Costing of strategies for long-term trunk main discolouration management.

Material continually accumulates throughout drinking water distribution systems, as a result episodic maintenance is essential to mitigate uncontrolled mobilisation leading to water quality failings. Focussing on discolouration as the primary issue observed by consumers, this risk is of particular significance in trunk mains that can supply large downstream populations. Long-term total costs are for the first time investigated here by considering future operational and capital interventions to sustain a defined hydraulic capability that limits the discolouration response. To achieve this, accumulation and mobilisation profiles of pipe wall material is simulated using the open source Variable Condition Discolouration Model (VCDM) to develop Pareto trade-off curves between discolouration resilience and maintenance intervention frequency and magnitude. As the rate at which material accumulates is considered a function of water quality, operational savings that could accrue from reduced maintenance following capital investment, such as water treatment upgrades, are also investigated. With the complexity, size and ageing nature of water distribution system infrastructure, the ability to forecast network discolouration behaviour and hence costs is vital for long-term delivery of safe water at least cost to customers.

Managing discolouration in drinking water distribution systems by integrating understanding of material behaviour.

Discoloured drinking water, caused by elevated concentrations of organic and inorganic particles, is unacceptable. It occurs due to accumulation and subsequent mobilisation of material from within drinking water distribution infrastructure. Discolouration is currently partially explained by either the theories of cohesive layers or gravitational sedimentation. It is proposed and shown here how the processes behind these two theories both occur and how to integrate them to better explain observed behaviour and inform operational interventions to reduce discolouration. Deficiencies in understanding regarding the process and factors that influence material accumulation are highlighted. Future research addressing these deficiencies will enable determination of long term sustainable management strategies balancing capital investment and operational maintenance to safeguard distribution of high quality drinking water.

Non-invasive Biofouling Monitoring to Assess Drinking Water Distribution System Performance.

Biofilms are endemic in drinking water distribution systems (DWDS), forming on all water and infrastructure interfaces. They can pose risks to water quality and hence consumers. Our understanding of these biofilms is limited, in a large part due to difficulties in sampling them without unacceptable disruption. A novel, non-destructive and non-disruptive biofilm monitoring device (BMD), which includes use of flow cytometry analysis, was developed to assess biofouling rates. Laboratory based experiments established optimal configurations and verified reliable cell enumeration. Deployment at three operational field sites validated assessment of different biofouling rates. These differences in fouling rates were not obvious from bulk water sampling and analysis, but did have a strong correlation with long-term performance data of the associated networks. The device offers the potential to assess DWDS performance in a few months, compared to the number of years required to infer findings from historical customer contact data. Such information is vital to improve the management of our vast, complex and uncertain drinking water supply systems; for example rapidly quantifying the benefits of improvements in water treatment works or changes to maintenance of the network.

Intermittent Water Supply Impacts on Distribution System Biofilms and Water Quality.

Intermittent water supplies (IWS) are routinely experienced by drinking water distribution systems around the world, either due to ongoing operational practices or due to one off interruptions. During IWS events changing conditions may impact the endemic biofilms leading to hydraulic mobilisation of organic and inorganic materials attached to pipes walls with a resulting degradation in water quality. To study the impact of IWS on the microbiological and physico-chemical characteristics of drinking water, an experimental full-scale chlorinated pipe facility was operated over 60 days under realistic hydraulic conditions to allow for biofilm growth and to investigate flow resumption behaviour post-IWS events of 6, 48 and 144 hours. Turbidity and metal concentrations showed significant responses to flow restarting, indicating biofilm changes, with events greater than 6 hours generating more turbidity responses and hence discolouration risk. The increase in pressure when the system was restarted showed a substantial increase in total cell counts, while the subsequent increases in flow led to elevated turbidity and metals concentrations. SUVA254 monitoring indicated that shorter times of non-water supply increased the risk of aromatic organic compounds and hence risk of disinfection-by-products formation. DNA sequencing indicated that increasing IWS times resulted in increased relative abundance of potential pathogenic microorganisms, such as Mycobacterium, Sphingomonas, and the fungi Penicillium and Cladosporium. Overall findings indicate that shorter IWS result in a higher proportion of aromatic organic compounds, which can potentially react with chlorine and increase risk of disinfection-by-products formation. However, by minimising IWS times, biofilm-associated impacts can be reduced, yet these are complex ecosystems and much remains to be understood about how microbial interactions can be managed to best ensure continued water safe supply.

The microbial ecology of a Mediterranean chlorinated drinking water distribution systems in the city of Valencia (Spain).

Drinking water distribution systems host extensive microbiomes with diverse biofilm communities regardless of treatment, disinfection, or operational practices. In Mediterranean countries higher temperatures can accelerate reactions and microbial growth that may increase aesthetic water quality issues, particularly where material deposits can develop as a result of net zero flows within looped urban networks. This study investigated the use of flow and turbidity monitoring to hydraulically manage mobilisation of pipe wall biofilms and associated material from the Mediterranean city of Valencia (Spain). Pipe sections of different properties were subjected to controlled incremental flushing with monitoring and sample collection for physico-chemical and DNA analysis with Illumina sequencing of bacterial and fungal communities. A core microbial community was detected throughout the network with microorganisms like Pseudomonas, Aspergillus or Alternaria increasing during flushing, indicating greater abundance in underlying and more consolidated material layers. Bacterial and fungal communities were found to be highly correlated, with bacteria more diverse and dynamic during flushing whilst fungi were more dominant and less variable between sampling sites. Results highlight that water quality management can be achieved through hydraulic strategies yet understanding community dynamics, including the fungal component, will be key to maintaining safe and ultimately beneficial microbiomes in drinking water distribution systems.

Unchartered waters: the unintended impacts of residual chlorine on water quality and biofilms.

Disinfection residuals in drinking water protect water quality and public heath by limiting planktonic microbial regrowth during distribution. However, we do not consider the consequences and selective pressures of such residuals on the ubiquitous biofilms that persist on the vast internal surface area of drinking water distribution systems. Using a full scale experimental facility, integrated analyses were applied to determine the physical, chemical and biological impacts of different free chlorine regimes on biofilm characteristics (composition, structure and microbiome) and water quality. Unexpectedly, higher free chlorine concentrations resulted in greater water quality degredation, observable as elevated inorganic loading and greater discolouration (a major cause of water quality complaints and a mask for other failures). High-chlorine concentrations also reduced biofilm cell concentrations but selected for a distinct biofilm bacterial community and inorganic composition, presenting unique risks. The results challenge the assumption that a measurable free chlorine residual necessarily assures drinking water safety.