Discolouration loose deposits in distribution systems: composition, behaviour and practical aspects

The occurrence of discoloured water at the tap is one of the most frequent reasons for customers to complain. Water discolouration arises from the resuspension of loose deposits (LD) that accumulate in drinking water pipes, due to velocity increase. Because of its typical brownish/reddish colour, discoloured water is traditionally perceived as a consequence of cast-iron pipes corrosion, even in networks devoid of cast-iron pipes. Current approaches for LD sampling imply the use of high flow velocities within the range of those used for pipe self-cleaning (0.40 m/s) and/or pipe cleaning (1.50 m/s). Although useful for quantitative identification of critical zones and diagnosis of LD accumulation, the application of such high-velocities as routine methodologies for LD sampling may be difficult to perform on-field and provide samples with materials other than discolouration causing LD. In this study, sampling velocities from 0.02 m/s to 0.52 m/s yielded relatively large samples (up to 2 g dry weight), thus allowing for extensive qualitative LD analysis and testing. Sample representativeness was analysed through the characterisation of LD key-constituents total iron (Fe t ) and volatile solids (VS). Turbidity decay trends showed the flocculent behaviour of discolouration causing LD.