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

181 Bacterial regrowth phenomena in the drinking water distribution system Bacterial water quality deterioration in the drinking water distribution system is due to biofilm activity. Biofilm kinetics is inlluenced 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 chlonne residuals, the amount of active attached bacteria decreases in residence time due to the kinetics of BDOC consumption. Free active bacteria concentrations change in residence time only in the mitial transitory period, the asymptote is determined by BDOC concentration and temperature Bacterial drinking water quality deterioration can be controlled by decreasmg BDOC concentration or maintainmg a closely constant chlorine concentration in the distribution system. drinking water, distribution system, biofilm, bacteria, regrowth, BDOC. Gábor Dombay Technical University of Budapest, Department of Water and Wastewater Engineering H-llll. Budapest, Műegyetem rkp. 3. Abstract: Keywords: 1. Introduction Drinking water quality changes in the distribution system, in space and in time. Water quality changes in the network are accounted to the activity of the distribution system as a complex physicochemical and biological reactor. The major phenomena can be charactenzed as - deterioration of organoleptic parameters (taste and odor, turbidity, color), - deterioration of bactenological parameters, - nitrification, - corrosion (chemical, electrochemical and microbial), and - deposition and scaling Of the various causes of quality deterioration in distribution systems, microbiological parameters are the most closely studied and monitored, because of the short-term risks regarding to public health. Public health risk of the drinking water could be associated with the presence of coliform bacteria. Several research showed, that coliform occurrence in the distribution system cannot be only accounted to an accidental contamination, but to coliform regrowth in the network as well (Geldreich and Rice, 1987; LeChevallier, 1990; Fass et al., 1996). Although the absence of coliforms does not necessarily indicate an adequate bacteriological water quality. Certain members of heterotrophic bacteria (HPC counts) can be considered opportunistic pathogens, and might contribute to gastrointestinal illness (Payment et al., 1993, 1994). Even if high HPC counts do not necessarily give rise to a health nsk, they are the signature of a network in which undesirable microbial water quality changes may occur. Substantial bacterial activity in the network might promote the presence of higher organisms (fungi, yeast, protozoa, microzooplankton, invertebrates) in the network, resulting fiirther water quality deterioration (Levy et al., 1986; Fass et al., 1996). Taste and odor problems can be a result of bacterial activity in the network (Burlingame and Anselme, 1995), or can be accounted to chlorine residuals (Duguet et al., 1995; Welte and Montiéi, 1997). In order to limit public health nsk in the drinking water, bacterial regrowth in the network has to be controlled. Due the growmg public health concern and expectations present study focuses on bacterial dynamics in drinking water distribution systems. 2. The distribution system as a biofilm reactor Bactenological water quality detenoration is mainly accounted to the biofilm activity in the distribution system (LeChevallier, 1990). In the distnbution system bulk water phase represents only a small fragment of the bacterial activity (Herson et al., 1991). In the network bacteria colonizes surfaces, this attached biomass is often referred as biofilm. In biofilms bacteria is embedded in an extracellular polymer mátrix, forming an inhomogeneous layer on the substratum. Consequently, a biofilm can be viewed as a polymer gel with organisms dispersed within it. Generally the biofilm is spatially heterogeneous, i.e. components within the gel are not necessarily uniformly distibuted (Characklis and Marshall, 1990). In drinking water mains, in case of a surface colonization of 10

7 bacteria/cm

2 and 10 bactena/ml in the bulk phase (Maul et al., 1991), the suspended fraction represents only 5% of the attached biomass in case of a 200 mm diameter pipe, and 0.5% for a 100 mm pipe. From the reactor engineenng view, the behavior of the biofilm reactor is influenced by - hydraulic conditions, determined by properties and operation of the distribution system, and - water quality charactenstics, determined by environmental parameters (raw water quality) and treatment technology. 3. The process of biofilm accumulation In oligotrophic environment surface colonization might be used as a strategic survival mechanism of organisms (Mueller, 1996). Attached to the substratum, bactena is exposed to an increased substrate flux, consequently it has a better availability to nutrients. The initial process of biofilm accumulation is the surface colonization by bacteria. Surfaces exposed to bacterial flux in the drinking water distribution system are coloni-