S. Husband

Field Studies of Discoloration in Water Distribution Systems: Model Verification and Practical Implications

Discoloration in water distribution systems has been studied in partnership with a number of U.K. water companies by measuring the turbidity response to changes in hydraulic conditions induced by systematic flushing. The resulting data was used to verify a predictive empirical model and hence the underlying assumptions made in its development. Model simulations, made using previously established parameters defined solely by pipe diameter and pipe material, are presented alongside measured data to demonstrate this verification. The primary cause of discoloration observed is the mobilization of material from cohesive layers bonded to pipe walls. These layers demonstrate a profile of increasing shear strength with increasing degree of discoloration. Differences are demonstrated in the layer and ultimate shear strength characteristics of the discoloration layers formed in iron and plastic pipes, with a modeled shear stress of 1.2 N/ m2 shown to exhaust material layers in plastic pipes. Based on the observed d...

DISCOLOURATION RISK MANAGEMENT FOR TRUNK MAINS

Results are presented from operations undertaken to clean a 13.3 km long 700 mm (27”) diameter lined steel trunk main. Since its commission a decade earlier this strategic reserve main had been operated with only a ‘trickle’ flow of 15 l/s (velocity of 0.04 m/s). Analysis by grab samples and internal inspection suggested significant material accumulation with the potential for a discolouration event should the main be called into service in its present condition. Incremental flushing, as a means of raising the boundary forces (shear stress) to condition the pipe to expected peak demand of 500 l/s (1.35 m/s), was selected as a rapid and simple technique to maintain asset integrity, minimise discolouration risk, and gain an understanding of the material layers present within the pipe. This paper details the ‘enhanced conditioning’ strategy adopted and presents results showing turbidity increasing for every rise in hydraulic loading. This supports the PODDS (Prediction of Discolouration in Distribution Systems) concepts, verified in smaller distribution pipes, that material layers develop throughout water supply networks exhibiting variable layer strength characteristics and that the current layer strength is conditioned by imposed flow. Calibration of the PODDS model, coded as a water quality element into EPANET, to measured data confirms the application of shear stress based analysis for larger diameter mains. Modelling simulations enable evaluation of operational strategies that predict discolouration response to flow increases in this main. Overall the trial demonstrates that discolouration risk may be managed in trunk mains and provides operators with the understanding and tools to facilitate this.

Dynamics of Biofilm Regrowth in Drinking Water Distribution Systems

ABSTRACT The majority of biomass within water distribution systems is in the form of attached biofilm. This is known to be central to drinking water quality degradation following treatment, yet little understanding of the dynamics of these highly heterogeneous communities exists. This paper presents original information on such dynamics, with findings demonstrating patterns of material accumulation, seasonality, and influential factors. Rigorous flushing operations repeated over a 1-year period on an operational chlorinated system in the United Kingdom are presented here. Intensive monitoring and sampling were undertaken, including time-series turbidity and detailed microbial analysis using 16S rRNA Illumina MiSeq sequencing. The results show that bacterial dynamics were influenced by differences in the supplied water and by the material remaining attached to the pipe wall following flushing. Turbidity, metals, and phosphate were the main factors correlated with the distribution of bacteria in the samples. Coupled with the lack of inhibition of biofilm development due to residual chlorine, this suggests that limiting inorganic nutrients, rather than organic carbon, might be a viable component in treatment strategies to manage biofilms. The research also showed that repeat flushing exerted beneficial selective pressure, giving another reason for flushing being a viable advantageous biofilm management option. This work advances our understanding of microbiological processes in drinking water distribution systems and helps inform strategies to optimize asset performance. IMPORTANCE This research provides novel information regarding the dynamics of biofilm formation in real drinking water distribution systems made of different materials. This new knowledge on microbiological process in water supply systems can be used to optimize the performance of the distribution network and to guarantee safe and good-quality drinking water to consumers.

Understanding and Managing Discolouration Risk in Trunk Mains

There is currently no accepted concept or approach for understanding and controlling discolouration risk associated with trunk mains. This paper assesses the applicability of cohesive layer theories to manage discolouration and a steady state empirical modelling tool that describes the process of particulate material accumulation. Results are presented from independent field experiments across the UK and internationally that evidence hydraulically induced mobilisation, or effectively cleaning, once imposed system shear stress exceeds normal conditions. Model calibration to measured data validates the cohesive layer concept with transferability in empirically derived parameters demonstrating a viable operational planning tool. The experiments highlight the accumulation of material layers as a continuous and ubiquitous process, such that fully clean pipes can never exist and helping explain how discolouration risk changes over time. A major practical implication of the novel understanding demonstrated in this paper is that discolouration risk in trunk mains can be simply managed by pro-active strategies that regularly vary the hydraulic conditions. This avoids the need for disruptive and expensive out of service invasive interventions yet offers operators a cost-effective long-term strategy to safeguard water quality.

Long Term Asset Condition and Discolouration Modelling in Water Distribution Systems with Epanet MSX

Water distribution systems are subjected to a continual low level background flux of particulate and soluble material despite the excellent performance of modern water treatment works. Over time this material accumulates ubiquitously on boundary surfaces, building up in layers with defined cohesive properties leading to asset performance deterioration. Gross mobilisation, primarily identified as being due to system shear forces exceeding the hydraulically conditioned shear strength of this attached material, entrains this material into the bulk flow where it can remain until exiting the system via customer connections, leading to customer contacts. Through planned operational strategies, such as flushing or more invasive maintenance interventions, it is possible to manage the extent of and risk associated with accumulated material. Such interventions effectively increase network flexibility and resilience by limiting potential discolouration incidents that could otherwise result from system increases in shear stress, planned or otherwise e.g. following a pipe burst, or seasonally increased demand. To optimise maintenance strategies and determine long term operating costs water quality modelling tools are essential. Such models should incorporate the processes of material mobilisation and regeneration as part of extended period hydraulic simulations. To achieve this, modelling of the regeneration process and the rate it occurs at is required. In this paper a differential solution of the verified PODDS turbidity model of mobilisation is coded in Epanet MSX together with modes of regeneration. Following preliminary assessment the model is calibrated against a years flow and turbidity from a 300 mm transmission main in England that has seasonal hydraulic variation and 7 identified discolouration events. With material mobilisation and regeneration functionality successfully simulated, the model has the potential to be used both as an investigative and predictive tool. Of operational value is the temporal tracking of the shear strength of bound material layers, highlighting flows at which discolouration via material mobilisation is likely.