Assimilable organic carbon cycling within drinking water distribution systems

A new conceptual model to describe and understand the role of assimilable organic carbon (AOC) within drinking water distribution systems is proposed. The impact of AOC on both drinking water biofilm and water quality was studied using bespoke pipe loop experimental facilities installed at three carefully selected operational water treatment works. Integrated physical, chemical and biological monitoring was undertaken that highlights the central role of biofilms in AOC cycling, forming the basis of the new conceptual model. Biofilms formed under high AOC conditions were found to pose the highest discoloration response, generating a turbidity (4.3 NTU) and iron (241.5 µg/l) response sufficient to have caused regulatory failures from only 20 m of pipe in only 12 months of operation. This new knowledge of the role of biofilms in AOC cycling, and ultimately impacts on water quality, can be used to inform management and help ensure the supply of high-quality, biostable drinking water

Implications of climate change: how does increased water temperature influence biofilm and water quality of chlorinated drinking water distribution systems?

Temperature variation can promote physico-chemical and microbial changes in the water transported through distribution systems and influence the dynamics of biofilms attached to pipes, thus contributing to the release of pathogens into the bulk drinking water. An experimental real-scale chlorinated DWDS was used to study the effect of increasing temperature from 16 to 24°C on specific pathogens, bacterial-fungal communities (biofilm and water samples) and determine the risk of material accumulation and mobilisation from the pipes into the bulk water. Biofilm was developed for 30 days at both temperatures in the pipe walls, and after this growth phase, a flushing was performed applying 4 gradual steps by increasing the shear stress. The fungal-bacterial community characterised by Illumina MiSeq sequencing, and specific pathogens were studied using qPCR: Mycobacterium spp., Mycobacterium avium complex, Acanthamoeba spp., Pseudomonas aeruginosa, Legionella pneumophilia, and Stenotrophomonas maltophilia. Sequencing data showed that temperature variation significantly modified the structure of biofilm microbial communities from the early stages of biofilm development. Regarding bacteria, Pseudomonas increased its relative abundance in biofilms developed at 24°C, while fungal communities showed loss of diversity and richness, and the increase in dominance of Fusarium genus. After the mobilisation phase, Pseudomonas continued being the most abundant genus at 24°C, followed by Sphingobium and Sphingomonas. For biofilm fungal communities after the mobilisation phase, Helotiales incertae sedis and Fusarium were the most abundant taxa. Results from qPCR showed a higher relative abundance of Mycobacterium spp. on day 30 and M. avium complex throughout the growth phase within the biofilms at higher temperatures. The temperature impacts were not only microbial, with physical mobilisation showing higher discolouration response and metals release due to the increased temperature. While material accumulation was accelerated by temperature, it was not preferentially to either stronger or weaker biofilm layers, as turbidity results during the flushing steps showed. This research yields new understanding on microbial challenges that chlorinated DWDS will undergo as global temperature rises, this information is needed in order to protect drinking water quality and safety while travelling through distribution systems

A worst-case-scenario in an unstable, chlorinated distribution network.

<p>Prediction of changes in the microbiological state of the water due to the depletion of residual chlorine and the concomitant growth of bacteria, potentially resulting in hygienic and sensory deterioration of the water quality.</p

Diurnal changes in bacterial parameters of WTP and DN points.

<p>Intensive sampling of one WTP (n = 19) and one point in the DN (n = 23) during 21 hours reveals steady cell concentrations at the treatment plant but clear variations in the distribution network. Intracellular adenosine tri-phosphate (ATP) data points were derived from duplicate measurements of extracellular and total ATP concentrations. FCM intact cells (after SYBR Green I and propidium iodide staining) were single measurements, with a relative standard deviation of 9% calculated from all data in the present study.</p

The impact of low velocity flushing on the water quality in a newly-opened fire hydrant.

<p>Intracellular adenosine tri-phosphate (ATP) data points were derived from duplicate measurements of extracellular and total ATP concentrations. FCM intact cells (after SYBR Green I and propidium iodide staining) were single measurements, with a relative standard deviation of 9% calculated from all data in the present study.</p

Changes in bacterial parameters between water treatment plant (WTP) and distribution network (DN) sampling points.

<p>(A) For comparison, all values at the WTP were set to 1, and values in the DN were expressed relative to their values at the WTP. The original raw data for these samples are shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0096354#pone.0096354.s002" target="_blank">Figure S2</a>. Data points are average values for duplicate FCM and ATP measurements and triplicate HPC measurements; (B) Flow cytometric density plots of samples stained with SYBR Green I and propidium iodide, showing the intact cell concentration at the plant and in the specific network point.</p

Intact cell concentrations of all samples measured from the distribution network (n = 49).

<p>(A) Intact cell concentrations arranged in ascending order and categorized into four main classes (colored bars) according to increasing concentrations. Data points are average values of duplicate measurements. Blue and purple stripes above and below data bars show the measured values. (B) Actual distribution of the classes of intact cells (colored circles) throughout the drinking water distribution network. WTP 1, WTP 2, WTP 3 represent location and productivity of the main water treatment plans supplying the city: WTP 1 operates using surface water, WTP 2 – artificially recharged ground water, WTP 3 – natural groundwater. WTP 4 – 6 indicates on other three pump stations with less significance for the city water supply.</p

Comparison of the various microbiological parameters.

<p>Clear correlations were observed between intact cells and intracellular ATP (n = 49) (A), but no obvious correlations between these two parameters and heterotrophic plate counts at 22°C (n = 38) (B) (C).</p

Average water quality parameters for the final effluents of the the three main treatment plants of Riga (Latvia).

a<p>Data supplied by the water utility or measured in previous sampling campaigns.</p>b<p>Data from present study.</p