Pilot studies on discolouration loose deposits' build-up

Abstract

Tap water discolouration occurs due to resuspension of loose deposits (LD) that accumulate in drinking water distribution systems. Strategies for discolouration control involve network pipe cleaning and replacement of cast-iron pipes. However, the sole application of such measures is not generally effective. Therefore, a deeper understanding on processes associated with LD origin and development is required. Transparent (but covered) test rig pipes (Ø = 23 mm) continuously supplied with drinking water at steady flow through mode (7–9 months) were used to investigate LD build-up under laminar flow (0.2–4.1 cm/s) conditions. Rather than continuously and homogeneously, LD developed as spots, and predominantly at the pipes' bottom. Results suggested that particle attachment may be added to settling as a LD build-up process. This is consistent with LD cohesive–adhesive properties, as shown by LD repose angle tests, as well as to the occurrence of flocculent extracellular polymeric substances-EPS in LD.

Keywords::

• discolouration
• drinking water
• loose deposits build-up
• pilot studies

Acknowledgements

The authors are grateful to Vítor Napier, João Vale, Paula Freire, Cristina Ramalho, Bruno Cordeiro and Dália Loureiro for their valuable technical support.

Funding

This work was funded by the Portuguese Foundation for Science and Technology (FCT) PTDC/ECM/108261/2008 and SFRH/BD/43715/2008 project and doctoral grants, respectively.