Pyrosequencing reveals bacterial communities in unchlorinated drinking water distribution system: an integral study of bulk water, suspended solids, loose deposits, and pipe wall biofilm.

The current understanding of drinking water distribution system (DWDS) microbiology is limited to pipe wall biofilm and bulk water; the contributions of particle-associated bacteria (from suspended solids and loose deposits) have long been neglected. Analyzing the composition and correlation of bacterial communities from different phases helped us to locate where most of the bacteria are and understand the interactions among these phases. In the present study, the bacteria from four critical phases of an unchlorinated DWDS, including bulk water, pipe wall biofilm, suspended solids, and loose deposits, were quantified and identified by adenosine triphosphate analysis and pyrosequencing, respectively. The results showed that the bulk water bacteria (including the contribution of suspended solids) contributed less than 2% of the total bacteria. The bacteria associated with loose deposits and pipe wall biofilm that accumulated in the DWDS accounted for over 98% of the total bacteria, and the contributions of bacteria in loose deposits and pipe wall biofilm were comparable. Depending on the amount of loose deposits, its contribution can be 7-fold higher than the pipe wall biofilm. Pyrosequencing revealed relatively stable bacterial communities in bulk water, pipe wall biofilm, and suspended solids throughout the distribution system; however, the communities present in loose deposits were dependent on the amount of loose deposits locally. Bacteria within the phases of suspended solids, loose deposits, and pipe wall biofilm were similar in phylogenetic composition. The bulk water bacteria (dominated by Polaromonas spp.) were clearly different from the bacteria from the other three phases (dominated by Sphingomonas spp.). This study highlighted that the integral DWDS ecology should include contributions from all of the four phases, especially the bacteria harbored by loose deposits. The accumulation of loose deposits and the aging process create variable microenvironments inside loose deposits structures for bacteria to grow. Moreover, loose deposits protect the associated bacteria from disinfectants, and due to their mobility, the associated bacteria reach taps easily.

Impact of particles on sediment accumulation in a drinking water distribution system.

Discolouration of drinking water is one of the main reasons customers complain to their water company. Though corrosion of cast iron is often seen as the main source for this problem, the particles originating from the treatment plant play an important and potentially dominant role in the generation of a discolouration risk in drinking water distribution systems. To investigate this thesis a study was performed in a drinking water distribution system. In two similar isolated network areas the effect of particles on discolouration risk was studied with particle counting, the Resuspension Potential Method (RPM) and assessment of the total accumulated sediment. In the 'Control Area', supplied with normal drinking water, the discolouration risk was regenerated within 1.5 year. In the 'Research Area', supplied with particle-free water, this will take 10-15 years. An obvious remedy for controlling the discolouration risk is to improve the treatment with respect to the short peaks that are caused by particle breakthrough.

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.

An experimental study on the influence of water stagnation and temperature change on water quality in a full-scale domestic drinking water system.

The drinking water quality changes during the transport through distribution systems. Domestic drinking water systems (DDWSs), which include the plumbing between the water meter and consumer's taps, are the most critical points in which water quality may be affected. In distribution networks, the drinking water temperature and water residence time are regarded as indicators of the drinking water quality. This paper describes an experimental research on the influence of stagnation time and temperature change on drinking water quality in a full-scale DDWS. Two sets of stagnation experiments, during winter and summer months, with various stagnation intervals (up to 168 h of stagnation) were carried out. Water and biofilms were sampled at two different taps, a kitchen and a shower tap. Results from this study indicate that temperature and water stagnation affect both chemical and microbial quality in DDWSs, whereas microbial parameters in stagnant water appear to be driven by the temperature of fresh water. Biofilm formed in the shower pipe contained more total and intact cells than the kitchen pipe biofilm. Alphaproteobacteria were found to dominate in the shower biofilm (78% of all Proteobacteria), while in the kitchen tap biofilm Alphaproteobacteria, Betaproteobacteria and Gammaproteobacteria were evenly distributed.

Radial transport processes as a precursor to particle deposition in drinking water distribution systems.

Various particle transport mechanisms play a role in the build-up of discoloration potential in drinking water distribution networks. In order to enhance our understanding of and ability to predict this build-up, it is essential to recognize and understand their role. Gravitational settling with drag has primarily been considered in this context. However, since flow in water distribution pipes is nearly always in the turbulent regime, turbulent processes should be considered also. In addition to these, single particle effects and forces may affect radial particle transport. In this work, we present an application of a previously published turbulent particle deposition theory to conditions relevant for drinking water distribution systems. We predict quantitatively under which conditions turbophoresis, including the virtual mass effect, the Saffman lift force, and the Magnus force may contribute significantly to sediment transport in radial direction and compare these results to experimental observations. The contribution of turbophoresis is mostly limited to large particles (>50 µm) in transport mains, and not expected to play a major role in distribution mains. The Saffman lift force may enhance this process to some degree. The Magnus force is not expected to play any significant role in drinking water distribution systems.