The quality of drinking water delivered to customers' taps has become a technological challenge for the worldwide water industry from both a quantity and a quality viewpoint. Customers require delivery, without interruption, of high-quality drinking water that meets increasingly stringent standards for aesthetic, bacteriological and chemical water quality. There is increased public awareness of water quality issues and reporting of water quality failures that is of major concern to the water service providers and of increasing interest to the regulators. In developed countries public complaint usually relates to problems of discoloration, taste and odour, but of increasing concern are outbreaks of illness, and sometimes death, caused by the presence of bacteria, pathogens and viruses. Cases of fatalities due to drinking water pollution such as in Walkerton, Canada in 2000, although rare, have important long-term consequences. In the less developed world, water quality in distribution systems is more vulnerable because of the poorer service and in many cases intermittent supply regime. During the periods of no flow (or air entry to the pipes) there is a high likelihood of ingress of polluted groundwater. This is often the cause of spread of waterborne diseases.
Most distribution systems comprise a complex network of pipes of different ages and material types, and frequently the system is supplied from a number of different treatment works, each with different source water(s) and treatment systems. Despite the fact that modern water treatment works produce high-quality water as it enters the distribution system, the quality of the water is known to change (deteriorate) as it is transported through the system. In that respect, the distribution system can be considered as a linear bio-chemical reactor under continuous change. Such changes are sometimes rapid and may have dramatic effects, especially in the cases when chlorination is not systematic or it is insufficient.
Let us mention a few processes that affect water quality in distribution systems. The processes of dispersion and diffusion influence water quality changes. Dispersion in the bulk water phase may result in a transformation of water quality from one point in the system to another, whereas turbulent diffusion is important near the flow/wall interface, particularly in respect of the transfer processes and exchanges between the bulk water and the pipe wall. Water age is also fundamental to understanding and predicting any change in water quality. Age may be predicted using travel time concepts, but the prediction of the way in which age is blended at pipe junctions and the way in which the blended ages influence water quality are key unknowns. Such processes are strongly influenced by the pipe material and pipe wall roughness, as, for example, old and corroded pipes provide excellent hospitable environments for the growth of micro-organisms and biofilm, and hence for the exchange processes to occur. The prediction of the age of water is further compounded by the presence of service reservoirs and pumping stations.
Pipe corrosion also results in the release of constituents into the bulk water that interact with the water chemistry, for example nutrients, thus promoting microbial re-growth. Conversely such re-growth may enhance the rate of corrosion.
Biofilms act as a refuge and harbour many types of micro-organisms, for example anaerobic bacteria, protozoa, copepods and nematodes and, although the theory of biofilm kinetics is well documented under strictly controlled laboratory conditions, its extrapolation to live distribution systems is poorly understood. The modelling of biofilm kinetics therefore requires special attention.
Environmental factors such as temperature, pH and dissolved oxygen also influence microbial growth and again result in changes in water quality within the system. The problem is further compounded in complex networks where water from different parts of the network (frequently from different sources and treatment works) mix and interact, thereby creating additional and complex process reactions.
Of major importance is the need to understand the timescales associated with each of these processes as they are different, for example, some chemical reactions take only seconds, biofilm regrowth and decay may take months whilst the build-up of pipe wall scale may take years. Clearly, therefore, there is a complex ecosystem that exists within our water distribution networks and that this is influenced by many dynamic hydrodynamic, physical and bio-chemical processes that change both temporally and spatially. Despite significant advancement in various aspects of water quality in distribution systems the status of our current scientific knowledge is far from satisfactory.
This special issue of the Urban Water Journal is devoted to the issue of water quality in distribution systems and aims to contribute to understanding the state-of-the-art in this area. The following six papers have been selected for presentation of the results of the research carried out recently by the authors:
| 1. | Water quality decay in distribution systems—problems, causes, and new modeling tools by J. Woolschlager, B. Rittmann and P. Piriou | ||||
| 2. | Evaluating the impacts of treatment modifications on regrowth potential in distribution systems: a new screening procedure using water quality modeling by P. Laurent, B. Barbeau and P. Servais | ||||
| 3. | Interactive analyser for understanding water quality problems in distribution systems by M. Besner, V. Gauthier, M. Trépanier, M. Leclair and M. Prévost | ||||
| 4. | A review of modeling water quantity in distribution system by A. Ostfield | ||||
| 5. | An assessment, by computational simulation, of random daytime sampling for the optimization of plumbosolvency control treatment measures by D. Van Der Leer, C. R. Hayes and N. P. Weatherill | ||||
| 6. | Hierarchical model predictive control of integrated quality and quantity in drinking water distribution systems by K. Duzinkiewicz, M. A. Brdys and T. Chang | ||||
As can be seen from the titles and content, the papers deal with key aspects of water quality in distribution including processes, modelling, databases and other forms of informatic support, predictive control and management. These represent contributions to research in their own right and should promote and provoke further research in this timely and important field.
We would like to express our appreciation for Dr John Woolschlager, of University of North Florida, who served as guest editor for this issue.