Discolouration loose deposits: balancing views and practices

Abstract

Tap water discolouration incidents may occur due to resuspension of loose deposits (LD) that accumulate in drinking water distribution systems (DWDS), due to velocity increases or rapid changes. While lowering the consumers’ confidence in tap water’s safety, they are often a cause of customer complaints, especially when recurrent. Owing to the lack of knowledge on LD dynamics and build-up processes in DWDS, however, LD control and prevention strategies are still ineffective to counteract LD accumulation and prevent tap water discolouration incidents. In this paper, supported by relevant literature and results from a four year study on LD sampling and characterisation, a conceptual model for LD dynamics and build-up processes in DWDS is proposed. The model description together with the expected LD build-up rates were used to interpret discolouration phenomena. The model’s practical implications and recommendations were also discussed, including the potential strategies for LD control and prevention in DWDS. Overall, it was concluded that LD accumulation is most critical at mid-range velocities and that, under this range, it may occur exponentially in DWDS. The possible changes in the relative composition of LDs occurring over accumulation were studied and related with LD resuspension potentials, along with LD ageing and residence times.