Initial stages of particulate iron oxide attachment on drinking water PVC pipes characterized by turbidity data and brightfield microscopy from a full-scale laboratory

Iron oxide particles produced by corroding metallic pipes in drinking water networks often accumulate and form material deposits on the wall of downstream polymeric pipes. The aim of this paper is to isolate and examine the physical processes of attachment and subsequent mobilization of iron oxide particles on new PVC pipes in a full-scale laboratory operated under hydraulic conditions that are representative of actual drinking water distribution systems. Particle accumulation was quantified using: 1) a mass balance approach based on the transformation of turbidity data into iron oxide particle concentration through calibration curves, and 2) the direct observation of particles accumulated on coupon samples with automated brightfield microscopy. Results showed that: 1) the iron oxide particle concentration in the bulk water had a major impact on the extent of material deposits observed on the pipe wall; 2) gravity greatly assisted in the attachment stage of fine iron oxide particles (1-10 mu m) in the invert position of the pipe. The experimental data led to the hypotheses that the fine particles were seated in the pipe wall roughness "valleys" which protected them from the flow shear force and might explain the different shear strength of material deposits.