pipe surface is greater than the cell's mean residence time, the cell is washed away prior to attachment. Wolfaardt et al. (1992) examined bactenal colonization rates in continous flow reactors under various rotation speeds simulatmg water velocities (0-6 m/s). Their results show that water velocities had an effect on colonization rates, an increase of flow rates causing a decrease of colonization rates. Although it is to be mentioned, that in the distn- The metabolism of the irreversibly adsorbed active bacteria begins, resulting in bactenal growth and product (EPS) formation; - Biofilm growth is continuing, balanced by biofilm detachment. Biofilm detachment is the key factor which - balances microbial growth hence governs the steady state biofilm condition, and - responsible for the increase of suspended bactena in the network. The major processes and influencing factors of biofilm kinetics are shown in Figure I. Nutnent conditions, temperature, chlonne residuals and pipe material alsó could influence initial processes of bactenal colonization. bution system the velocity rangé is much lower (0-1 m/s), consequently its possible effect on colonization rates should be much lower, if there is any. Substratum matenal does not have a sigmficant effect on steady-state biofilm growth (Haudidier-Bornert, 1989), unless the material is growth-promoting, such as plasticized PVC (van der Kooij and Veenendaal, 1994). In initial biofilm processes pipe material does have an effect on the adsorption of bactena (Mózes and Rouxhet, 1992). This influence on initial surface colonization rate can be entitled to surface electric potential (Gillis and Gillis, 1996), and to surface roughness (Percival et al., 1998). Furthermore, certain bacteria tend to attach in larger numbers to hydrophobic than to hydrophilic surfaces, for others the opposite is true (Characklis and Marshall, 1990). flow velocity residence time chlorine BDOC bacteria chlorine BDOC bacteria Figure 1. Major processes of biofilm kinetics
