Calibrating and validating a combined accumulation and mobilisation model for water distribution system discolouration using particle swarm optimisation

Abstract

A discolouration concept is proposed describing simultaneous pipe wall material accumulation and mobilisation processes that define discolouration in drinking water distribution systems, one of the biggest causes of customer dissatisfaction. Validation of these mathematical forms is presented. The model formulation was shown to maintain the mobilisation functionality of previous validated shear-stress-dependant modelling tool, but requiring only two empirical parameters. Two distinct operational datasets are then analysed and robust empirical model parameter calibration is obtained utilising a refined particle swarm optimisation technique. The model is shown to make usefully accurate simulations for flow mediated events, providing evidence of predictive capabilities. The combined tracking of accumulation and mobilisation behaviour enables assessment of the current and future discolouration risk posed by any pipe irrespective of age or material, allowing pro-active, risk based planning and prioritisation of maintenance interventions to protect the quality of delivered water.

Simulating long term discolouration behaviour in large diameter trunk mains

“…Similar conclusions were suggested from previous studies. 35 The accumulation period parameter value of 1.65-1.85 years found here for trunk mains is comparable with that previously for pipes in reticulation or local distribution systems, which found around 1.5 to 2 years in UK systems with cast iron pipes 15 and within 1.5 years in a network in Netherland. 18 Both these studies used calculations from turbidity data to estimate the accumulation period, rather than via model fitting as reported here.…”

“…34 The VCDM simulates discolouration using two validated mobilisation parameters (βe and α; a rate term and a scaling factor respectively), initially tested through synthetic data and then successfully calibrated for small diameter pipes and a single trunk main. 35 The calibration results demonstrated that the VCDM retains the mobilisation functionality of the PODDS model, with comparable quality of fit to measured data. A third parameter is a linear accumulation rate.…”

“…The values found are also comparable with previous VCDM modelling assessment where values were found between 0.5–5.0 years depending on water source and pipe materials. 34,35 However, these previous studies have limited repeated turbidity mobilisation events and hence low confidence. The ability to determine an accumulation period, or transition to maximum risk, by fitting to flow and turbidity data highlights significant potential for this modelling approach as this offers the possibility to assess and quantify network performance and discolouration risk with respect to water quality.…”

“…Model calibration was undertaken by manually tuning the three parameters (βr, βe, α) with initial values extrapolated from previous findings. 35 Manual calibration enabled understanding of the effects of the parameters on simulation behaviour. The measured treated water turbidity was used to confirm that there were no turbidity events originating from the WTW that could be erroneously attributed to the pipe network with the result incoming turbidity was set as zero in all simulations.…”