How Effective Is Flushing of Cast Iron Pipes?

Discolouration incidents in the drinking water distribution networks are caused by the resuspension of loose particles. Removing particles will consequently reduce the discolouration risk. Unidirectional water flushing can be an effective method when strict boundary conditions are met like a minimum velocity of 1.5 m/s, flushed volume 2 to 3 times the content of the pipe and using a clear water front. Supposedly adverse effects of cleaning of cast iron pipes are that the protective layer of corrosion products can be damaged leading to increased particle production post cleaning. The Resuspension Potential Measurement gives unbiased and reproducible information on the discolouration risk. This methodology is used to assess the efficacy of the flushing program of the Water Company of Limburg in the Netherlands (WML). The program is followed with an extensive measuring campaign involving 701 RPM measurements at more than 120 locations. The adverse effects of cleaning of ferrous pipes were observed in the period immediately following the cleaning. This effect was neutralized within a year which led to the conclusion that water flushing is practically as effective in ferrous pipes as it is in non-ferrous pipes provided that the operational boundaries are met on velocity, volume and order of flushing.