Biofilm accumulation in drinking water distribution systems

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Abstract
In order to classify the relative importance of the parameters governing the accumulation of biofilm in drinking water distribution systems, a study has been carried out, using an industrial pilot plant fed with finished waters, with three main objectives, viz. (i) determination of biofilm density on pipe surfaces (PVC or cement lined cast iron) as a function of the distance from the water treatment plant, (ii) evaluation of the growth rate of attached bacteria along the distribution system, and (iii) measurement of the behavior of the dissolved organic matter (DOC) in the water stream for system residence times of 40 to 240 h. Biofilms were found to comprise a complex mixture of microorganisms, viz. fungi, yeasts, protozoa (Bodo sp.), free amoebae (Hartmannela, Vannella, Cochliopodium and Naegleria), and bacteria (up to 106 cells.cm−2), 19% of which were actively respiring as determined by the INT method. The densities of attached bacteria and the proportions of active bacteria decreased slowly along the distribution system and fitted a non‐linear polynomial regression model with six significant terms. The highest growth rate of attached bacteria was observed at the beginning of the experimental network (μ = 0.0017 h−1 or g = 17 d with no residual chlorine at 25°C), i.e. where the system receives the highest flux of biodegradable organic matter. The apparent doubling time of attached cells in drinking water networks might be expected to be generally relatively low (more than 100 d) because of the rapid consumption of biodegradable organic matter (within 40 to 80 h at 25°C when there is no chlorine in the water). In large water distribution systems, far from the water treatment plant, the analysis of drinking water quality gives more circumstantial information on the ecology of attached cells (biofilm) than on water treatment efficiency.