Field investigation of discolouration material accumulation rates in live drinking Water distribution systems.

Discolouration is the biggest cause of aesthetic customer contacts in the water industry. This project was designed to develop new insight and understanding into discolouration, by investigating the impact ofdifferent factors on discolouration material accumulation rates and the effectiveness of traditionaJly used mains cleaning and rehabilitation methods, through detailed studies of live distribution systems. Analysis ofdiscolouration customer contacts and burst incidents covering a five year period for two water companies and pipe asset data base for a water company's entire region, at the District Meter Area (DMA) level, shows little correlation between the number ofdiscolouration events and pipe properties, indicating complex inter relationships between a number offactors. Discolouration accumulation rates were investigated through repeat full zonal flushing in two DMAs. Discolouration material was seen to accumulate at the same rate in all areas ofthe networks, as a factor ofwater quality, until equilibrium was reached between accumulation and erosion rates. This equilibrium was as a factor ofdaily conditioning shear stress, whereby pipes affected by a low daily conditioning shear stress continued to accumulate material longer than pipes with a higher daily conditioning shear stress and thicker layers ofdiscolouration material were fonned. Long tenn turbidity monitoring in five DMAs has showed that the amplitude ofa daily turbidity cycle, based on resuspension and corrosion processes, can be used to assess the effectiveness of network rehabilitation. A greater improvement in the reduction ofdiscolouration potential was seen in the full zonal flush DMAs, than in DMAs that were rehabilitated under the Distribution Operation and Maintenance Strategy (DOMS), at far greater cost. Using the principles of change in shear the methodology for a simple modelling tool was designed and field tested to predict the discolouration response to valve movements, to enable valve operations to be managed to reduce discolouration risk.