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INTRODUCTION
Despite its previous treatment and filtration, potable water in water distribution systems (WDSs) accumulates particles due to various mechanisms, such as corrosion of iron-based pipes and equipment, precipitation of dissolved compounds or introduction of exogenous material (Gauthier 1998; Vreeburg & Boxall 2007). Sedimented particles offer a protection against disinfectants, which favour bacterial proliferation and thus may aggravate corrosion, generate taste and flavour or allow the development of pathogenic species. Particles also accumulate toxic compounds such as heavy metals and organic contaminants (De Rosa 1993; Gauthier 1998). Changes in water flow may resuspend sedimented particles and generate water discolouration events, which are the main cause of customer complaints, and could also expose customers to released contaminants (Hasit 2004). Additionally, WDSs commonly develop incrustations which are a problematic source of energy loss. As an example, iron tubercles are prevalent in the province of Quebec (Canada) where water pipes are commonly composed of cast iron (36% grey cat iron and 32% ductile iron; CERIU 2017). Tubercles can grow to several centimetres in thickness and thus induce very significant pressure losses, generating complaints from customers and firefighters due to low water pressure (Ellison 2003; Sarin et al. 2004).
Several cleaning methods have been developed to fight particle accumulation and incrustation growth, such as unidirectional flushing (UDF), air scouring (AS), swabbing, pigging or chemical methods. The most appropriate cleaning method to apply is generally selected based on the objective of the cleaning, e.g. preventing discolouration events, removing tubercles before rehabilitation or dislocating the biofilm. UDF and AS, which restore clean water (Ellison 2003; Vitanage et al. 2004) and thus prevent discolouration events, are less invasive methods than swabbing, pigging and chemical methods. They are similar in their set-up, where a hydrant (this could be several for UDF) is opened to accelerate the water within the pipe section desired for cleaning. Over time, incrustations and sedimented particles are flushed away due to the increased shear stress resulting from these higher water velocities. For sections in looped areas of WDSs, some valves are closed to direct water to an opened hydrant from a single direction. For AS sequences, water flow is reduced by partially closing an upstream valve, then compressed and filtered air is injected through an upstream hydrant....