Dual estimation of iron oxide deposition on drinking water PVC pipes using calibrated turbidity data and brightfield microscopy in a full-scale laboratory system

Abstract

[EN] The assessment of accumulated sediments inside drinking water pipes is an important step for determining the risk of water quality deterioration for a sector of a distribution network and for scheduling the required maintenance activities that minimize this risk. Water utilities and researchers have traditionally used turbidity data collected during flushing operations to quantify the discolouration potential in isolated pipe lengths. Flushing has an elevated cost of specialized personnel, consumes large quantities of drinking water, and offers poor information about sediment conditions prior to mobilization (e.g. structure, position on the pipes). The last problem must be overcome by gaining a better understanding of the processes driving material accumulation, which might help in the development of strategies to prevent sediment deposits. In addition, a complex relationship between turbidity and SSC also makes it difficult to accurately translate turbidity units (NTU) into physical units of concentration (e.g. mg/L). This paper aims to consolidate the macroscopic estimations of sediment deposits in drinking water pipes using turbidity data and to propose a microscopic complement that provides richer data about sediment deposits at the pipe wall. The research was developed through a controlled experiment using a full-scale PVC pipe system that mimics the operational conditions of drinking water distribution systems. In the experiments, the drinking water was amended with iron oxide particles that progressively adhered to the pipe walls during 30 days of steady flow conditioning. After the conditioning period, the pipes were flushed to mobilize the sediment deposits. The SSC of water samples collected during the experiments were used to produce translation factors for the online turbidity data. Macroscopic sediment loads were estimated based on the difference between suspended sediments at the inlet and outlet of the pipe loop, while microscopic loads were estimated through the direct observation of particles on pipe wall samples using automated brightfield microscopy. Physical metrics were proposed to adequately represent the sediment load data. Results from the turbidity data analysis produced insights about the impacts of experimental conditions on the SSC translation factors, while microscopy images allowed a detailed assessment of particles deposited on the pipe walls including information about their particle size distribution and dispersion.