Control Of Sewer Systems Research Articles

Energy optimization of the urban drainage system by integrated real-time control during wet and dry weather conditions

ABSTRACTAn extension of the frequently applied equal filling degree control algorithm is proposed for integrated control of sewer system and wastewater treatment plant (WWTP). Under dry weather conditions (DWC), this control synchronizes sewer storage volume activation at pumping stations (PS) throughout the sewer system with the intermittent aeration at the WWTP. This is in contrast with the frequently advocated influent load equalization at WWTPs. The concept is demonstrated on a case study using an integrated system composed of detailed models for sewer system and WWTP where it reveals considerable potential for energy savings and effluent quality improvement. Moreover, the integrated control allows for the reduction of combined sewer overflows during wet weather events and decreased sedimentation potential in the sewer system. Neither weather predictions nor structural changes at the WWTP or in the sewer system are required. The control concept is applicable to about 50 Flemish WWTPs.

Temporal asymmetry in precipitation time series and its influence on flow simulations in combined sewer systems

A property of natural processes is temporal irreversibility. However, this property cannot be reflected by most statistics used to describe precipitation time series and, consequently, is not considered in most precipitation models. In this paper, a new statistic, the asymmetry measure, is introduced and applied to precipitation enabling to detect and quantify irreversibility. It is used to analyze two different data sets of Singapore and Germany. The data of both locations show a significant asymmetry for high temporal resolutions. The asymmetry is more pronounced for Singapore where the climate is dominated by convective precipitation events. The impact of irreversibility on applications is analyzed on two different hydrological sewer system models. The results show that the effect of the irreversibility can lead to biases in combined sewer overflow statistics. This bias is in the same order as the effect that can be achieved by real time control of sewer systems. Consequently, wrong conclusion can be drawn if synthetic time series are used for sewer systems if asymmetry is present, but not considered in precipitation modeling.

Controlling sewer systems – a critical review based on systems in three EU cities

The term Real Time Control (RTC) is widely used to describe all types of control systems in sewer systems. Today the term covers everything from the simplest to the most advanced types of control systems, making it difficult to communicate about sewer system control in a precise manner, as well as search and find specific types of control systems for comparison. Through a survey of implemented control systems in three EU cities today and with the perspectives of current research within the field of sewer system control, the needs for a new control system design framework is identified. With the basis of existing frameworks for control system design, a new time-scale dependent framework is proposed. We believe this comprehensive time-scale dependent framework can help water utilities to retrofit and design new control solutions and facilitate knowledge sharing about existing designs.

A generic methodology for the optimisation of sewer systems using stochastic programming and self-optimizing control

The design of sewer system control is a complex task given the large size of the sewer networks, the transient dynamics of the water flow and the stochastic nature of rainfall. This contribution presents a generic methodology for the design of a self-optimising controller in sewer systems. Such controller is aimed at keeping the system close to the optimal performance, thanks to an optimal selection of controlled variables. The definition of an optimal performance was carried out by a two-stage optimisation (stochastic and deterministic) to take into account both the overflow during the current rain event as well as the expected overflow given the probability of a future rain event. The methodology is successfully applied to design an optimising control strategy for a subcatchment area in Copenhagen. The results are promising and expected to contribute to the advance of the operation and control problem of sewer systems.

Regulatory control analysis and design for sewer systems

A systematic methodology for regulatory control analysis and design is adapted for sewer system operation and evaluated. The main challenge with adapting the methodology is the handling of the stochastic and transient nature of the rainfall disturbances, inherent to sewer system operation. To this end, four distinct modes of operation are identified (dry weather, filling, saturation and emptying) and for each of these the process gain matrix is found. Based on the gain matrices a controllability analysis is performed, to screen for suitable pairings between measurements and actuators in the case study area of Copenhagen. The analysis effectively reduces the number of potential controlled variables, by considering the sensitivity of the measurements towards changes in the manipulated variables. Several potential pairings are generated and the best alternative is chosen for closed-loop testing. The methodology is a promising tool for systematic generation of solutions for sewer system control.

Self-optimising control of sewer systems

Self-optimising control is a useful concept to select optimal controlled variables from a set of candidate measurements in a systematic manner. In this study, use self-optimizing control tools and apply them to the specific features of sewer systems, e.g. the continuously transient dynamics, the availability of a large number of measurements, the stochastic and unforeseeable character of the disturbances (rainfall). Using a subcatchment area in the Copenhagen sewer system as a case study we demonstrate, step by step, the formulation of the self-optimising control problem. The final result is an improved control structure aimed at optimizing the losses for a given control objective, here the minimization of the combined sewer overflows despite rainfall variations.

Real-time control of sewer systems using turbidity measurements

Real-time control (RTC) of urban drainage systems has been proven useful as a means to reduce pollution by combined sewer overflow discharges. So far, RTC has been investigated mainly with a sole focus on water quantity aspects. However, as measurement techniques for pollution of wastewater are advancing, pollution-based RTC might be of increasing interest. For example, turbidity data sets from an extensive measurement programme in two Paris catchments allow a detailed investigation of the benefits of using pollution-based data for RTC. This paper exemplifies this, comparing pollution-based RTC with flow-based RTC. Results suggest that pollution-based RTC indeed has some potential, particularly when measurements of water-quality characteristics are readily available.

Agile Design of Sewer System Control

AbstractWe describe the first part of an attempt to include stakeholder participation in the design of a central automatic controller for a sewer system in a small pilot project (five subcatchments) and present lessons learned so far. The pilot is part of a project aimed at the improvement of water quality management through central automatic control of sewer systems and surface water systems.

Fuzzy Neural Network for Flow Estimation in Sewer Systems During Wet Weather

Estimation of the water flow from rainfall intensity during storm events is important in hydrology, sewer system control, and environmental protection. The runoff-producing behavior of a sewer system changes from one storm event to another because rainfall loss depends not only on rainfall intensities, but also on the state of the soil and vegetation, the general condition of the climate, and so on. As such, it would be difficult to obtain a precise flowrate estimation without sufficient a priori knowledge of these factors. To establish a model for flow estimation, one can also use statistical methods, such as the neural network STORMNET, software developed at Lyonnaise des Eaux, France, analyzing the relation between rainfall intensity and flowrate data of the known storm events registered in the past for a given sewer system. In this study, the authors propose a fuzzy neural network to estimate the flowrate from rainfall intensity. The fuzzy neural network combines four STORMNETs and fuzzy deduction to better estimate the flowrates. This study's system for flow estimation can be calibrated automatically by using known storm events; no data regarding the physical characteristics of the drainage basins are required. Compared with the neural network STORMNET, this method reduces the mean square error of the flow estimates by approximately 20%. Experimental results are reported herein.

Comparison of conventional rule based flow control with control processes based on fuzzy logic in a combined sewer system

While conventional rule based, real time flow control of sewer systems is in common use, control systems based on fuzzy logic have been used only rarely, but successfully. The intention of this study is to compare a conventional rule based control of a combined sewer system with a fuzzy logic control by using hydrodynamic simulation. The objective of both control strategies is to reduce the combined sewer overflow volume by an optimization of the utilized storage capacities of four combined sewer overflow tanks. The control systems affect the outflow of four combined sewer overflow tanks depending on the water levels inside the structures. Both systems use an identical rule base. The developed control systems are tested and optimized for a single storm event which affects heterogeneously hydraulic load conditions and local discharge. Finally the efficiencies of the two different control systems are compared for two more storm events. The results indicate that the conventional rule based control and the fuzzy control similarly reach the objective of the control strategy. In spite of the higher expense to design the fuzzy control system its use provides no advantages in this case.

PID and PLC units for the real-time control of sewer systems

Moveable gates for the real-time control (RTC) of sewer systems storage capacities are usually operated by controllers which, on the basis of local water level or flow velocity measurements, calculate the regulation errors of the monitored variables, determining the necessary regulator movements to lead the flow conditions to the desired set point. In this paper the results of an investigation on the influence of proportional-integral-derivative (PID) controllers and programmable logic controllers (PLC) for the RTC of moveable sluice gates positioned into sewer systems are presented. The analysis of response time, damping and regulation errors provided information on the PID parameter calibration values and on the PLC control function performances. A comparison between the two kind of controllers has also been carried out.

Effects of real time control of sewer systems on treatment plant performance and receiving water quality

Four individual mathematical submodels simulating different subsystems of urban drainage were intercoupled to an integral model. The submodels (for surface runoff, flow in sewer system, wastewater treatment plant and receiving water) were calibrated on the basis of field data measured in an existing urban catchment investigation. Three different strategies for controlling the discharge in the sewer network were defined and implemented in the integral model. The impact of these control measures was quantified by representative immission state-parameters of the receiving water. The results reveal that the effect of a control measure may be ambivalent, depending on the referred component of a complex drainage system. Furthermore, it is demonstrated that the drainage system in the catchment investigation can be considerably optimised towards environmental protection and operation efficiency if an appropriate real time control on the integral scale is applied.

Investigations of the dynamic behaviour of the composition of combined sewage using on-line analyzers

Continuously analyzing sensors today permit the recording of the dynamic behaviour pattern of pollutants in combined sewage through physical parameters. This report presents the results of such measurements. They were done in a storm water tank which is equipped with a combined sewage overflow and located at the end of a 360 ha catchment area. The results show that by using a combination of sensors for dissolved solids (using UV absorption) and of sensors for particulate solids (using scattered light measurement), reliable information on the composition of the combined sewage can be obtained. A statistical relationship between these two parameters and the chemical oxygen demand was established. The analyzing methods presented permit the real-time control of sewer systems on the basis of the pollution carried in the combined sewage. Through the information on the actual concentration of primary pollutants provided by on-line analysis, heavily polluted streams can be retained selectively in storm water tanks or directed to the treatment plant. However, due to the adverse conditions in a sewer system operational disturbances and stringent maintenance requirements are still an impediment to long-term operation.

Control of sewer systems and wastewater treatment plants using pollutant concentration profiles

On-line measurements of pollutants in the wastewater combined with grey-box modelling are used to estimate the amount of deposits in the sewer system. The pollutant mass flow at the wastewater treatment plant is found to consist of a diurnal profile minus the deposited amount of pollutants. The diurnal profile is found to be a second order harmonic function and the pollutants deposited in the sewer are identified using first order ordinary differential equations.

Control of sewer systems and wastewater treatment plants using pollutant concentration profiles

On-line measurements of pollutants in the wastewater combined with grey-box modelling are used to estimate the amount of deposits in the sewer system. The pollutant mass flow at the wastewater treatment plant is found to consist of a diurnal profile minus the deposited amount of pollutants. The diurnal profile is found to be a second order harmonic function and the pollutants deposited in the sewer are identified using first order ordinary differential equations.

Model based real-time control of sewer system using fuzzy-logic

This paper describes the use of the sewer model HYSTEM-EXTRAN in combination with a rule based control device using fuzzy-logic to simulate the real-time control of a sewer system. The rules for the control of the system were set up with the help of optimization procedures. The advantage of the procedure is proved by comparing the uncontrolled versus the controlled state in a simulated mode for an existing sewer system. The final system was installed and tested within the sewer system.

Model based real-time control of sewer system using fuzzy-logic

This paper describes the use of the sewer model HYSTEM-EXTRAN in combination with a rule based control device using fuzzy-logic to simulate the real-time control of a sewer system. The rules for the control of the system were set up with the help of optimization procedures. The advantage of the procedure is proved by comparing the uncontrolled versus the controlled state in a simulated mode for an existing sewer system. The final system was installed and tested within the sewer system.

Predictive control of sewer systems by means of grey-box models

Grey-box modelling is a new methodology which can be used for obtaining important information from on-line data. This information can be used in control for optimizing the performance of the sewer system as well as the treatment plant during rain events. The objective is to minimize the effect from discharges on the receiving water without increasing the risk of flooding. This is the concept used in the control part of the EUREKA-financed project MUST.

Predictive control of sewer systems by means of grey-box models

Grey-box modelling is a new methodology which can be used for obtaining important information from online data. This information can be used in control for optimizing the performance of the sewer system as well as the treatment plant during rain events. The objective is to minimize the effect from discharges on the receiving water without increasing the risk of flooding. This is the concept used in the control part of the EUREKA-financed project MUST.

Model Based On-Line Control of Sewer Systems

A new software package has been developed for use in real-time control of the flow in combined sewer systems (CSS). A central feature of the new package is forecast modelling of the flows and volumes in the sewer system. Based upon the forecast model predictions, an expert system or an optimisation module determines the set-points for all regulators in the system. The new software package includes a comprehensive tool-set for use in the design of on-line control systems, including a model-based real-time simulator, which can be applied in the testing of control strategies. The first practical application of the software package for a sewer system in the city of Aalborg is described.