Modeling and optimization of production and distribution of drinking water at VMW

We develop and discuss an operational planning model aiming at minimizing production and distribution costs in large drinking water networks containing buffers with free inflow. Modeling drinking water networks is very challenging due of the presence of complex hydraulic constraints, such as friction losses and pump curves. Non-linear, non-convex constraints result from the relationships between pressure and flow in power terms. Also, binary variables are needed to model the possibility of free inflow or re-injection of water at reservoirs. The resulting model is thus a non-convex Mixed-Integer Non-Linear Program (MINLP). A discrete-time setting is proposed to solve the problem over a finite horizon made of several intervals. A commercial solver, BONMIN, suited for convex MINLP models is used to heuristically solve the problem. We are able to find a good solution for a small part of an existing network operated by the Vlaamse Maatschappij voor Watervoorziening (VMW), a major drinking water company in Flanders

WaterBox: A Testbed for Monitoring and Controlling Smart Water Networks

Copyright 2015 ACM.Smart water distribution networks are a good example of a large scale Cyber-Physical System that requires monitoring for precise data analysis and network control. Due to the critical nature of water distribution, an extensive simulation of decision making and control algorithms are required before their deployment. Although some aspects of water network behaviour can be simulated in software such as hydraulic responses in valve changes, software simulators are unable to include dynamic events such as leakages or bursts in physical models. Furthermore, due to safety concerns, contemporary large-scale testbeds are limited to the monitoring processes or control methods with well established safety guarantees. Sophisticated algorithms for dynamic and optimal water network reconfiguration are not yet widespread. This paper presents a small-scale testbed, WaterBox, which allows the simulation of emerging/advanced monitoring and control algorithms in a fail-safe environment. The flexible hydraulic, hardware, and software infrastructure enables a substantial number of experiments. On-going experiments are related to in-node data processing and decision making, energy optimization, event-driven communication, and automatic control

Multiobjective optimization of water distribution systems accounting for economic cost, hydraulic reliability, and greenhouse gas emissions

In this paper, three objectives are considered for the optimization of water distribution systems (WDSs): the traditional objectives of minimizing economic cost and maximizing hydraulic reliability and the recently proposed objective of minimizing greenhouse gas (GHG) emissions. It is particularly important to include the GHG minimization objective for WDSs involving pumping into storages or water transmission systems (WTSs), as these systems are the main contributors of GHG emissions in the water industry. In order to better understand the nature of tradeoffs among these three objectives, the shape of the solution space and the location of the Pareto-optimal front in the solution space are investigated for WTSs and WDSs that include pumping into storages, and the implications of the interaction between the three objectives are explored from a practical design perspective. Through three case studies, it is found that the solution space is a U-shaped curve rather than a surface, as the tradeoffs among the three objectives are dominated by the hydraulic reliability objective. The Pareto-optimal front of real-world systems is often located at the "elbow" section and lower "arm" of the solution space (i.e., the U-shaped curve), indicating that it is more economic to increase the hydraulic reliability of these systems by increasing pipe capacity (i.e., pipe diameter) compared to increasing pumping power. Solutions having the same GHG emission level but different cost-reliability tradeoffs often exist. Therefore, the final decision needs to be made in conjunction with expert knowledge and the specific budget and reliability requirements of the system. © 2013. American Geophysical Union. All Rights Reserved.Wenyan Wu, Holger R. Maier, and Angus R. Simpso

Burst Detection in Water Distribution Systems via Active Identification Procedure

This paper considers an approach to detect unreported pipe bursts in water distribution systems via active identification procedure. The approach involves carrying out an e-FAVOR test; results of the test are used together with a hydraulic model of the network as the inputs to a software tool, which is under development. New bursts indicators are proposed, which are considered to be more resilient to modelling errors and to inaccurate reading of the pressure logger elevation. The methodology was tested in practice in a manual manner and proved to be effective, but time consuming. In this paper some automatic analysis algorithms, developed to speed up the burst detection process, are described and tested via simulations. Results to date indicate suitability of the proposed burst indicators and the developed algorithms

Improving Stability of Electronically Controlled Pressure 1 Reducing Valves

The file attached to this record is the author's final peer reviewed version.9 This paper explains the root cause of instabilities which tend to arise in pressure reducing 10 valves (PRVs) under low flow conditions. It was found that the loss of stability in PRVs is a direct 11 result of an increase in the static valve/network gain as the valve position gets smaller, thus making 12 pressure changes more sensitive to valve position adjustments. If the valve controller is tuned at 13 medium valve openings characteristic of normal operating conditions, the increased gain at low 14 valve openings can cause the control system to be too aggressive in its valve position adjustments 15 leading to oscillations. The manuscript provides a mathematical derivation of the gain equation 16 for a simplified pipe-PRV-pipe model. The obtained gain equation curve is then used to derive the 17 formula for a gain compensator whose purpose is to keep the static gain constant across an entire 18 range of permitted valve openings. A simplified network transient model is then used to recreate a 19 real-life PRV instability event and show the remedial effects of the gain compensator

Integrated benchmark simulation model of an immersed membrane bioreactor

The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.This paper presents a new integrated model of an immersed membrane bioreactor (iMBR)for wastewater treatment. The model is constructed out of three previously published sub-models describing the bioreactor, the membrane, and the interface between them. Thebioreactor submodel extends a conventional activated sludge model with soluble and boundbiopolymers which have been found to cause irreversible and reversible fouling. The mem-brane model describes fouling as a function of biopolymer concentrations, permeate flow,and shear stresses on the membrane surface. The interface describes the dependency ofoxygen transfer rate on suspended solids concentrations and calculates shear stresses onthe membrane surface from air-scour rates. The paper serves three purposes. First, the inte-grated model is simulated on a plant layout of a previously published MBR benchmark modelwhich did not consider any interactions between the submodels. Hence, this paper presentsa new and upgraded MBR benchmark model. Secondly, the simulation results showcase howsimulations with an integrated model can be used to optimise plant performance and min-imise energy consumption. Finally, the paper introduces new measures of fouling whichcan be used for benchmarking different MBR plant layouts and control strategies

Optimal Scheduling of Variable Speed Pumps Using Mixed Integer Linear Programming -- Towards An Automated Approach

This article describes the methodology for formulating and solving optimal pump scheduling problems with variable-speed pumps (VSPs) as mixed integer linear programs (MILPs) using piece-linear approximations of the network components. The water distribution network (WDN) is simulated with an initial pump schedule for a defined time horizon, e.g. 24 hours, using a nonlinear algebraic solver. Next, the network element equations including VSPs are approximated with linear and piece-linear functions around chosen operating point(s). Finally, a fully parameterized MILP is formulated in which the objective is the total pumping cost. The method was used to solve a pump scheduling problem on a a simple two variable speed pump single-tank network that allows the reader to easily understand how the methodology works and how it is applied in practice. The obtained results showed that the formulation is robust and the optimizer is able to return global optimal result in a reliable manner for a range of operating points.Comment: Presented at 19th Computing and Control for the Water Industry Conference, CCWI 202

Bursts Identification in Water Distribution Systems

The leakage reduction problem as a whole is complex and requires co-ordinated actions in different areas of water network management, such as: direct detection and repair of existing bursts, general pipe rehabilitation programmes and operational pressure control. Water companies undertake a mixture of these complimentary actions. General pipe rehabilitation is the most costly and long term action, but is undertaken to improve a number of different factors including leakage and water quality. Operational pressure control is a cost-effective action for reducing leakage over whole sub-networks, and for reducing the risk of further leaks by smoothing pressure variations and is the subject of ongoing research. Detection and repair actions are targeted at sub-networks where bursts are present. Benefits of quick burst repair include reduced water losses, reduced disruption to traffic, reduced consequent losses (e.g. from flooding), and also reduced disruption to customers' supplies, which is an important water industry performance measure. The existing methods typically use passive identification approach whilst the presented approach is based on the active identification procedure. The proposed burst location algorithm is based on comparing data by means of statistical analysis from a simple field experiment with results of water network simulation. An extended network hydraulic simulator is used to model pressure dependent leakage terms. The presence of a burst changes the flow pattern and also pressure at network nodes, which may be used to estimate the burst size and its location. The influence of such random factors as demand flows and background leakage on the process of burst detection is also considered. The field experiment is an extended fixed and variable orifice (e-FAVOR) test. During this test inlet pressure is being stepped up and down and the following variables are measured: inlet flow, inlet pressure (head) and pressure at a number of selected sensitive nodes. The method consists of three stages and uses two different models; one is inlet flow model (IFM) to represent the total inlet flow and another is the extended hydraulic model to simulate different burst locations. Initially the presence of a potential burst is investigated. If this is confirmed values of the demand, background leakage flow and burst flow in IFM are subsequently estimated. These are used to identify the burst site at the third stage of the method. The approach has been validated by solving a practical case study with correct diagnosis of the existing problems

Online Simplification of Water Distribution Network Models

This paper presents an implementation of the simplification algorithm of water distribution network (WDN) models for the purpose of inclusion to the online optimisation strategy for the energy and leakage management in WDN, formulated within a model predictive control framework. The advantage of the online model reduction is adaptation to abnormal situations and structural changes in a network. The implementation was carried out with the utilisation of nowadays parallel programing techniques to distribute the simplification tasks across multiple CPU treads. This resulted in significant reduction of the computational time required for the simplification process of the large–scale WDN models. The authors also highlighted a problem of the energy distribution when the reduced and original models were compared

Pump schedules optimisation with pressure aspects in complex large-scale water distribution systems

This paper considers optimisation of pump and valve schedules in complex large-scale water distribution networks (WDN), taking into account pressure aspects such as minimum service pressure and pressuredependent leakage. An optimisation model is automatically generated in the GAMS language from a hydraulic model in the EPANET format and from additional files describing operational constraints, electricity tariffs and pump station configurations. The paper describes in details how each hydraulic component is modelled. To reduce the size of the optimisation problem the full hydraulic model is simplified using module reduction algorithm, while retaining the nonlinear characteristics of the model. Subsequently, a nonlinear programming solver CONOPT is used to solve the optimisation model, which is in the form of Nonlinear Programming with Discontinuous Derivatives (DNLP). The results produced by CONOPT are processed further by heuristic algorithms to generate integer solution. The proposed approached was tested on a large-scale WDN model provided in the EPANET format. The considered WDN included complex structures and interactions between pump stations . Solving of several scenarios considering different horizons, time steps, operational constraints, demand levels and topological changes demonstrated ability of the approach to automatically generate and solve optimisation problems for a variety of requirements