Report No DWI0401
EVOLUTION OF DRINKING WATER QUALITY IN DISTRIBUTION
Biodegradable Organic Hatter and Biological Stability
THE FRENCH EXPERIENCE
The quality of the drinking water leaving the treatment plants is generally high, according to the current standards, but deteriorates in distribution. Studies carried out in the last decade have demonstrated that the deterioration of the treated waters microbiological characteristics in distribution is due to the formation of a biofilm on the surface of the pipes. The control of the distributed water quality thus necessarily requires the control of the biofilm growth.
A lot of work has been done to endeavour to understand the biofilm formation, to determine its effects on the water quality in distribution and to investigate means of controlling its negative effects. Part of this considerable work has been carried out by French Research Teams, including Universities, as well as private Water Companies. These Research Teams were met in May - June 1992.
The studies carried out on real or pilot distribution systems showed that the residual oxidant (chlorine or chloramines) present in the water leaving the treatment plant at levels used in drinking water production cannot be sufficient to control the biofilm growth.
The Biodegradable Organic Matter (BOM) entering the distribution system was identified as a key parameter of the biofilm growth. This growth can thus be slowed down by the removal of the BOM present in treated water. Several methods have been developed to assess the concentration of BOM in water. The BDOC methods (measuring the Biodegradable Dissolved Organic Carbon) seem to be more relevant than the AOC methods (Assimilable Organic Carbon). Furthermore, the BDOC method developed by Dr Joret and Dr Levi is the more convenient at the present time. It was used to assess treatment efficiencies to remove the BOM and to observe the evolution of the BOM in distribution.
Mathematical models are being elaborated to explain the removal of the BOM in distribution. These models rely on chemical, biological and physical parameters including the retention time and the velocity of the water in the pipes. A predictive model of coliform occurrences is also being developed.
If the water quality in distribution can be maintained partly by adequate hydraulic conditions and by good maintenance of the mains, efforts have to be made to remove the BOM at the treatment plant. It was found that, in the absence of residual oxidant, a BDOC as low as 0.1 mg C/l can lead to the growth of the biofilm.
The biological stability of the drinking water produced can be improved by the optimisation of the existing classical treatment mainly coagulation/ flocculation) and by the use of new treatment processes such as an ozonation followed by a biological filtration. The lowering of the BOM also reduces the negative effects due to the subsequent use of chlorine in post-disinfection.
The density of micro-organisms in the biofilm is also linked to the amount of micro-organisms entering the distribution system. The nanofiltration, still in development, is a promising process as it is able to remove completely both the organic matter and the micro-organisms of the clarified water. This process is to replace both the ozonation, granular activated carbon filtration and final disinfection.Copies of this report may be available as an Acrobat pdf download under the 'Pre 2000 Reports' heading on the DWI website.