An experimental study of how hydraulic transients cause mobilisation of material within drinking water distribution systems.

This paper provides new insight into how the hydraulic transients that occur within drinking water distribution networks can mobilise material adhered to the pipe wall and hence cause unacceptable water quality and customer dissatisfaction. Results are reported from extensive, representative, physical experiments covering a wide range of repeatable rapidly accelerating and decelerating hydraulic conditions. Novel time synchronous analysis shows that mobilisation always occurs in the first dynamic surge of the transient; however, differences in the physical processes that govern mobilisation were observed between the two groups of transient type studied. A function to estimate the mobilising force is proposed and applied to the physical experiments performed. The research provides important insights for identifying and understanding the mechanisms and forces induced during transients, vital for ensuring the supply of safe drinking water in operational distribution systems.

Impact of hydraulic interventions on chronic and acute material loading and discolouration risk in drinking water distribution systems.

This paper presents results from an intensive long term investigation in three comparable trunk mains and downstream impact of non-invasive, in-service flow conditioning to manage discolouration risk. Findings show that flow conditioning, the careful regular increase in flows to mobilise small amounts of material from cohesive layers formed at the pipe wall, provides immediate risk mitigation and system resilience benefits. Evidence is presented showing longer term risk reduction in the trunk mains and a 25% discolouration risk reduction in the downstream networks. Whilst the flow conditioning produced an acute but short duration controlled mobilisation of material from the trunk main, longer term downstream monitoring showed reduced chronic or background material loading. It is proposed this change is due to altering the material exchange behaviour and volumes bound within cohesive layers that develop on bulk water/infrastructure interfaces. The paper provides evidence that flow conditioning is an efficient strategy to manage discolouration risk and improve consumer water quality throughout water distribution systems.

Succession of bacterial and fungal communities within biofilms of a chlorinated drinking water distribution system.

Understanding the temporal dynamics of multi-species biofilms in Drinking Water Distribution Systems (DWDS) is essential to ensure safe, high quality water reaches consumers after it passes through these high surface area reactors. This research studied the succession characteristics of fungal and bacterial communities under controlled environmental conditions fully representative of operational DWDS. Microbial communities were observed to increase in complexity after one month of biofilm development but they did not reach stability after three months. Changes in cell numbers were faster at the start of biofilm formation and tended to decrease over time, despite the continuing changes in bacterial community composition. Fungal diversity was markedly less than bacterial diversity and had a lag in responding to temporal dynamics. A core-mixed community of bacteria including Pseudomonas, Massillia and Sphingomonas and the fungi Acremonium and Neocosmopora were present constantly and consistently in the biofilms over time and conditions studied. Monitoring and managing biofilms and such ubiquitous core microbial communities are key control strategies to ensuring the delivery of safe drinking water via the current ageing DWDS infrastructure.

Biofilm structures (EPS and bacterial communities) in drinking water distribution systems are conditioned by hydraulics and influence discolouration.

High-quality drinking water from treatment works is degraded during transport to customer taps through the Drinking Water Distribution System (DWDS). Interactions occurring at the pipe wall-water interface are central to this degradation and are often dominated by complex microbial biofilms that are not well understood. This study uses novel application of confocal microscopy techniques to quantify the composition of extracellular polymeric substances (EPS) and cells of DWDS biofilms together with concurrent evaluation of the bacterial community. An internationally unique, full-scale, experimental DWDS facility was used to investigate the impact of three different hydraulic patterns upon biofilms and subsequently assess their response to increases in shear stress, linking biofilms to water quality impacts such as discolouration. Greater flow variation during growth was associated with increased cell quantity but was inversely related to EPS-to-cell volume ratios and bacterial diversity. Discolouration was caused and EPS was mobilised during flushing of all conditions. Ultimately, biofilms developed under low-varied flow conditions had lowest amounts of biomass, the greatest EPS volumes per cell and the lowest discolouration response. This research shows that the interactions between hydraulics and biofilm physical and community structures are complex but critical to managing biofilms within ageing DWDS infrastructure to limit water quality degradation and protect public health.

Portable LED fluorescence instrumentation for the rapid assessment of potable water quality.

Characterising the organic and microbial matrix of water are key issues in ensuring a safe potable water supply. Current techniques only confirm water quality retrospectively via laboratory analysis of discrete samples. Whilst such analysis is required for regulatory purposes, it would be highly beneficial to monitor water quality in-situ in real time, enabling rapid water quality assessment and facilitating proactive management of water supply systems. A novel LED-based instrument, detecting fluorescence peaks C and T (surrogates for organic and microbial matter, respectively), was constructed and performance assessed. Results from over 200 samples taken from source waters through to customer tap from three UK water companies are presented. Excellent correlation was observed between the new device and a research grade spectrophotometer (r(2)=0.98 and 0.77 for peak C and peak T respectively), demonstrating the potential of providing a low cost, portable alternative fluorimeter. The peak C/TOC correlation was very good (r(2)=0.75) at low TOC levels found in drinking water. However, correlations between peak T and regulatory measures of microbial matter (2 day/3 day heterotrophic plate counts (HPC), E. coli, and total coliforms) were poor, due to the specific nature of these regulatory measures and the general measure of peak T. A more promising correlation was obtained between peak T and total bacteria using flow cytometry. Assessment of the fluorescence of four individual bacteria isolated from drinking water was also considered and excellent correlations found with peak T (Sphingobium sp. (r(2)=0.83); Methylobacterium sp. (r(2)=1.0); Rhodococcus sp. (r(2)=0.86); Xenophilus sp. (r(2)=0.96)). It is notable that each of the bacteria studied exhibited different levels of fluorescence as a function of their number. The scope for LED based instrumentation for in-situ, real time assessment of the organic and microbial matrix of potable water is clearly demonstrated.

The bacteriological composition of biomass recovered by flushing an operational drinking water distribution system.

This study investigates the influence of pipe characteristics on the bacteriological composition of material mobilised from a drinking water distribution system (DWDS) and the impact of biofilm removal on water quality. Hydrants in a single UK Distribution Management Area (DMA) with both polyethylene and cast iron pipe sections were subjected to incremental increases in flow to mobilise material from the pipe walls. Turbidity was monitored during these operations and water samples were collected for physico-chemical and bacteriological analysis. DNA was extracted from the material mobilised into the bulk water before and during flushing. Bacterial tag-encoded 454 pyrosequencing was then used to characterize the bacterial communities present in this material. Turbidity values were high in the samples from cast iron pipes. Iron, aluminium, manganese and phosphate concentrations were found to correlate to observed turbidity. The bacterial community composition of the material mobilised from the pipes was significantly different between plastic and cast iron pipe sections (p < 0.5). High relative abundances of Alphaproteobacteria (23.3%), Clostridia (10.3%) and Actinobacteria (10.3%) were detected in the material removed from plastic pipes. Sequences related to Alphaproteobacteria (22.8%), Bacilli (16.6%), and Gammaproteobacteria (1.4%) were predominant in the samples obtained from cast iron pipes. The highest species richness and diversity were found in the samples from material mobilised from plastic pipes. Spirochaeta spp., Methylobacterium spp. Clostridium spp. and Desulfobacterium spp., were the most represented genera in the material obtained prior to and during the flushing of the plastic pipes. In cast iron pipes a high relative abundance of bacteria able to utilise different iron and manganese compounds were found such as Lysinibacillus spp., Geobacillus spp. and Magnetobacterium spp.

Influence of hydraulic regimes on bacterial community structure and composition in an experimental drinking water distribution system.

Microbial biofilms formed on the inner-pipe surfaces of drinking water distribution systems (DWDS) can alter drinking water quality, particularly if they are mechanically detached from the pipe wall to the bulk water, such as due to changes in hydraulic conditions. Results are presented here from applying 454 pyrosequencing of the 16S ribosomal RNA (rRNA) gene to investigate the influence of different hydrological regimes on bacterial community structure and to study the potential mobilisation of material from the pipe walls to the network using a full scale, temperature-controlled experimental pipeline facility accurately representative of live DWDS. Analysis of pyrosequencing and water physico-chemical data showed that habitat type (water vs. biofilm) and hydraulic conditions influenced bacterial community structure and composition in our experimental DWDS. Bacterial community composition clearly differed between biofilms and bulk water samples. Gammaproteobacteria and Betaproteobacteria were the most abundant phyla in biofilms while Alphaproteobacteria was predominant in bulk water samples. This suggests that bacteria inhabiting biofilms, predominantly species belonging to genera Pseudomonas, Zooglea and Janthinobacterium, have an enhanced ability to express extracellular polymeric substances to adhere to surfaces and to favour co-aggregation between cells than those found in the bulk water. Highest species richness and diversity were detected in 28 days old biofilms with this being accentuated at highly varied flow conditions. Flushing altered the pipe-wall bacterial community structure but did not completely remove bacteria from the pipe walls, particularly under highly varied flow conditions, suggesting that under these conditions more compact biofilms were generated. This research brings new knowledge regarding the influence of different hydraulic regimes on the composition and structure of bacterial communities within DWDS and the implication that this might have on drinking water quality.

Bacterial water quality and network hydraulic characteristics: a field study of a small, looped water distribution system using culture-independent molecular methods.

AIMS: To determine the spatial and temporal variability in the abundance, structure and composition of planktonic bacterial assemblages sampled from a small, looped water distribution system and to interpret results with respect to hydraulic conditions. METHODS AND RESULTS: Water samples were collected from five sampling points, twice a day at 06:00 h and 09:00 h on a Monday (following low weekend demand) and a Wednesday (higher midweek demand). All samples were fully compliant with current regulated parameter standards. This study did not show obvious changes in bacterial abundance (DAPI count) or community structure Denaturing gradient gel electrophoresis analysis with respect to sample site and hence to water age; however, the study did show temporal variability with respect to both sampling day and sample times. CONCLUSIONS: Data suggests that variations in the bacterial assemblages may be associated with the local system hydraulics: the bacterial composition and numbers, over short durations, are governed by the interaction of the bulk water and the biofilm influenced by the hydraulic conditions. SIGNIFICANCE AND IMPACT OF THE STUDY: This study demonstrates general stability in bacterial abundance, community structure and composition within the system studied. Trends and patterns supporting the transfer of idealized understanding to the real world were evident. Ultimately, such work will help to safeguard potable water quality, fundamental to public health.

Asset deterioration and discolouration in water distribution systems.

Water Distribution Systems function to supply treated water safe for human consumption and complying with increasingly stringent quality regulations. Considered primarily an aesthetic issue, discolouration is the largest cause of customer dissatisfaction associated with distribution system water quality. Pro-active measures to prevent discolouration are sought yet network processes remain insufficiently understood to fully justify and optimise capital or operational strategies to manage discolouration risk. Results are presented from a comprehensive fieldwork programme in UK water distribution networks that have determined asset deterioration with respect to discolouration. This is achieved by quantification of material accumulating as cohesive layers on pipe surfaces that when mobilised are acknowledged as the primary cause of discolouration. It is shown that these material layers develop ubiquitously with defined layer strength characteristics and at a consistent and repeatable rate dependant on water quality. For UK networks iron concentration in the bulk water is shown as a potential indicator of deterioration rate. With material layer development rates determined, management decisions that balance discolouration risk and expenditure to maintain water quality integrity can be justified. In particular the balance between capital investment such as improving water treatment output or pipe renewal and operational expenditure such as the frequency of network maintenance through flushing may be judged. While the rate of development is shown to be a function of water quality, the magnitude (peak or average turbidity) of discolouration incidents is shown to be dominated by hydraulic conditions. From this it can be proposed that network hydraulic management, such as regular periodic 'stressing', is a potential strategy in reducing discolouration risk. The ultimate application of this is the hydraulic design of self-cleaning networks to maintain discolouration risk below acceptable levels.

Laboratory studies investigating the processes leading to discolouration in water distribution networks.

Results are reported from laboratory experiments conducted to investigate the processes of discolouration within a water distribution system and specifically the concepts underpinning an empirical model proposed by Boxall et al. [Boxall, J.B., Saul, A.J., Skipworth, P.J., 2001. A novel approach to modelling sediment movement in distribution mains based on particle characteristics. Water Software Systems 1, 263-273.] and field validated by Boxall and Saul [Boxall, J.B., Saul, A.J., 2005. Modelling discolouration in potable water distribution systems. Journal of Environmental Engineering ASCE 131(5).]. The model is based on the hypothesis that discolouration is caused by the erosion and transport of fine particles, typically dominated by iron and manganese in the UK, that are attached to the pipe walls of the system by forces in addition to self-weight. These particles display cohesive-like properties and build up in layers on the pipe wall, conditioned by the usual daily flow patterns within the system. Discolouration events are caused by erosion of these layers due to changes in the system hydraulics and specifically changes in shear stress at the pipe wall, for example due to change in demand, a burst or the opening of a fire hydrant. Once cleaned from the pipe walls the layers re-accumulate under the usual conditions within the system. Experiments to determine cohesive layer behaviour and strength characteristics involved development periods followed by the measurement of the resultant discolouration when accumulated material was eroded by an increase in pipe-wall shear stress. The results support the empirical model concepts and hence its application. The results also suggest that the generation of material layers is influenced by the range of daily flow patterns, with greater variability reducing material accumulation, but not by the magnitude of steady state hydraulic conditions.

Discolouration in potable water distribution systems: a review.

A large proportion of the customer contacts that drinking water supply companies receive stem from the occurrence of discoloured water. Currently, such complaints are dealt with in a reactive manner. However, water companies are being driven to implement planned activities to control discolouration prior to contacts occurring. Hence improved understanding of the dominant processes and predictive and management tools are needed. The material responsible for discolouration has a variety of origins and a range of processes and mechanisms may be associated with its accumulation within distribution systems. Irrespective of material origins, accumulation processes and mechanisms, discolouration events occur as a result of systems changes leading to mobilisation of the accumulations from within the network. Despite this conceptual understanding, there are very few published practicable tools and techniques available to aid water companies in the planned management and control of discolouration problems. Two recently developed and published, but different approaches to address this are reviewed here: the PODDS model which was developed to predict levels of turbidity as a result of change in hydraulic conditions, but which is semi-empirical and requires calibration; and the resuspension potential method which was developed to directly measure discolouration resulting from a controlled change in hydraulic conditions, providing a direct assessment of discolouration risk, although intrinsically requiring the limited generation of discoloured water within a live network. Both these methods support decision making on the need for maintenance operations. While risk evaluation and implementation of appropriate maintenance can be implemented to control discolouration risk, new material will continue to accumulate and hence an ongoing programme of maintenance is required. One sustainable measure to prevent such re-accumulation of material is the adoption of a self-cleaning threshold, an hydraulic force which a pipe experiences on a regular basis that effectively prevents the accumulation of material. This concept has been effectively employed for the design of new networks in the Netherlands. Alternatively, measures could be implemented to limit or prevent particles from entering or being generated within the network, such as by improving treatment or preventing the formation of corrosion by-products through lining or replacing ferrous pipes. The cost benefit of such capex investment or ongoing opex is uncertain as the quantification and relative significance of factors possibly leading to material accumulation are poorly understood. Hence, this is an area in need of significant further practical research and development.

Longitudinal mixing in meandering channels: new experimental data set and verification of a predictive technique.

Evaluation of longitudinal mixing processes in open channel flows is important in environmental management, requiring the quantification of mixing coefficients. Estimates of these coefficients sufficiently accurate for environmental impact assessments cannot be achieved using current theoretical or semi-empirical methods for natural channels. This inaccuracy is caused by a limited understanding and quantification of the interaction of the dominant mechanisms resulting from natural channel features, such as plan form curvature and changes in cross-sectional shape. Experimental results are presented here from studies conducted in three self-formed channels, developed by known discharges. Longitudinal mixing was investigated at various flow rates within each of the channels by monitoring the development of tracer plumes during transit through the channels. Using an optimisation procedure, coefficients required for solution of the one-dimensional advection dispersion equation (1D-ADE) were found in the range 0.02-0.2m(2)/s. The coefficients were found to vary as functions of longitudinal meander location, channel form and discharge. Predictions of these longitudinal mixing coefficients were made using a mathematical technique requiring only channel form properties and flow rate as inputs. Predicted values were typically within 20% of the measured values, although deviation of up to 50% was found for the lowest discharge in each channel. This large error is likely to have been caused by increased dead zone effects associated with channel bathymetry at low discharges that are not captured by the method. The method was shown to be capable of capturing the variation in the longitudinal mixing coefficient with longitudinal meander location, with channel form and with discharge.