Application of system dynamics for developing financially self-sustaining management policies for water and wastewater systems

Recently enacted regulations in Canada and elsewhere require water utilities to be financially self-sustaining over the long-term. This implies full cost recovery for providing water and wastewater services to users. This study proposes a new approach to help water utilities plan to meet the requirements of the new regulations. A causal loop diagram is developed for a financially self-sustaining water utility which frames water and wastewater network management as a complex system with multiple interconnections and feedback loops. The novel System Dynamics approach is used to develop a demonstration model for water and wastewater network management. This is the first known application of System Dynamics to water and wastewater network management. The network simulated is that of a typical Canadian water utility that has under invested in maintenance. Model results show that with no proactive rehabilitation strategy the utility will need to substantially increase its user fees to achieve financial sustainability. This increase is further exacerbated when price elasticity of water demand is considered. When the utility pursues proactive rehabilitation, financial sustainability is achieved with lower user fees. Having demonstrated the significance of feedback loops for financial management of water and wastewater networks, the paper makes the case for a more complete utility model that considers the complexity of the system by incorporating all feedback loops.     

Optimisation of total water coverage in a network with priority using max-min ant system algorithm

A general mathematical programming algorithm is presented to optimise the total cost for the whole water network achieving total water coverage to all demand points in a water network with priority considering penalties associated with failure to meet the demand of two classes of consumers. The algorithm accommodates water leakages and pressure-driven demand and their effects on allocating water. Max-min ant system (MMAS) is used to solve the model using MIDACO 3.0 after interfacing MATLAB 7.0.4 and GAMS 23.9.4. The model is first solved using a particle swarm optimisation (PSO) algorithm. MMAS proved to be an effective water network optimisation tool with priority evidenced in an analysis of the Bulawayo water network by a lower total cost, inclusive of penalties for failure to meet demand for senior priority water users, which is reduced by 25.4%.     

Generation of synthetic water demand time series at different temporal and spatial aggregation levels

This paper presents a procedure for the generation and spatial-temporal aggregation of synthetic water demand time series which reproduce the main statistics - mean, variance and (spatial and temporal) covariance - of the corresponding observed series. Starting from observed historical time series taken at low levels of temporal aggregation (e.g., one minute) and relating to individual users, the procedure enables a) the generation of synthetic water demand time series for every individual user with a time step of one minute, b) the temporal aggregation of these synthetic series in order to obtain synthetic water demand time series with a time step, for example, of one hour, and which are such as to reproduce the hourly mean, variance and temporal covariances of the corresponding temporally aggregated historical time series, and c) the spatial aggregation of the synthetic hourly water demand time series of every user in order to generate a synthetic water demand time series that is representative of the entire group of users considered, and is such as to reproduce the mean, variance and temporal covariance observed at that level of spatial aggregation; The entire procedure was parameterized and applied to a case study on the water demands of 21 users of the water distribution system of Milford (Ohio). The results obtained show that the temporal aggregation procedure is effective in generating hourly water demand time series that preserve the mean, variance and temporal correlation of the historical time series for every individual user, while the spatial aggregation method shows good level of effectiveness in preserving the statistics of the aggregated series. Overall, the proposed procedure is demonstrated to be a valid tool for the bottom-up generation of synthetic water demand time series at various levels of spatial-temporal aggregation which reproduce the mean, variance and covariance statistics of the historical time series.     

Accurate water demand spatial allocation for water networks modelling using a new approach

The paper presents an alternative approach regarding the spatial allocation of the actual water demand (at node level) when developing a pipe network's hydraulic simulation model. The process takes into account the respective demand patterns of the various types of water users, considering the water being lost through leaks/breaks occurring, as a competitive use. This new method accurately approximates the demand allocation of a network when there is no GIS data, thus having a significant impact on its cost effectiveness. Kos Town (Greece) water pipe network is used as the case study to demonstrate the entire process and the problems encountered. Finally, to prove its effectiveness the results of the new method were compared to MW-Voronoi diagram method's results and to field measurements.     

Accuracy of single-jet water meters during filling of the pipe network in intermittent water supply

In intermittently operated water distribution systems the air intruded into the pipe network through the filling and emptying process must inevitably exit the system at some point. Part of this air is discharged through service connections and thus through water meters. Due to this air flow, water meters are being used contrary to their actual conception. This article presents a study in which the measurement error of single-jet water meters due to the filling process of an empty service connection is experimentally examined. It shows that this error is caused mainly by the air flow before the water front reaches the water meter. The correlation between the measurement error and the air volume in front of the water meter is almost linear. The impact of the water front, the presence of water-air mixtures or unsteady flow processes however, is shown to have very little influence on the results.     

OPTIMAL DESIGN OF WATER DISTRIBUTION NETWORKS USING FUZZY OPTIMIZATION

A new heuristic approach for the design of water distribution networks involving a robust fuzzy linear program optimization in which the capital costs of the network are minimized while maintaining the nodal heads at demand nodes within a satisfactory region as defined by the customers at those nodes is presented. Iterative interaction between the fuzzy linear program and a network solver is used to ensure hydraulic consistency. Level of service is modelled by the residual nodal head available at demand nodes with the subjective nature of customers’ satisfaction with the nodal head being represented through fuzzy sets which reflect more realistically consumers’ attitudes toward pressure variations in the supply of water. Non-probabilistic uncertainty in the demand is modelled by a trapezoidal possibility distribution function. The model is demonstrated by application to an example network.     

Rationalisation and reliability improvement of fire-fighting systems – the Novi Sad case study

Water supply systems are commonly used to supply fire-fighting water in most EU countries. To provide fast access to water, the standard practice is to install a huge number of hydrants along the entire water supply network. Pipe diameters and working pressures in the network are designed for a combination of the maximum hourly water demand and fire-fighting needs. However, in poorly managed water supply systems most of the hydrants are out of order. Although hundreds of hydrants exist in the network, the reality is that fire water tankers are usually refilled at just a few locations that are equipped with reliable hydrants maintained by the fire brigades. This paper presents a somewhat different approach for the provision of fire-fighting water. The approach discussed in the paper is based on the concept being introduced in the city of Novi Sad, Serbia, by cooperative efforts of the municipal Water Supply and Sewerage Company and the fire-fighting service. In addition to presenting the experience gained in the case study, some generic conclusions are drawn. As an alternative to having a number of unreliable and small hydrants distributed throughout the network, the new concept proposes the construction of several strategically positioned filling stations, with high-flow-rate pillar hydrants and good access roads for the manoeuvering of fire water tankers. If properly designed and distributed around the city, such filling stations would increase the reliability of fire-fighting operations. The filling stations' design methodology presented in this paper is composed of a fire-risk spatial assessment, a hydraulic check of water supply network operations and a worst-case traffic load analysis. By employing the proposed methodology, not only can the reliability of fire-fighting operations be increased, but if accepted and implemented the methodology can lead to leakage reduction and improvements in water quality and the system's energy efficiency. In the test case of Novi Sad, a large number of hydrants were replaced by 14 filling stations that have been designed and constructed. A hydraulic model of the water supply network was calibrated using pressure-drop tests and flow capacity measurements of the existing hydrants. The model was used to examine the performance of the designed filling stations during their parallel operation. The paper presents the results of the test application and recommendations for the possible implementation of the concept.     

城市二次供水改造对节水节能、安全供水的效果分析

随着我国城市化发展水平的提高,城市建设楼层也日益增高,城市供水也越来越多地采用二次供水系统向高层用户供水。二次供水即是在高密度人口的城市集中供水单位或者楼层,为解决供水压力的不足而建设高低位的蓄水设施,并连接配水管网,通过水泵进行加压,在水处理设备处理供水以后再将这种集中式供水供应到高层建筑中,实现对高层用户或配水管网末端用户的供水。论文就针对城市二次供水改造中节水节能和安全供水的效果进行简要的分析。     

水生态系统服务供需视角下的水网乡村 适应性规划策略——以吴江张鸭荡片区为例

苏南乡村地区一直是中国乡村建设的先行区域,特殊的纵横交错的水网结构构成了一个错综复杂的系统,呈现出其他地区不具有的复杂性和生态特殊性。随着城镇的扩张,乡村的发展建设使其水网空间的平衡发展面临极大的挑战。传统单一静态的规划方法逐渐显示出无法适应经济、社会等发展要求的缺陷。苏南乡村地区以水为核心,从水生态系统服务供需关系的视角下探究水网乡村的适应性规划策略更加适应当前的乡村现状和需求。以传统水网乡村空间形态转译为基础,建立水生态系统供需服务评估体系,在评估水生态系统服务供需能力的基础上,分析供需分异模式及供需矛盾。从构建乡村水域空间生态格局、乡村水域空间功能分区规划,以及乡村水域空间多情景预判3个方面,提出苏南水网乡村的适应性规划策略,并为水网乡村的生态实践提出新思路。     

Segment identification in water distribution systems

This paper presents a new method for identifying the segments that are formed after the installation and closure of isolation valves in a water distribution network. This method is able to identify segments also when one-way devices are installed in the network. Thanks to its short computing times, the method enables the analysis of real networks which always comprise a large number of nodes and pipes.

The numerical examples presented in this paper refer to two real water distribution networks. The first network is a part of a provincial network where two one-way devices are present; the second is a complex urban network without one-way devices. The method was first used to analyse the existing situation in both networks, i.e. the set of segments that are formed as a consequence of the present valve system. The method was subsequently used for the problem of the hypothetic redesign of the isolation valve system in the second urban network, i.e. the search for the optimal positions of the isolation valves in the network; in the redesign phase it provided solutions which are more cost-effective than the configuration of isolation valves currently present, the level of water service reliability being the same.     

A Review of: “Learning from Failure: The Systems Approach”, Alan N Beard,Joyce Fortune and Geoff Peters,Published by Wiley,Chichester, 1995 (ISSN No. 0379-7112)

Models for water distribution networks have become indispensable tools for understanding system behaviour by simulating pressures and flows at different locations and times in the networks. This helps inform actions that should be taken in order to improve network performance. The rationale behind traditional modelling of water networks uses a simplifying assumption that pressures in the network are a function of demand, so that it is demand that drives network performance. This demand-driven approach yields good results when pressures in the network are generally sufficient, but exhibits profound weaknesses as it produces unreliable results when network pressures are low. It therefore appears that if the network to be modelled has low pressures, then rather than employing the aforementioned demand-driven approach, the pressure-/head-driven approach should be used. In demand-driven analysis, the higher the outflow the lower the pressure while in head-driven analysis, the higher the pressure, the higher the outflow. The key difference and advantage of the head-driven approach is observed when a more realistic network model is analysed. This paper uses a real-world network to clarify the difference between the demand-driven and head-driven approach, and highlights the superiority of the latter during low-pressure conditions.     

燃气公司开发工商业用户的策略

以气源供应充足为契机,大力开发工商业用户和汽车加气用户,是燃气企业科学发展的着力点和增长点。论述了工商业用户的筛选,开发工商业用户的思路与办法,运用市场化的价格机制开发工商业用户的价格策略,开发工商业用户的工程投资策略,加强为用户服务的措施。     

Using EPA-SWMM to simulate intermittent water distribution systems

ABSTRACT

EPA Storm Water Management Model (EPA-SWMM), developed for the analysis of urban drainage systems, was used in a novel way to simulate the filling phase in intermittent water distribution systems. Firstly, the model was validated against field experimental observations from a filling test in a real water distribution network in Sicily. Then, model results were compared to those obtained by the use of a model based on the theory of the rigid water column, revealing the potential and limitations of the two approaches. The obtained results open new perspectives for the use of EPA-SWMM for the analysis of processes occurring in intermittent water distribution systems.     

用于城市燃气抢修的移动式LNG应急装置

为了避免小范围内部分用户因管网抢修而引起停气，在具有灌装功能的城镇LNG应急站的基础上，提出了开发移动式LNG应急装置的设想和工艺设计方案。     

供水管网清洗技术及其应用

周期性清洗管网对提高管网水质、恢复管道通水能力、抑制腐蚀发生、维护管网正常运行等具有重要意义，为此对管网冲洗的必要性、冲洗技术、冲洗方法以及冲洗效果评价等进行了探讨，分析了其不足之处并提出了改进建议。     

改进的RBF网络在区域需水预测中的应用

利用径向基函数神经网络,建立了区域用水量预测模型,改进了RBF网络学习方法;根据某地区近年来影响用水量主要影响因素的数据对该网络进行训练,并用训练好的网络模型对该区域以往和今后不同年份的用水量进行预测;对以往用水量预测结果表明该模型有较高预测精度、通用性和客观性.     

Optimal sensor locations for contamination detection in pressure-deficient water distribution networks using genetic algorithm

A new sensor placement problem is formulated to cover two objectives of: (1) assuring quality of water delivered to consumers; and (2) detection of any contamination event at the earliest so as to minimize its consequences, through maximization of: (1) demand coverage; and (2) time-constrained detection likelihood for pressure deficient networks. The network may become pressure-deficient owing to continued use of water distribution network beyond its design life. The two objectives are combined using weights. Genetic algorithm is used to obtain optimum sensor locations. The methodology is applied to a pressure- deficient network in the Dharampeth zone of Nagpur city (India). The pressure- dependent analysis is carried out using WaterGem v8i to simulate the system hydraulics. Performance objectives are evaluated considering the availability of flows at nodes and velocity of flow in pipes under pressure-deficient conditions. Comparison of optimal sensor network design is carried out with that obtained by demand-dependent analysis.     

A Joint US and UK Approach to Water Supply and Distribution Modelling

Network analysis has been practised in the water industry for many years, both in the UK and US, especially for capital investment planning and operational strategies. Driven now by the need to improve and maintain customer service levels, the management of water distribution is beginning to rely more on network analysis techniques. The combination of large, detailed models and easy-to-use software is moving network analysis into new areas of benefits in Severn Trent Water using the Stoner workstation service. By providing a simple, geographic interface with the user, the use of network simulation facilities has been increased tenfold in the company. In addition to the hydraulic benefits, Severn Trent Water has followed a lead from the US with water quality modelling, including nitrate blending, source water mixing and chlorine residual analysis.     

Robust optimization of valve management to improve water quality in WDNs under demand uncertainty

ABSTRACT

Water quality deterioration in water distribution networks can be associated with high water residence time in the network. To this end, some previous studies have proposed optimization procedures for valve management. However, these studies generally come up with operational configurations assuming deterministic user demand patterns that may never occur in reality. In consequence, the proposed solutions may not be effective for improving water quality or do not comply with pressure constraints if different demand patterns are observed. This study proposes a methodology to determine robust configurations of the valves to keep water residence time at acceptable levels regardless of the variability in demand patterns. The methodology is tested on four different distribution systems of varying topology and size. Results show the importance of executing robust – instead of deterministic, optimization to find valve configurations that guarantee the performance of the networks in terms of hydraulics and water quality.