Biofilm and microbial water quality were studied on 4 middle size full-scale distribution systems (DSs) (maximumresidence time from 23 to 160 h) deserving from 5,000 to 30,000 inhabitants. Three samplingcampaigns have been performed over a 1 year period on each DS. Biofilm was studied on castiron coupons incubated for more than one month in devices continuously fed with water fromthe DS. Biofilm was quantified using the potential exoproteolytic analysis method (PEPA)and by total (TDC) and heterotrophic plate counts (HPC) after detachment of the biofilm fromthe support by sonication. Microbiological water quality (TDC and plate counts) wasestimated in parallel. In each DS, large variations of bacterial abundance in water wereobserved: the ratio between minimum and maximum values in a given DS was on average100 for the TDC and 1000 for the cultivable bacteria when all sampling points and campaignswere considered. Very different biofilm densities in each DS were also recorded whatever themethod used to quantify the biofilm. The main factor controlling the biofilm was thedisinfectant residual: when all biofilm data were considered, microbial counts tended tofollow a logarithmic increase as the disinfectant residual decreased. A multiple linearregression confirmed the impact of disinfectant residual on biofilm and also showedsignificant effects on cultivable bacteria concentration in water. The maximum level ofbiofilm measured during the present study by the PEPA method was higher than thoserecently recorded in large full-scale DSs in Paris and Brussels. In order to study the impact ofwater residence times, 4 groups (0-20h; 20-40h; 40-80h; 80-160h) were considered. A bellsharpcurve was observed in water for each microbial method whereas the bacterialabundance in the biofilm tended to increase up to a maximum for the residence time class 80-160h. The impact of temperature on microbial counts in water and biofilm was measurablebut quite limited. Includes 12 references, tables, figures.
Keywords: Biofilm; Water Quality; Disinfection Residuals; Distribution Systems; Paris, France; Heterotrophic Plate Count
