Valve Distribution and Impact Analysis in Water Distribution Systems

In water distribution systems, valves play a crucial role in system reliability and security by providing a shutoff function when it is necessary to isolate subsystems. In this paper failure of shutoff valves to close is considered. The failure impact is quantified in terms of the expanding subnetwork and the increased number of customers out of service. To evaluate a system-wide valve failure impact, three methods are suggested: segment–valve matrix, decision tree analysis, and simulation. The segment–valve matrix shows the propagation of failure through the network as valves fail to close. The decision tree enumerates all possible valve failure combinations and corresponding event probabilities. Because the complete enumeration can be unwieldy, simulation procedures are offered that approximate the enumeration results closely. The details of the methods are illustrated with the aid of an example.     

Real-Time Connectivity Modeling of Water Distribution Networks to Predict Contamination Spread

This paper presents a new approach to analyzing water distribution networks during a contamination event. Previous computer models for predicting the extent of contamination spread in water distribution networks are demand-driven models. The new approach makes use of supervisory control and data acquisition (SCADA) data to create connectivity matrices, which encapsulate the worst-case projection of the potential spread of contamination obtained by combining the effects of all possible scenarios. Two methods for creating connectivity matrices are described, the first based on operating modes, and the second on fundamental paths. Both methods produce identical results, although the method of fundamental paths is more efficient computationally. The connectivity- and hydraulic-based approaches are compared using an example problem.     

Adjoint Sensitivity Analysis of Contaminant Concentrations in Water Distribution Systems

Sensitivity analysis is used to determine how a system state or a model output changes due to a change in the value of a system parameter or a model input. We present the adjoint approach for determining the sensitivity of the concentration of a contaminant in a water distribution system to a change in a system parameter such as the location of the source of contamination, the reaction rate of the contaminant, and others. With the adjoint method, the sensitivity of the model output to any number of parameters can be obtained with one simulation of the adjoint model. If the number of parameters of interest exceeds the number of model outputs for which the sensitivity is desired, the adjoint method is more efficient than traditional direct methods of calculating sensitivities. We develop the adjoint equations for water quality in a water distribution system, verify the adjoint-based sensitivity equation using an analytical example, and demonstrate the numerical calculation of adjoint sensitivities using EPANET.     

Quality Modeling of Water Distribution Systems Using Sensitivity Equations

In this paper, unsteady water quality modeling and the associated sensitivity equations are solved for water distribution systems. A new solution algorithm is proposed, designed for slow varying velocity and based on a time splitting method to separate and solve efficiently each phenomenon such as advection and chemical reaction. This numerical approach allows simultaneous solution of both the direct problem and the sensitivity equations. Special attention is given to the treatment of advection, which is handled with a total variation diminishing scheme. The general model presented in this study permits global sensitivity analysis of the system to be performed and its efficiency is illustrated on two pipe networks. The importance of the sensitivity analysis is shown as part of the calibration process on a real network.     

Pseudotransient Continuation Method in Extended Period Simulation of Water Distribution Systems

This paper presents and discusses a new static solver that implements the pseudotransient continuation method for the quasi-steady state analysis, or extended-period simulation of water distribution systems. The implementation is based on the concept of virtual tanks and has a clear physical meaning. The steady state solver described in this paper can analyze a pipe network under pressure deficient conditions and is free from some convergence problems that occur in the Newton-Raphson method-based solvers when analyzing a pipe network with control devices. The numerical examples considered in the paper demonstrate the convergence of the proposed method in cases where existing static solvers (e.g., that of the EPANET 2 hydraulic simulator) fail.     

Dealing with Zero Flows in Solving the Nonlinear Equations for Water Distribution Systems

Three issues concerning the iterative solution of the nonlinear equations governing the flows and heads in a water distribution system network are considered. Zero flows cause a computation failure (division by zero) when the Global Gradient Algorithm of Todini and Pilati is used to solve for the steady state of a system in which the head loss is modeled by the Hazen-Williams formula. The proposed regularization technique overcomes this failure as a solution to this first issue. The second issue relates to zero flows in the Darcy-Weisbach formulation. This work explains for the first time why zero flows do not lead to a division by zero where the head loss is modeled by the Darcy-Weisbach formula. In this paper, the authors show how to handle the computation appropriately in the case of laminar flow (the only instance in which zero flows may occur). However, as is shown, a significant loss of accuracy can result if the Jacobian matrix, necessary for the solution process, becomes poorly conditioned, and so it is recommended that the regularization technique be used for the Darcy-Weisbach case also. Only a modest extra computational cost is incurred when the technique is applied. The third issue relates to a new convergence stopping criterion for the iterative process based on the infinity-norm of the vector of nodal head differences between one iteration and the next. This test is recommended because it has a more natural physical interpretation than the relative discharge stopping criterion that is currently used in standard software packages such as EPANET. In addition, it is recommended to check the infinity norms of the residuals once iteration has been stopped. The residuals test ensures that inaccurate solutions are not accepted.     

Modeling Residual Chlorine Response to a Microbial Contamination Event in Drinking Water Distribution Systems

Changes in chlorine residual concentrations in water distribution systems could be used as an indicator of microbial contamination. Consideration is given on how to model the behavior of chlorine within the distribution system following a microbial contamination event. Existing multispecies models require knowledge of specific reaction kinetics that are unlikely to be known. A method to parameterize a rate expression describing microbially induced chlorine decay over a wide range of conditions based on a limited number of batch experiments is described. This method is integrated into EPANET-MSX using the programmer’s toolkit. The model was used to simulate a series of microbial contamination events in a small community distribution system. Results of these simulations showed that changes in chlorine induced by microbial contaminants can be observed throughout a network at nodes downstream from and distant to the contaminated node. Some factors that promote or inhibit the transport of these chlorine demand signals are species-specific reaction kinetics, the chlorine concentration at the time and location of contamination, and the system’s unique demand patterns and architecture.     

Systematic Surge Protection for Worst-Case Transient Loadings in Water Distribution Systems

Estimating appropriate water demands for the design of a distribution system is itself difficult, but the continuously fluctuating nature of these demands has the added potential of creating water hammer problems that might result in catastrophic pipeline or system failure. To first identify and then avoid these eventualities, this paper searches a predefined set of possible water hammer events in water distribution systems to identify the most severe transient loadings and then conducts a search for suitable surge protection strategies. Genetic algorithms and particle swarm optimization are combined with transient analysis first to identify a set of worst-case loads and then to seek an optimal protection strategy to cope with them. Case studies show that the worst case is not always obvious and cannot always be assumed a priori to correspond with high or low demand scenarios. Both the search for the worst-case loading and its associated optimal protection strategies are strongly sensitive to the characteristics of both the pipe system and the candidate transient events.     

Nitrification Modeling in Pilot-Scale Chloraminated Drinking Water Distribution Systems

A suspended growth nitrification model was developed to describe nitrification dynamics in terms of chloramine, ammonia, nitrite, nitrate, and nitrifying bacteria concentrations in pilot-scale chloraminated drinking water systems. The model provided a semimechanistic base to study the regrowth and persistence of nitrifiers in chloraminated distribution systems. Results showed that the developed suspended growth model, without a biofilm nitrification component, was able to simulate and predict nitrification episodes in the pilot-scale systems. In the restricted low nutrient drinking water environment, growth kinetic parameters for nitrifiers were estimated to be significantly lower than ranges reported in the literature. The maximum specific growth rate and ammonia half-saturation constant for ammonia oxidizing bacteria were estimated to be 0.46?day?1and 0.023?mg NH3–N/L, respectively. In addition, an estimated reaction rate of 70±32?L/(mg?HPC?day) between chloramines and soluble microbial products suggests that heterotrophic growth can be a significant contributor to chloramine decay in some chloraminated distribution systems.     

Institutional Analysis of Infrastructure Problems: Case Study of Water Quality in Distribution Systems

Civil engineers working with public infrastructure face institutional problems, but they are hard to explain and no effective methodology for analyzing them has been available to civil engineers. As applied to public infrastructure, the term “institution” includes more than agencies and organizations, and extends to laws, customs, and management behaviors. A methodology for institutional analysis should provide a systematic way to answer questions about infrastructure that include: what are the laws and controls; what are the incentives; who has control and which roles; and what is the management culture? The methodology is presented and a case study of institutional problems with water quality in distribution systems identifies technical issues and gaps in institutional arrangements that inhibit solutions to them: fragmented authority, inadequate legal controls stemming from poor technical understanding, faulty incentive structures, management cultures, and unclear roles and responsibilities, made worse by difficulties in enabling the players to undertake their responsibilities. It was evident from the case study that unless institutional problems are addressed, progress is not possible on the technical and management issues. Whereas the elements of institutional analysis are not new, the methodology offers a repeatable way to structure the analysis.     

Field Studies of Discoloration in Water Distribution Systems: Model Verification and Practical Implications

Discoloration in water distribution systems has been studied in partnership with a number of U.K. water companies by measuring the turbidity response to changes in hydraulic conditions induced by systematic flushing. The resulting data was used to verify a predictive empirical model and hence the underlying assumptions made in its development. Model simulations, made using previously established parameters defined solely by pipe diameter and pipe material, are presented alongside measured data to demonstrate this verification. The primary cause of discoloration observed is the mobilization of material from cohesive layers bonded to pipe walls. These layers demonstrate a profile of increasing shear strength with increasing degree of discoloration. Differences are demonstrated in the layer and ultimate shear strength characteristics of the discoloration layers formed in iron and plastic pipes, with a modeled shear stress of 1.2?N/m2 shown to exhaust material layers in plastic pipes. Based on the observed data it is theorized that accumulation of material to the pipe walls is primarily dependent on two mechanisms; ubiquitous background concentrations in the bulk water, and if present corrosion by-products from iron pipes and fittings. A consequence of this is that all pipes within a water distribution system are susceptible to the development of material layers. In the formulation of operation and maintenance strategies it is suggested that iron and plastic pipes should be treated differently to obtain optimum operational effectiveness and minimize discoloration risk.     

Impact of Water Blending on Calcium Carbonate Equilibrium in Water Distribution Systems

A set of equations describing calcium carbonate equilibrium in water was applied to water mixtures found in an existing water supply system. A theoretical model was used to calculate the total alkalinity and pH resulting from the mixing of two waters of different origin. The accuracy of the results was tested experimentally. The experimental results confirmed the theoretical calculations with acceptable accuracy. Two different sources of possible errors in the Langelier saturation index computations were investigated and compared graphically. The first source resulted from possible inaccuracy of data, assumed to be equal to the standard errors of laboratory measurements. The second was caused by neglecting the inorganic complexes of calcium and magnesium in calculations.     

Modeling and Control of Vinyl Chloride in Drinking Water Distribution Systems

In water distribution systems containing PVC pipe manufactured in the “early era” (prior to 1977), vinyl chloride can leach into drinking water resulting in vinyl chloride concentrations exceeding the 2 μg?L?1 maximum contaminant level. Field testing of dead-end segments of water distribution systems consisting of early-era PVC pipe was conducted to examine their initial intrapipe vinyl chloride monomer (VCM) concentrations based on a Fickian-diffusion-based leaching model. The experiments showed a wide range of VCM concentrations within early-era PVC pipe ranging from less than 50 to more than 600 mg?kg?1. Based on the diffusion modeling approach, a protocol was designed that provides a means for utility managers to calibrate the model for specific dead-end lines. The paper delineates procedures to determine which dead ends require flushing to control vinyl chloride, examines the effects of system parameters such as temperature on vinyl chloride leaching, and provides a method to devise flush schedules and volumes. Through a properly designed, tested, and maintained flush protocol such as that developed in this research, public water systems with dead-end lines consisting of early-era PVC pipe can control vinyl chloride concentrations using either manual or automatic flush valves.     

Occurrence of Protozoan and Bacterial Pathogens in Water Samples from Small Water Systems in Mountainous Areas of Taiwan

Giardia, Cryptosporidium, Salmonella, and Shigella have emerged as waterborne pathogens of concern. Thirty water samples were collected from 10 small water systems in mountainous areas of Taiwan and investigated for the occurrence of these pathogens. In addition, the basic characteristics of each small water system were recorded, and each water sample was tested for several water quality parameters. The occurrence frequency was 10.0% for Giardia spp., 3.3% for Cryptosporidium spp. and Shigella flexneri, and 16.7% for Salmonella enterica. The highest correlation was found between the presence of Salmonella enterica and low water temperature, followed by the correlation between water turbidity and fecal coliform concentration. A low correlation was found between the presence of Giardia and the concentration of heterotrophic bacteria. The water samples from small water systems with filtration devices had a lower occurrence of pathogen microorganisms than those without filtration devices. The proportion of water samples with pathogenic microorganisms increased with the consumer population of the water systems, and the pathogen occurrence differed among the regions of the sampling sites. In order to prevent parasitic infection, the use of disinfection devices in small water systems would be needed.     

Improved Control of Pressure Reducing Valves in Water Distribution Networks

The behavior of transients in water pipe networks is well understood but the influence of modulating control valves on this behavior is less well known. Experimental work on networks supplied through pressure reducing valves (PRVs) has demonstrated that, in certain conditions, undesirable phenomena such as sustained or slowly decaying oscillation and large pressure overshoot can occur. This paper presents results from modeling studies to investigate interaction between PRVs and water network transients. Transient pipe network models incorporating random demand are combined with a behavioral PRV model to demonstrate how the response of the system to changes in demand can produce large or persistent pressure variations, similar to those seen in practical experiments. A proportional-integral-derivative (PID) control mechanism, to replace the existing PRV hydraulic controller, is proposed and this alternative controller is shown to significantly improve the network response. PID controllers are commonly used in industrial settings and the methods described are easy to implement in practice.     

Tank Simulation for the Optimization of Water Distribution Networks

In this paper an original approach to the simulation of floating-on-the-system tanks as decision variables for water distribution system design optimization is presented, aiming to bridge the gap between traditional engineering practice and mathematical considerations needed for genetic algorithms (GAs). The paper includes a systematic and detailed critical overview of various mathematical approaches in literature, as well as a novel, more “engineering oriented” approach to the simulation of tanks as decision variables for water distribution system design optimization, describing in detail assumptions and impacts to the evaluation of potential solutions. Tank simulation is based on two decision variables: capacity and minimum normal operational level, omitting risers. Shape and ratio between emergency/total capacities are taken into consideration as design parameters. Assessment of tank performance is carried out by four criteria for the normal daily operational cycle, differentiating between operational and filling capacity, as well as two further criteria for emergency flows. The original design and operational mathematical assumptions are implemented in a fuzzy multiobjective GA model, which is applied to the well-known example from literature “Anytown” water distribution network to benchmark the results.     

Modeling the Behavior of Flow Regulating Devices in Water Distribution Systems Using Constrained Nonlinear Programming

Currently the modeling of check valves and flow control valves in water distribution systems is based on heuristics intermixed with solving the set of nonlinear equations governing flow in the network. At the beginning of a simulation, the operating status of these valves is not known and must be assumed. The system is then solved. The status of the check valves and flow control valves are then changed to try to determine their correct operating status, at times leading to incorrect solutions even for simple systems. This paper proposes an entirely different approach. Content and co-content theory is used to define conditions that guarantee the existence and uniqueness of the solution. The work here focuses solely on flow control devices with a defined head discharge versus head loss relationship. A new modeling approach for water distribution systems based on subdifferential analysis that deals with the nondifferentiable flow versus head relationships is proposed in this paper. The water distribution equations are solved as a constrained nonlinear programming problem based on the content model where the Lagrangian multipliers have important physical meanings. This new method gives correct solutions by dealing appropriately with inequality and equality constraints imposed by the presence of the flow regulating devices (check valves, flow control valves, and temporarily closed isolating valves). An example network is used to illustrate the concepts.     

Detection of Patterns in Water Distribution Pipe Breakage Using Spatial Scan Statistics for Point Events in a Physical Network

Infrastructure systems of many U.S. cities are in poor condition, with many assets reaching the end of their service life and requiring significant capital investments. One primary requirement to optimize the allocation of investments in such systems is an effective assessment of the physical condition of assets. This paper addresses the physical condition assessment of drinking water distribution systems by analyzing pipe breakage data as the main source of evidence about the current physical condition of water distribution pipes over space. From this spatial perspective, the primary questions are whether data sets present unexpected clustering of pipe breaks, and where those break clusters are located if they do exist. This paper presents a novel approach that aims to detect and locate clusters of break points in a water distribution network. The proposed approach extends existing spatial scan statistic approaches, which are commonly used for detection of disease outbreaks in a two-dimensional spatial framework, to data collected from networked infrastructure systems. This proposed approach is described and tested in a data set that consists of 491 breaks that occurred over six years in a 160-mi water distribution network. The results presented in this paper indicate that the adapted spatial scan statistic approach applied to points in physical networks is able to detect clusters of noncompact shapes, and that these clusters present significantly higher than expected breakage rates even after accounting for pipe age and diameter. Several possible hypotheses are explored for potential causes of these clusters.     

Comparative Analysis of Chlorine Dioxide, Free Chlorine and Chloramines on Bacterial Water Quality in Model Distribution Systems

Drinking water utilities may be required to change disinfectant to improve water quality and meet more stringent disinfection regulations. This research was conducted to assess and compares chlorine dioxide to free chlorine and chloramines on bacterial water quality monitored within model distribution systems (i.e., annular reactors). Following colonization with nondisinfected water, annular reactors containing either polycarbonate or cast iron coupons were treated with free chlorine, chlorine dioxide or chloramines. Two disinfectant doses (low/high) were tested for each disinfectant. Under specific environmental conditions, bacterial inactivation varied as a function of the disinfectant type and dose, sample type (bulk water versus biofilm bacteria) and coupon material. The ranking by efficiency was as follows: chlorine dioxide > chlorine > chloramines. On preformed biofilms of 106–107?cfu/cm2, the continuous application of a disinfectant led to a log removal of heterotrophic bacteria concentrations for suspended and biofilm bacteria ranging from 1.1 to 4.0, and from 0.2 to 2.5, respectively. Doubling the amount of disinfectant doses led to an additional log inactivation of 1–2.5 of heterotrophic bacteria levels. This study demonstrates that bacterial inactivation in distribution systems is governed by various inter-related parameters. The data indicate that chlorine dioxide represents a viable alternative for secondary disinfection in distribution systems.     

Water Distribution in Laterals and Units of Subsurface Drip Irrigation. II: Field Evaluation

The performance of drip irrigation and subsurface drip irrigation (SDI) laterals has been compared. Two emitter models (one compensating and the other noncompensating) were assessed. Field tests were carried out with a pair of laterals working at the same inlet pressure. A procedure was developed that recorded head pressures at both lateral extremes and inlet flow during irrigation. Both models showed similar behavior and soil properties affected their discharge. On the other hand, the performance of a field SDI unit of compensating emitters was characterized by measuring pressures at different points and inlet flow. Finally, the distribution of water and soil pressure in the laterals and the unit were predicted and irrigation uniformity and soil pressure variability were also determined. Predictions agreed reasonably well with the experimental observations. Thus, the methodology proposed could be used to support the decision making for the design and management of SDI systems.