MICROBIAL CHARACTERISTICS OF A DISTRIBUTION-SYSTEM FED WITH NANOFILTERED DRINKING-WATER

This work was designed to study the transition from water treated with O-3 and granular activated carbon to nanofiltered water with special regard to biofilm sloughing and turbidity, and to measure the extent o f biofouling in an ''old'' network fed with non-chlorinated nanofilter ed water. The experimental drinking water distribution system containe d six loops in series, with a total hydraulic residence time (HRT) of 72 h. II was first fed with biologically treated water (GAG water) hav ing organic carbon (1.6 mg-C litre(-1), of which 22% was biodegradable ) and microorganisms (1.12 x 10(5) cells ml(-1), of which 7% was cultu rable after 15 days of incubation). It was then fed with nanofiltered water, which was contaminated with bacteria in transit from the nanofi ltration unit to the network. This nanofiltered water 1 contained 0.23 mg-C litre(-1) (biodegradable fraction < 0.1 mg-C litre(-1)) and 3.0 x 10(4) cells ml(-1) (22% was culturable after 15 days). Finally, the system was fed with nanofiltered water 2 that contained few microorgan isms (50 cells ml(-1)) and few nutrients (0.21 mg-C litre(-1), biodegr adable fraction <0.1 mg-C litre(-1)). The changes in the water and bio film quality in the experimental network fed successively with the thr ee waters, and the water quality during transport through the network, were determined. The bacterial population in the system showed a grad ual drop in the total number of cells and their culturability with the water changes. The total number of cells and the colony forming units (CFU) after 3-15 days decreased respectively about 10-fold and 100-fo ld after 1 year of feeding with nanofiltered water 2 compared to GAC w ater to reach 1.3 x 10(4) cells ml(-1), 2 CFU ml(-1) (after 3 days of incubation at 22 degrees C) and 121 CFU ml(-1) (after 15 days of incub ation at 22 degrees C) in the first loop of the experimental network ( HRT 12 h). No massive biofilm sloughing was observed. The quality of n anofiltered water 2 changed during its transport through the network ( total HRT of the water 72 h) with immediate bacterial contamination, B ut the total number of cells increased slowly and reached 1.4 x 10(5) cells ml(-1) at the end of the system. The number of culturable bacter ia decreased slowly and regularly as a function of the HRT to reach 8 CFU (3 days) ml(-1) and 260 CFU (15 days) ml(-1) at the end of the net work. (C) 1997 Elsevier Science Ltd.