The role of trunk mains in discolouration

Understanding and managing the causes of drinking water discolouration are key challenges for the worldwide water industry. Discolouration is a major cause of customer contacts, undermining consumer confidence. Despite significant investment in the UK in the last 15 years in replacing/lining cast iron pipes and improving water treatment processes, discolouration continues to be an issue. Evidence is presented suggesting that trunk mains, which have largely been ignored in terms of rehabilitation and maintenance strategies for controlling discolouration, play an important role in discolouration. Three main datasets are presented to evidence this: discolouration-related customer contact data, analysed utilising a novel cluster procedure; results from distribution network flushing studies pre- and post-cleaning of the supplying trunk main; and associated long-term trunk main turbidity and flow data. It is shown that a significant proportion of discolouration events affect multiple areas simultaneously, that trunk mains can act as a store of material, which can migrate downstream seeding the distribution systems and that the presence of material layers on the trunk main pipe surfaces demonstrates an inherent discolouration risk. Overall the results suggest that if discolouration is to be addressed, then any effective strategy must include the risk presented by trunk mains.